King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102 - Introduction to computing
Lecture 1 : Introduction
Components of a Personal Computer
Questions :
- what are the input devices ?
- what are the output devices ?
Motherboard
A motherboard, provides the electrical connections by
which the other components of the system
communicate and hosts the central processing unit
as well as other subsystems and devices .
Motherboard example : Acer E360
CPU (Central Processing Unit)
The Central Processing Unit (CPU) or
processor is the portion of a computer system
that carries out the instructions of a computer
program and is the primary element carrying
out the computer's functions.
Example: (3 + 2) = 5
In an addition operation, the arithmetic
logic unit (ALU) will be connected to a
set of inputs and a set of outputs. The
inputs provide the numbers to be added,
and the outputs will contain the final
sum .
3
5
2
Memory
 Memory refers to computer components, devices, and recording
media that hold digital data used for computing for some interval of
time.
 There are mainly three types of memory :



CPU memory

Registers

Cache
Main memory (RAM):

Data has to be in main memory so that CPU can access it

Volatile: lost when program exits; computer shuts off
Hard Disk, CD, etc.

Persistent

This is where you keep the data for long-term storage
Main Memory …
Address
1002
1003
1004
1005
Cell
Memory is divided into
Many memory locations (cells)
Each memory cell has a
numeric address, which
uniquely identifies it
Each cell contains a data
value, e.g. 22
Main Memory …
Main Memory …
The word “Hello.” stored in 6 memory cells
Memory units : Bits and Bytes

Bit -- most basic unit of memory

1 or 0, on or off

1 Byte = 8 bits

In a computer, data values are stored as a sequence of bits
1004
2
1004
00000010
1005
7
1005
00000111
Program / CPU / Memory Interaction

Example:

Input read a number from keyboard

Add 1 to it

Output it on screen
Keyboard
1
RAM
1
1
CPU
2
RAM
2
2
Monitor
What is programming?
What is a program?

A (software) program is a list of instructions intended to a computer

The list must be ordered correctly

A program has inputs and outputs

Each instruction tells the computer to do something (an action, a
calculation, a comparison)
Program Execution

A program tells the CPU how to manipulate and/or move information

Programming is about processing information

Take some input, manipulate it in some way, and produce a particular
output
Program
Inputs
Manipulation
Outputs
Example 1 : Recipe for Scrambled Eggs



Ingredients (Inputs) : two eggs, tablespoon of
oil, salt
Instructions (program):

Add oil to pan

Heat pan on stove

Crack eggs into pan

Add salt

Mix until light and flakey
Output: scrambled eggs
Example 2 : Currency Exchange
Task : convert an amount of money in some currency (e.g. US
Dollars) to another one (e.g. Saudi Riyals).

Input:




•
Amount
Source Currency
Desired Currency
Instructions

Look up in table current exchange rate for the selected
currencies

Calculate result as amount * exchange rate
Output: result
Programming language


A programming language is the language used to write programs
A programming language employs a set of rules that dictate how the words and
symbols can be put together to form valid program statements

A programming language has a syntax and semantics

There are several types of languages (functional, Object-Oriented, etc.)

In this course we focus on Java programming language.
Java
• Java is a programming language originally
developed by James Gosling at Sun Microsystems
• It was first released in 1995.
• The language derives much of its syntax from C
and C++.
• But has a simpler object model and fewer lowlevel facilities than C and C++.
Why Java ?
Currently, Java is the most
popular language in the
world !
Why Java ?
1. Simple
• Get started quickly
2. Concise
• Write less code
3. Object-oriented
• Better quality code
4. Portable
• Architecture neutral (write once run anywhere)
5. Secure
• More appropriate for Internet
Portability of Java
Classical model :
Java model :
Portability of Java
Write once, run anywhere :Because applications written in the
Java programming language are compiled into machineindependent bytecodes, they run consistently on any Java
platform .
A Java Program
A Java Program



A Java program consists of one
or more classes
A Java class consists of one or
more methods
A Java method consists of one
or more statements
Java
classes
Java
Methods
A Java Program

A Java program resides in one or more files.

The file name of a Java program has extension .java.

One of the classes of a Java program is called the driver class.


The name of the driver class must be the same as the name of
its Java file. (Java is case sensitive. So EX1 is different from
ex1.)
The diver class must contain a method called main. The
execution of Java program starts from the main method of the
driver class.
Example of a Java Program
Example of a Java Program
Class name
Main method
Class body
Instruction
Example of a Java Program
Also notice:
Curly braces { }
Example of a Java Program
Also notice:
Curly braces { }
Parentheses ( )
Example of a Java Program
Also notice:
Curly braces { }
Square brackets [ ]
Parentheses ( )
Example of a Java Program
Also notice:
A pair of braces { }
define a block
Compiling and running a program

Type in your program

Save the program


Store all your files in one directory for now

Give the program the same name as the class
Compile the program


this produces a .class file
Translates the program into something the computer can understand and
execute (Java bytecode)

Run the program

Observe the result and adjust the program if necessary
Edit
Public class Adven
/*
This program is an Arit
*/
Public class
/*
Text Editor
public static void main
/* Program statements g
System.out.print(“Wel
}
}
Adventure.java
Compile - With Errors
Public class Adven
/*
This program is an Arit
*/
public static void main
/* Program statements g
System.print(“Welcome
}
Compiler
}
Adventure.java
Errors and Warnings
------------------Error : The method print(
String) is undefined for
Type System
Compile - Success
Public class Adven
/*
This program is an Arit
*/
public static void main
/* Program statements g
System.out.print(“Wel
}
Compiler
}
Adventure.java
001011010001011101
1011010001011101110
101000101110111010110100010
111011101011010001011101110
101101000101110111010110100
010111011100010111011101011
010001011101110010111011101
011010001011101110010111011
101011010001011101110010111
011101011010001011101110010
111011101011010001011101110
0101110
Adventure.class
Run Program
Fred
1
Adventure
Welcome to the Arithm
The date is Monday Sep
What is your name?Fred
Well Fred, after a day
The cube appears to be
You find a Green door,
The door closes behind
There is a feel of mat
End
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 1 : Expressions and
Assignment
Outline











July 17, 2016
Declaration Statements
Primitive Data Types
Identifiers
Assignment Statements
Arithmetic Operators and Expressions
Precedence Rules
Round-Off Errors in Floating-Point Numbers
Integer and Floating-Point Division
The % Operator
Type Casting
Increment and Decrement Operators
ICS102: Expressions & Assignment
38
Variable

variable
In mathematics:
x + y = 17
variable

variable
July 17, 2016
In Computer Science: a variable is an identifier
(usually a letter, word, or phrase) that is linked to
a value stored in the computer's memory
x = 5;
ICS102: Expressions & Assignment
39
Variable Declaration

Every variable in a Java program must be declared
before it is used
int numberOfBeans;
numberOfBeans = 10;


A variable declaration tells the compiler what kind of data
(type) will be stored in the variable
Basic types in Java are called primitive types
- Primitive Data Types
July 17, 2016
ICS102: Expressions & Assignment
41
Names of variables (Identifiers)


int variable = 3;
The name of a variable must not start with a digit, and all the
characters must be letters, digits, or the underscore symbol


Java is a case-sensitive language: Rate, rate, and RATE are
the names of three different variables
It is not allowed to use reserved words as variable names:
public
July 17, 2016
class
void
ICS102: Expressions & Assignment
static
42
Assignment Statements


In Java, the assignment statement is used to change the
value of a variable
An assignment statement consists of a variable on the left
side of the operator, and an expression on the right side of
the operator
Variable = Expression;

Example
temperature = 98.6;
count = numberOfBeans;
July 17, 2016
ICS102: Expressions & Assignment
44
Assignment Statements With Primitive Types

When an assignment statement is executed, the expression is
first evaluated, and then the variable on the left-hand side of
the equal sign is set equal to the value of the expression
distance = rate * time;

Note that a variable can occur on both sides of the assignment
operator
count = count + 2;

The assignment operator is automatically executed from rightto-left, so assignment statements can be chained
number2 = number1 = 3;
July 17, 2016
ICS102: Expressions & Assignment
45
Initialize Variables …
int initialCount = 50;
int finalCount;
What is the value of initialCount?
What is the value of finalCount?


A variable that has been declared but that has not yet been given
a value by some means is said to be uninitialized
In certain cases an uninitialized variable is given a default value

July 17, 2016
It is best not to rely on this and always initialize your variables
ICS102: Expressions & Assignment
46
Shorthand Assignment Statements …

e.g.  variable += 3;

Shorthand assignment notation combines the assignment
operator (=) and an arithmetic operator

The general form is
Variable Op = Expression
July 17, 2016
Example:
Equivalent To:
count += 2;
count = count + 2;
sum -= discount;
sum = sum – discount;
bonus *= 2;
bonus = bonus * 2;
time /= rushFactor;
time = time / rushFactor;
change %= 100;
change = change % 100;
amount *= count1 + count2;
amount = amount * (count1 + count2);
ICS102: Expressions & Assignment
47
Arithmetic Operators and Expressions




These are : + (addition), - (subtraction), * (multiplication), /
(division), and % (modulo, remainder)
If an arithmetic operator is combined with int operands, then
the resulting type is int
If an arithmetic operator is combined with one or two double
operands, then the resulting type is double
If different types are combined in an expression, then the
resulting type is the right-most type on the following list that is
found within the expression
byteshortintlongfloatdouble
Char
July 17, 2016
ICS102: Expressions & Assignment
48
Assignment Compatibility

Is this legal ?
int x = 2.99;

And this ?
double variable = 2;
July 17, 2016
ICS102: Expressions & Assignment
49
Assignment Compatibility

More generally, a value of any type in the following list can
be assigned to a variable of any type that appears to the
right of it
byteshortintlongfloatdouble

Note that as your move down the list from left to right,
the range of allowed values for the types becomes
larger
int x = 5.8;
error
double y = 6; ok
short i = 14.3; error
float s = 9.2; ok
For these, you need type cast.
next slide
July 17, 2016
ICS102: Expressions & Assignment
50
Type Casting

A type cast takes a value of one type and produces
a value of another type with an "equivalent" value
int x = (int) 2.9;

When type casting from a floating-point to an
integer type, the number is truncated, not
rounded:
(int) 2.9 evaluates to 2, not 3
July 17, 2016
ICS102: Expressions & Assignment
51
Integer and Floating-Point Division

When one or both operands are a floating-point type,
division results in a floating-point type
15.0/2 evaluates to 7.5

When both operands are integer types, division results
in an integer type

Any fractional part is discarded

The number is not rounded
15/2 evaluates to 7

July 17, 2016
Be careful to make at least one of the operands a
floating-point type if the fractional portion is needed
ICS102: Expressions & Assignment
52
y 17, 2016
Precedence Rules

If some or all of the parentheses in an expression are omitted,
Java will follow precedence rules to determine, in effect, where
to place them.
High precedence
First: The unray operators: +, -, ++, --, and !
Second: The binary arithmetic operators: *, /. And %
Third: The binary arithmetic operators: + and Low precedence
e.g.  int x = y = -2 + 5 * 7 – 7 / 2 % 5;
y 17, 2016
Precedence Rules

If some or all of the parentheses in an expression are omitted,
Java will follow precedence rules to determine, in effect, where
to place them.
High precedence
First: The unray operators: +, -, ++, --, and !
Second: The binary arithmetic operators: *, /. And %
Third: The binary arithmetic operators: + and Low precedence
e.g.  int x = y = -2 + 5 * 7 – 7 / 2 % 5;
This will be evaluated as..
int x = (y = ((-2 + (5 * 7)) – ((7 / 2) % 5)));
35
3
3
33
30
ICS102: Expressions & Assignment
54
The % (modulo) Operator

The % operator is used with operands of type int to recover
the information lost after performing integer division
15/2 evaluates to the quotient 7
15%2 evaluates to the remainder 1

The % operator can be used to count by 2's, 3's, or any other
number

July 17, 2016
To count by twos, perform the operation number % 2, and
when the result is 0, number is even
ICS102: Expressions & Assignment
55
Increment and Decrement Operators

The increment operator (++) adds one to the value
of a variable


The decrement operator (--) subtracts one from the
value of a variable

July 17, 2016
If n is equal to 2, then n++ or ++n will change the value of
n to 3
If n is equal to 4, then n-- or --n will change the value of
n to 3
ICS102: Expressions & Assignment
56
Increment and Decrement Operators

When either operator precedes its variable, and is part
of an expression, then the expression is evaluated using
the changed value of the variable


When either operator follows its variable, and is part of
an expression, then the expression is evaluated using
the original value of the variable, and only then is the
variable value changed

July 17, 2016
If n is equal to 2, then 2*(++n) evaluates to 6
If n is equal to 2, then 2*(n++) evaluates to 4
ICS102: Expressions & Assignment
57
The end
Important to do at home :
- read section 1.2 of the textbook
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 3 : Math
What is a library?



Most programming languages come with a library.
A library is a collection of classes for common system
services such as : math operations, input/output
routines, etc.
The Java library is called : API (Application Programming
Interface).

Java API is organized into packages.

Today we will see a class of this library : Math.
The Math Class


The Math class provides a number of standard
mathematical methods
The Math class has two predefined constants, E (e,
the base of the natural logarithm system) and PI (,
3.1415 . . .)
area = Math.PI * radius * radius;
Some Methods in the Class Math …
Some Methods in the Class Math …
Some Methods in the Class Math …
Some Methods in the Class Math …
Some Methods in the Class Math
The end
Exercises
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 4 : String Class
Outline

The String Class

Explicit String Objects

String variables are References

String Methods

Escape Sequences
July 17, 2016
ICS102: The class String
70
The String Class



There is no primitive type for strings in Java
The class (type) String is a predefined class in Java that is used to
store and process strings
A String object is a sequence of characters that is treated as a
single value.
 When any “ ” (quote) appears, a String object is created
automatically.
System.out.println("Hello, how are you?");

A variable of type String can be given the value of a String
object
String sentence = "Java is Fun!";
71
July 17, 2016
ICS102: The class String
Explicit String Objects

A declaration and object creation are needed for
instances of the String class. For example,
String name1;
name1 = new String("Salam");

We normally use shorthand notation
(only for Strings):
These two
statements are
equivalent.
String name1;
name1 = "Salam";
July 17, 2016
ICS102: The class String
72
String variables are References…
Code
A
String
word1, word2;
word1 = new String("Java");
word2 = word1;
State of
Memory
word1
L
word2
L
After A
is executed
Both word1 and word2
are allocated memory
(to store references),
but the objects
themselves are not yet
created, so they both
contain null.
String variables are References…
Code
String
B
One String object is
created and assigned to
word1, so word1
contains the address of
this object.
word1, word2;
word1 = new String("Java");
word2 = word1;
word1
String
Java
State of
Memory
word2
L
After B
July 17, 2016
is executed
ICS102: The class String
74
String variables are References.
Code
String
Content of word1,
which is an address, is
assigned to word2,
making word2 refer to
the same object.
word1, word2;
word1 = new String("Java");
C
word2 = word1;
word1
String
Java
State of
Memory
word2
After C
July 17, 2016
is executed
ICS102: The class String
75
String Methods

String Concatenation

String Length

String Starting Position

Other Useful String Operators
July 17, 2016
ICS102: The class String
76
String Concatenation

Concatenation: Using the + operator on two (or more) strings in
order to connect them to form one longer string
String greeting = "Hello ";
String course
= "ICS102";
System.out.println(greeting + course);


Hello ICS102
A new String object is created operands are not affected.
When a string is combined with almost any other type of item, the
result is a string
int x = 6, y = 10;
String s = ".Hi.";
System.out.println(s
System.out.println(x
System.out.println(s
System.out.println(s
July 17, 2016
+
+
+
+
x);
y + s);
(x + y));
x + y);
ICS102: The class String
.Hi.6
16.Hi.
.Hi.16
.Hi.610
77
String Length

We determine the number of characters in a String with
the length method.
String name = "Espresso!";
String str2 = "";
String str3;
name.length();
9
str2.length();
0
str3.length();
Error!
Error because no
object is created for
str3, so it is a null.
String Starting Position


Individual characters in a String can be accessed with the charAt
method.
Position of a character or String can be found with indexOf
method.
String text = "Java is fun.";
text
This variable refers to the
whole string.
text.indexOf("is")
The method returns the
position of the string “is”.
text.charAt(8)
The method returns the
character at position # 8.
Other Useful String Operators
Method
equals
Meaning
Checks if two strings are equal. (Use equalsIgnoreCase for case
insensitive)
str1.equals( str2 )
compareTo
Compares the two strings.
str1.compareTo( str2 )
substring
Extracts the a substring from a string.
str1.substring( 1, 4 )
str1.substring( 5 )
trim
toUpperCase
Removes the leading and trailing spaces.
str1.trim( )
Converts a string to all caps string. (Use toLowerCase for all small)
str1.toUpperCase( )
Escape Sequences …

A backslash (\) immediately preceding a character (i.e.,
without any space) denotes an escape sequence or an
escape character


July 17, 2016
The character following the backslash does not have its usual
meaning
Although it is formed using two symbols, it is regarded as a
single character
ICS102: The class String
81
Escape Sequences
July 17, 2016
ICS102: The class String
82
The end
Important to do at home :
- read section 1.3 (pages 65-77)
Exercises

For each of these expressions determine its result
String text = "Java Programming";





text.substring(0,
text.length( )
text.substring(8,
text.substring(0,
text.substring(5,
4)
12)
1) + text.substring(7, 9)
6) + text.substring(text.length() – 3, text.length())
Exercises

Write a program that initializes a String object to
“Hello the World”, prints its length, then erases the
word “the” from that String.
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 5 : Screen Output
and Console Input
System.out.println

To write of the screen:
System.out.println(“Hello World");
Package name

Class name
Method name
The item to be printed on the screen
It is possible to print more than one item:

A plus sign is used to connect more than one item
System.out.println("The answer is " + 42);

Every invocation of println generates a new line after it finishes
System.out.println(“Hello World”);
System.out.println(“Hello World”);
Hello World
Hello World
println Versus print

To stay on the same line, use print
System.out.print(“Hello World ”);
System.out.print(“Hello World ”);

Hello World Hello World
The print method is like println, except that it does not generate
a new line

With println, the next output goes on a new line

With print, the next output goes on the same line
Console Input
What if we want to let
the user choose the
values he wants to use ?
We can prompt the user by console (keyboard) input
Console Input
Import instruction
Create Scanner
object
Read a first integer and
assign it to variable a
Read a second integer and
assign it to variable b
Console Input
Details ..


Java includes a class for doing simple keyboard input named the
Scanner class
In order to use the Scanner class, a program must include the
following line near the start of the file:
import java.util.Scanner

This statement tells Java to


Make the Scanner class available to the program
Find the Scanner class in a library of classes (i.e., Java
package) named java.util
Details ..

The following line creates an object of the class Scanner and
names the object keyboard :
Scanner keyboard = new Scanner(System.in);

Although a name like keyboard is often used, a Scanner object
can be given any name

For example, in the following code the Scanner object is
named scannerObject
Scanner scannerObject = new Scanner(System.in);

Once a Scanner object has been created, a program can then use
that object to perform keyboard input using methods of the
Scanner class
Details ..

The method nextInt reads one int value typed in at the keyboard
and assigns it to a variable:
int numberOfPods = keyboard.nextInt();

The method nextDouble reads one double value typed in at the
keyboard and assigns it to a variable:
double d1 = keyboard.nextDouble();

Multiple inputs must be separated by whitespace and read by
multiple invocations of the appropriate method

Whitespace is any string of characters, such as blank spaces, tabs, and
line breaks that print out as white space on paper
Details ..


The method next reads one string of non-whitespace characters
delimited by whitespace characters such as blanks or the beginning or
end of a line
Given the code
String word1 = keyboard.next();
String word2 = keyboard.next();
and the input line
jelly beans
The value of word1 would be jelly, and the value of word2 would
be beans
Details ..

The method nextLine reads an entire line of keyboard input

The code,
String line = keyboard.nextLine();
reads in an entire line and places the string that is read into the
variable line

The end of an input line is indicated by the escape sequence '\n‘



This is the character input when the Enter key is pressed
On the screen it is indicated by the ending of one line and the beginning of the next
line
When nextLine reads a line of text, it reads the '\n' character, so
the next reading of input begins on the next line

However, the '\n' does not become part of the string value returned (e.g.,
the string named by the variable line above does not end with the '\n'
character)
Dealing with the Line Terminator, '\n‘ …

The method nextLine of the class Scanner reads the remainder of a line
of text starting wherever the last keyboard reading left off

This can cause problems when combining it with different methods for
reading from the keyboard such as nextInt

Given the code,
Scanner keyboard = new Scanner(System.in);
int n = keyboard.nextInt();
String s1 = keyboard.nextLine();
String s2 = keyboard.nextLine();
and the input,
2
Heads are better than
1 head.
what are the values of n, s1, and s2?
Dealing with the Line Terminator, '\n'

Given the code and input on the previous slide
n will be equal to 2,
s1 will be equal to "", and
s2 will be equal to "heads are better than“

If the following results were desired instead
n equal to 2,
s1 equal to "heads are better than", and
s2 equal to "1 head“
then an extra invocation of nextLine would be needed to get
rid of the end of line character ('\n')
Dealing with the Line Terminator, '\n'

Given the code
Scanner keyboard = new Scanner(System.in);
int n = keyboard.nextInt();
keyboard.nextLine();
String s1 = keyboard.nextLine();
String s2 = keyboard.nextLine();
and the input,
2
Heads are better than
1 head.

We get the desired result:
n equal to 2,
s1 equal to "heads are better than", and
s2 equal to "1 head“
Programming Tip …

Prompt for Input

A program should always prompt the user when he or she needs
to input some data:
System.out.println( "Enter the number of pods followed by");
System.out.println("the number of peas in a pod:");
The end
Important to do at home :
- read chapter 2 (pages 90-118)
Methods in the Class Scanner …
Methods in the Class Scanner ….
Methods in the Class Scanner
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 6 : File Input Output
Outline

Text Files

Stream

Reading from a Text File using Scanner

Writing to a File

IOException

Pitfalls
Text Files

Files that are designed to be read by human beings, and
that can be read or written with an editor are called text files


Text files can also be called ASCII files because the data they
contain uses an ASCII encoding scheme
An advantage of text files is that they are usually the same on
all computers, so that they can move from one computer to
another
ASCII Encoding
Streams

A stream is an object that enables the flow of data
between a program and some I/O device or file


If the data flows into a program, then the stream is called
an input stream
If the data flows out of a program, then the stream is
called an output stream
file
program
Streams

Input streams can flow from the keyboard or from a file

System.in is an input stream that connects to the
keyboard
Scanner keyboard = new Scanner(System.in);

Output streams can flow to a screen or to a file

System.out is an output stream that connects to the
screen
System.out.println("Output stream");
Reading From a Text File Using Scanner

The class Scanner can be used for reading from the
keyboard as well as reading from a text file

Simply replace the argument System.in (to the Scanner
constructor) with a suitable stream that is connected to the
text file
Scanner StreamObject =
new Scanner(new FileInputStream(FileName));

Methods of the Scanner class for reading input behave the
same whether reading from the keyboard or reading from a
text file

For example, the nextInt and nextLine methods
Reading Input from a Text File Using Scanner
Reading Input from a Text File Using Scanner
Reading Input from a Text File Using Scanner
Reading Input from a Text File Using Scanner
Writing to a Text File

The class PrintWriter is a stream class that can be
used to write to a text file


An object of the class PrintWriter has the methods
print and println
These are similar to the System.out methods of the
same names, but are used for text file output, not screen
output
Writing to a Text File

All the file I/O classes that follow are in the package
java.io, so a program that uses PrintWriter will start
with a set of import statements:
import java.io.PrintWriter;
import java.io.FileOutputStream;
import java.io.FileNotFoundException;

The class PrintWriter has no constructor that takes a file
name as its argument

It uses another class, FileOutputStream, to convert a file
name to an object that can be used as the argument to its (the
PrintWriter) constructor
Writing to a Text File

A stream of the class PrintWriter is created and
connected to a text file for writing as follows:
PrintWriter outputStreamName;
outputStreamName =
new PrintWriter(new FileOutputStream(FileName));


The class FileOutputStream takes a string representing the
file name as its argument
The class PrintWriter takes the anonymous
FileOutputStream object as its argument
Writing to a Text File


This produces an object of the class PrintWriter that is
connected to the file FileName

The process of connecting a stream to a file is called opening

If the file already exists, then doing this causes the old
contents to be lost

If the file does not exist, then a new, empty file named
FileName is created
the file
After doing this, the methods print and println can be
used to write to the file
Writing to a Text File
Path Names

When a file name is not in the same directory, the path name
must be given

First way:
Scanner in = new Scanner(new
FileInputStream(“c:/user/data.txt"));

Second way:
Scanner in = new Scanner(new
FileInputStream(“c:\\user\\data.txt"));
The end
Important to do at home :
- read chapter 10 from 602 to 615
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 7 : Selection
Structures
Outline

Block statement

Branching Statements

July 17, 2016

Simple if statement

if-else statement

if-else-elseif statement

switch statement
Nested if statements
ICS102: The course
124
Introduction to selection structures

Write a program that accepts an integer value from
the user and then:

prints “positive” if the integer is larger or equal to zero

Prints “negative” if the integer is less than zero
Block Statement


A block statement consists of one or more Java statements
enclosed in braces.
Example of a block statement:
{
}


statement 1;
statement 2;
…
statement n;
Blocks can be nested.
A block statement can be used anywhere that a single statement
can be used.
July 17, 2016
ICS102: The course
126
Branching Statement



July 17, 2016
A branching statement consists of one or more block
statements
The execution of a block statement in a branching statement is
controlled by a boolean expression which we call a condition.
There are mainly the following four types of branching
statement.

Simple if

If-else

If-elseif-else

switch
ICS102: The course
127
Simple if Statement …

Simple if statement has the following structure:
if ( <boolean_expression> ) {
<then_block>
}


The boolean_expression must be enclosed in parentheses
If the boolean_expression is true, then the then_block is executed.
Otherwise it will NOT execute.
Boolean Expression
if (
testScore >= 95
) {
System.out.println("You are a good student");
Then Block
}
July 17, 2016
ICS102: The course
128
-- Simple if Statement …
Control Flow of if:
true
testScore >=
95?
false
July 17, 2016
System.out.println
("You are a good student");
ICS102: The course
Chapter 6 - 129
… -- Simple if Statement

}
Example: Design and write a Java program prints the absolute
value of a number.
import java.util.Scanner;
class absolute {
public static void main(String [] args) {
Scanner kb = new Scanner(System.in);
System.out.print(“Enter a number: “);
double x = kb.nextDouble();
double y = x;
if( y < 0)
{
y = -y;
}
System.out.print(“The absolute value of “ + x + “ is “ + y);
}
July 17, 2016
ICS102: The course
130
-- if-else Statement …


An if-else statement chooses between two alternative statements
based on the value of a Boolean expression
If the boolean_expression is true, then the then_block is executed,
otherwise the else_block is executed.
if (<boolean_expression>) {
<then_block>
} else {
<else_block>
}
Boolean Expression
if (testScore < 50) {
Then Block
System.out.println("You did not pass");
} else {
Else Block
July 17, 2016
System.out.println("You did pass");
}
ICS102: The course
131
-- if-else Statement …
false
true
testScore < 50 ?
System.out.println
("You did pass");
July 17, 2016
System.out.println
("You did not pass");
ICS102: The course
Chapter 6 - 132
Compound Statements


You have to use braces if the <then> or <else> block has multiple
statements.
if only one statement is there braces are optional but it is advisable to
always use them to enhance readability
if (testScore < 70)
{
System.out.println("You did not pass");
System.out.println("Try harder next time");
Then Block
}
else
{
System.out.println("You did pass");
System.out.println("Keep up the good work");
Else Block
}
July 17, 2016
ICS102: The course
Chapter 6 - 133
… -- if-else Statement

Design and write a Java program which prints the difference of two
numbers.
Scanner kb = new Scanner(System.in);
System.out.println(“Enter the value of the first number: “);
double first = kb.nextDouble();
System.out.println(“Enter the value of the second number: “);
double second = kb.nextDouble();
if ( first > second)
{
double diff = first – second;
System.out.println(diff);
}
else
{
double diff = second – first;
System.out.println(diff);
}
July 17, 2016
ICS102: The course
134
-- if-elseif-else Statement …

The multiway if-else statement is simply a
normal if-else statement that nests another ifelse statement at every else branch




July 17, 2016
It is indented differently from other nested statements
All of the Boolean_Expressions are aligned with one
another, and their corresponding actions are also aligned
with one another
The Boolean_Expressions are evaluated in order until
one that evaluates to true is found
The final else is optional
ICS102: The course
135
if - else- if
if (score >= 85) {
System.out.println(”Grade is A");
} else if (score >= 75) {
System.out.println(”Grade is B");
Test Score
85  score
75  score  85
65  score  75
} else if (score >= 65) {
System.out.println(”Grade is C");
} else if (score >= 50) {
50  score  65
score  50
System.out.println(”Grade is D");
Grade
A
B
C
D
N
} else {
System.out.println(”Grade is N");
}
July 17, 2016
ICS102: The course
Chapter 6 - 137
- The switch Statement …

The switch statement is the only other kind of Java
statement that implements multiway branching


When a switch statement is evaluated, one of a number of
different branches is executed
The choice of which branch to execute is determined by a
controlling expression enclosed in parentheses after the
keyword switch

July 17, 2016
The controlling expression must evaluate to a char, int, short, or byte
ICS102: The course
138
Syntax for the switch Statement
switch ( <arithmetic expression> ) {
<case label 1> : <case body 1>
…
<case label n> : <case body n>
}
switch (
Arithmetic Expression
fanSpeed
) {
case 1:
Case
Label
System.out.println("That's low");
break;
case 2:
System.out.println("That's medium");
break;
Case
Body
case 3:
System.out.println("That's high");
break;
July 17, 2016
}
ICS102: The course
Chapter 6 - 139
switch With break Statements
switch ( N ) {
case 1: x = 10;
break;
case 2: x = 20;
break;
case 3: x = 30;
break;
}
N ==
1?
false
N ==
2?
x = 10;
break;
true
x = 20;
false
N ==
3?
false
July 17, 2016
true
ICS102: The course
break;
true
x = 30;
break;
Chapter 6 - 140
The switch Statement with default
switch ( <arithmetic expression> ) {
<case label 1> : <case body 1>
…
<case label n> : <case body n>
default: <default body>
}
switch (
binaryDigit
) {
case 0:
System.out.println("zero");
break;
case 1:
System.out.println("one");
break;
default:
System.out.println("That's not a binary digit");
break;
}
July 17, 2016
ICS102: The course
Chapter 6 - 141
switch With No break Statements
N ==
1?
switch ( N ) {
case 1: x = 10;
false
case 2: x = 20;
N ==
2?
case 3: x = 30;
}
true
x = 10;
true
x = 20;
false
N ==
3?
true
x = 30;
false
July 17, 2016
ICS102: The course
Chapter 6 - 142
… -- The switch Statement
double y = 30;
double z = 20;
Scanner kb = new Scanner(System.in);
System.out.println("1. add ");
System.out.println("2. Subtract ");
System.out.println("3. Multiply ");
System.out.println("Enter a value: between 1 and 3 ");
int x = kb.nextInt();
switch (x) {
case 1:
System.out.println(z + y);
break;
case 2:
System.out.println(z - y);
break;
case 3:
System.out.println(z * y);
break;
default: System.out.println("Wrong Choice.");
break;
}
July 17, 2016
ICS102: The course
143
- Nested if Statements …



One of the block statements of a branching statement can be
another if statement.
The inner if statement is executed when the enclosing block
statement is executed
If statements can be nested to many levels
If(<boolean_expresion_1>)
{
Statement_1;
<block_statement_2>;
Statement_3;
}
if(<boolean_expression_2>)
{
Statement_2A;
Statement_2B:
}
 Statement_2A and 2B are only executed if boolean_expresions 1 and
2 are true.
July 17, 2016
ICS102: The course
144
- Nested if Statements …
if (testScore >= 50) {
if (studentAge < 10) {
System.out.println("You did a great job");
} else {
Nested if
System.out.println("You did pass");
}
} else { //test score < 50
System.out.println("You did not pass");
}
July 17, 2016
ICS102: The course
Chapter 6 - 145
- Nested if Statements …
false
System.out.println
("You did not
pass");
true
testScore >= 50 ?
false
System.out.println
("You did pass");
July 17, 2016
ICS102: The course
studentAge <
10 ?
inner if
true
System.out.println
("You did a great
job");
Chapter 6 - 146
Nested simple-if
if (x < y) {
if (x < z)
System.out.println("Hello");
} else {
System.out.println("Good bye");
}
means
if (x < y) {
if (x < z) {
System.out.println("Hello");
}
} else {
System.out.println("Good bye");
}
July 17, 2016
ICS102: The course
Chapter 6 - 147
Nested if-else
if (x < y)
if (x < z)
System.out.println("Hello");
else
System.out.println("Good bye");
really means
if (x < y) {
if (x < z) {
System.out.println("Hello");
} else {
System.out.println("Good bye");
}
}
July 17, 2016
ICS102: The course
Chapter 6 - 148
The end
Important to do at home :
- read section 3.1 (pages 128-140)
Exercises

Write a Java program that reads an integer using
Scanner then tests if it is odd or even. The format
of the input and output should be as follows:
Please enter a number: 10
10 is even.

Write a program that reads three integers and
prints them in increasing order. Example:
Please enter three integers: 9, 5, 21
The integers in increasing order are: 5 9 21
Exercises

Write a program that reads an integer between 0
and 10 and then prints the word corresponding to
the number. Example:
Please enter a number between 0 and 10: 7
You entered: seven
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 8 : Boolean
Expressions
Outline











Introduction
Java Comparison Operators
Evaluating Boolean Expressions
Pitfall: Using == with Strings
Lexicographic and Alphabetical Order
Building Boolean Expressions
Truth Tables
Short-Circuit and Complete Evaluation
Precedence and Associativity Rules
Evaluating Expressions
Rules for Evaluating Expressions
July 17, 2016
ICS102: The course
153
- Introduction


A Boolean expression is an expression that is either true or false
The simplest Boolean expressions compare the value of two
expressions
time < limit
yourScore == myScore

July 17, 2016
Note that Java uses two equal signs (==) to perform equality
testing: A single equal sign (=) is used only for assignment
ICS102: The course
154
- Java Comparison Operators
July 17, 2016
ICS102: The course
155
- Evaluating Boolean Expressions

Even though Boolean expressions are used to control branch and
loop statements, Boolean expressions can exist independently as
well


A Boolean variable can be given the value of a Boolean expression by
using an assignment statement
A Boolean expression can be evaluated in the same way that an
arithmetic expression is evaluated

The only difference is that arithmetic expressions produce a number
as a result, while Boolean expressions produce either true or false as
their result
boolean madeIt = (time < limit) && (limit < max);
July 17, 2016
ICS102: The course
156
- Pitfall: Using == with Strings



The equality comparison operator (==) can correctly test two
values of a primitive type
However, when applied to two objects such as objects of the
String class, == tests to see if they are stored in the same
memory location, not whether or not they have the same value
In order to test two strings to see if they have equal values, use
the method equals, or equalsIgnoreCase
string1.equals(string2)
string1.equalsIgnoreCase(string2)
July 17, 2016
ICS102: The course
157
- Lexicographic and Alphabetical Order

Lexicographic ordering is the same as ASCII ordering, and includes
letters, numbers, and other characters


All uppercase letters are in alphabetic order, and all lowercase
letters are in alphabetic order, but all uppercase letters come
before lowercase letters
If s1 and s2 are two variables of type String that have been
given String values, then s1.compareTo(s2) returns:




A negative number if s1 is before s2 in lexicographic ordering
zero if the two strings are equal.
A positive number if s2 comes before s1
When performing an alphabetic comparison of strings (rather than
a lexicographic comparison) that consist of a mix of lowercase and
uppercase letters, use the compareToIgnoreCase method
instead
July 17, 2016
ICS102: The course
158
- Building Boolean Expressions

When two Boolean expressions are combined using the "and" (&&)
operator, the entire expression is true provided both expressions are true


When two Boolean expressions are combined using the "or" (||) operator,
the entire expression is true as long as one of the expressions is true


The expression is false only if both expressions are false
Any Boolean expression can be negated using the ! Operator


Otherwise the expression is false
Place the expression in parentheses and place the ! operator in front
of it
Unlike mathematical notation, strings of inequalities must be joined by &&

July 17, 2016
Use (min < result) && (result < max) rather than
result < max
ICS102: The course
min <
159
- Truth Tables
July 17, 2016
ICS102: The course
160
- Short-Circuit and Complete Evaluation




Consider
x > y || x > z
The expression is evaluated left to right. If x > y is true,
then there’s no need to evaluate x > z because the
whole expression will be true whether x > z is true or
not.
This also happens when we use
first expression is false.
&&
operator and the
To stop the evaluation once the result of the whole
expression is known is called short-circuit evaluation or
lazy evaluation
July 17, 2016
ICS102: The course
161
- Short-Circuit and Complete Evaluation


What would happen if the short-circuit evaluation is not done for the
following expression?

kids != 0 && toys/kids >= 2

There are times when using short-circuit evaluation can prevent a
runtime error
Sometimes it is preferable to always evaluate both expressions, i.e.,
request complete evaluation

July 17, 2016
In this case, use the & and | operators instead of && and ||
ICS102: The course
162
- Precedence and Associativity Rules …


Boolean and arithmetic expressions need not be fully
parenthesized
If some or all of the parentheses are omitted, Java will follow
precedence and associativity rules (summarized in the following
table) to determine the order of operations


July 17, 2016
If one operator occurs higher in the table than another, it has
higher precedence, and is grouped with its operands before the
operator of lower precedence
If two operators have the same precedence, then associativity
rules determine which is grouped first
ICS102: The course
163
Precedence and Associativity Rules
Instead of relying on precedence and associativity rules, it is best to include
most parentheses, except where the intended meaning is obvious
July 17, 2016
ICS102: The course
164
The end
Important to do at home :
- read chapter 3 from 141 to 153
165
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 9 : While and DoWhile loops
Outline

Introduction

while Loop

do-while Loop
July 17, 2016
ICS102: while & do-while
167
- Introduction

Loops in Java are similar to those in other high-level
languages

Java has three types of loop statements:





The while
The do-while
The for
The code that is repeated in a loop is called the body of
the loop
Each repetition of the loop body is called an iteration of
the loop
July 17, 2016
ICS102: while & do-while
168
- while loop

A while statement is used to repeat a portion of code (i.e.,
the loop body) based on the evaluation of a Boolean
expression

The Boolean expression is checked before the loop body is
executed



July 17, 2016
When false, the loop body is not executed at all
Before the execution of each following iteration of the loop
body, the Boolean expression is checked again

If true, the loop body is executed again

If false, the loop statement ends
The loop body can consist of a single statement, or multiple
statements enclosed in a pair of braces ({ })
ICS102: while & do-while
169
-- while Loop Syntax
while ( <boolean expression> )
<statement> //only one statement
OR
while ( <boolean expression> ) {
<statement> //many
Boolean Expression
}
while (
sum
Statement
(loop body)
number <= 100
=
sum + number;
number = number + 1;
) {
These statements are
executed as long as
number is less than or
equal to 100.
}
July 17, 2016
ICS102: while & do-while
170
-- while Loop Control flow
int sum = 0, number = 1
number <= 100 ?
false
true
sum = sum + number;
number = number + 1;
July 17, 2016
ICS102: while & do-while
171
-do-while Loop

A do-while statement is used to execute a portion of code (i.e., the
loop body), and then repeat it based on the evaluation of a Boolean
expression

The loop body is executed at least once


The Boolean expression is checked after each iteration of the loop
body




July 17, 2016
The Boolean expression is checked after the loop body is executed
If true, the loop body is executed again
If false, the loop statement ends
Don't forget to put a semicolon after the Boolean expression
Like the while statement, the loop body can consist of a single
statement, or multiple statements enclosed in a pair of braces ({ })
ICS102: while & do-while
172
-- do-while Loop Syntax
do {
<statement>
} while (<boolean expression>);
do
{
sum += number;
Statement
(loop body)
number++;
These statements are
executed as long as
sum is less than or
equal to 1,000,000.
} while (sum <= 1000000) ;
Boolean Expression
July 17, 2016
ICS102: while & do-while
173
-- do-while Loop Control Flow
int sum = 0, number = 1
sum += number;
number++;
sum <= 1000000 ?
true
false
July 17, 2016
ICS102: while & do-while
174
Examples
July 17, 2016
ICS102: while & do-while
175
Questions
1.
2.
3.
July 17, 2016
Write a Java program which computes the sum of all the odd
numbers between 0 and 100.
Write a Java program which reads 20 numbers using a
scanner and computes their average.
Write a Java program which reads unknown number of
integers using a scanner and counts the number of odd
numbers and the number of even numbers. Assume the input
integers are all positive. Use a negative number as a sentinel.
ICS102: while & do-while
176
Solution using while loop
July 17, 2016
ICS102: while & do-while
177
Q1 Solution
Write a Java program which computes the sum of all the odd numbers
between 0 and 100.
int n =1;
int sum = 0;
while (n < 100) {
sum += n;
n = n + 2;
}
System.out.println(“The sum is “ +
sum);
July 17, 2016
ICS102: while & do-while
178
Q2 Solution
Write a Java program which reads 20 numbers using a scanner and
computes their average.
Scanner kb = new Scanner(System.in);
int cnt = 0;
double x;
double sum = 0;
While (cnt < 20) {
x = kb.nextDouble();
sum += x;
cnt++;
}
System.out.println(“The Average is “ + sum/cnt);
July 17, 2016
ICS102: while & do-while
179
Q3 Solution
Write a Java program which reads unknown number of integers using a scanner and counts
the number of odd numbers and the count of even numbers. Assume the input integers are
all positive. Use any negative number as a sentinel.
Scanner kb = new Scanner(System.in);
int even_cnt = 0;
int odd_cnt = 0;
double x = kb.nextInt();
while (x > 0) {
if ( mod(x,2) == 0)
even_cnt++;
else
odd_cnt++;
x = kb.nextInt();
}
System.out.println(“Even numbers are = “ + even_count);
System.out.println(“Odd numbers are = “ + odd_count);
July 17, 2016
ICS102: while & do-while
180
Solution using do-while loop
July 17, 2016
ICS102: while & do-while
181
Q1 Solution
Write a Java program which computes the sum of all the odd numbers
between 0 and 100.
int n = 1;
int sum = 0;
do {
sum += n;
n = n + 2;
} While ( n < 100)
System.out.println(“The sum is “ +
sum);
July 17, 2016
ICS102: while & do-while
182
Q2 Solution
Write a Java program which reads 20 numbers using a scanner and computes
their average.
Scanner kb = new Scanner(System.in);
int cnt = 0;
double x;
double sum = 0;
do {
System.out.println(“Enter a number”);
x = kb.nextDouble();
sum += x;
cnt++;
} while (cnt < 20);
System.out.println(“The Average is “ + sum/cnt);
July 17, 2016
ICS102: while & do-while
183
Q3 Solution
Write a Java program which reads unknown number of integers using a scanner and counts
the number of odd numbers and the count of even numbers. Assume the input integers are
all positive. Use any negative number as a sentinel.
Scanner kb = new Scanner(System.in);
int even_cnt = 0;
int odd_cnt = 0;
double x = kb.nextInt();
if (x > 0) {
do {
if ( mod(x,2) == 0)
even_cnt++;
else
odd_cnt++;
x = kb.nextInt();
} while ( x > 0)
}
System.out.println(“Even numbers are = “ + even_count);
System.out.println(“Odd numbers are = “ + odd_count);
July 17, 2016
ICS102: while & do-while
184
Additional Slides
July 17, 2016
ICS102: while & do-while
185
while Loop Pitfall - 1
1
int product = 0;
while ( product < 500000 ) {
product = product * 5;
}
Infinite Loops
2
Both loops will not
terminate because the
boolean expressions will
never become false.
int count = 1;
while ( count != 10 ) {
count = count + 2;
}
July 17, 2016
ICS102: while & do-while
Chapter 7 - 186
while Loop Pitfall - 2
1
double count = 0.0;
while ( count != 1.0 ) {
count = count + 1.0/3.0;
}
Using Real Numbers
2
double count = 0.0;
Loop 2 terminates, but Loop
1 does not because only an
approximation of a real
number can be stored in a
computer memory.
while ( count <= 1.0 ) {
count = count + 1.0/3.0;
}
July 17, 2016
ICS102: while & do-while
Chapter 7 - 187
while Loop Pitfall - 3

1
Goal: Execute the loop body 10 times.
count = 1;
2 count =
while (count < 10) {
while (count <= 10) {
. . .
. . .
count++;
count++;
}
3
1;
}
count = 0;
4
while (count <= 10) {
count = 0;
while (count < 10) {
. . .
. . .
count++;
count++;
}
}
1 and 3 exhibit off-by-one error.
July 17, 2016
ICS102: while & do-while
Chapter 7 - 188
Checklist for Repetition Control
1. Watch out for the off-by-one error (OBOE).
2. Make sure the loop body contains a statement that
will eventually cause the loop to terminate.
3. Make sure the loop repeats exactly the correct
number of times.
July 17, 2016
ICS102: while & do-while
Chapter 7 - 189
The end
Important to do at home :
- read section 3.3 (pages 160-166)
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 10 : The for-loop
and nested loops
Outline




The for Statement Syntax
Semantics of the for Statement
Nested Loops
continue, break, and exit Statements
- The for Statement Syntax
for (Initializing; Boolean_Expression; Update)
Block


Note that the three control expressions are separated by two,
not three, semicolons
Note that there is no semicolon after the closing parenthesis at
the beginning of the loop
- The for Statement


The for statement is most commonly used to step through an
integer variable in equal increments
It begins with the keyword for, followed by three expressions in
parentheses that describe what to do with one or more controlling
variables



The first expression tells how the control variable or variables
are initialized or declared and initialized before the first iteration
The second expression determines when the loop should end,
based on the evaluation of a Boolean expression before each
iteration
The third expression tells how the control variable or variables
are updated after each iteration of the loop body
Example
- Nested Loops

Loops can be nested, just like other Java structures

When nested, the inner loop iterates from beginning to end for
each single iteration of the outer loop
for (Initializing; Boolean_Expression; Update)
Block 1

Block 1 can contain other loop statements as follows
Block 1  for (Initializing; Boolean_Expression; Update)
Block 2
- Nested Loops

Loops can be nested, just like other Java structures

When nested, the inner loop iterates from beginning to end for
each single iteration of the outer loop
int rowNum, columnNum;
for (rowNum = 1; rowNum <=3; rowNum++)
{
for (columnNum = 1; columnNum <=2; columnNum++)
System.out.print(" row " + rowNum + " column " + columnNum);
System.out.println();
}
Continue, break, and exit Statements
Class test {
public static void main( String [] args) {
for (int i = 0; i < 10; i++) {
statement 1;
statement 2;
if( cond) continue;
statement 3;
statement 4;
}
statement 5;
statement 6;
}
}
Continue, break, and exit Statements
Class test {
public static void main( String [] args) {
for (int I = 0; I < 10; i++) {
statement 1;
statement 2;
if( cond) break;
statement 3;
statement 4;
}
statement 5;
statement 6;
}
}
Continue, break, and exit Statements
Class test {
public static void main( String [] args) {
for (int I = 0; I < 10; i++) {
statement 1;
statement 2;
if( cond) System.exit(0);
statement 3;
statement 4;
}
statement 5;
statement 6;
}
}
For-loop examples
Questions
1.
2.
3.
July 17, 2016
Write a Java program which computes the sum of all the odd
numbers between 0 and 100.
Write a Java program which reads 20 numbers using a
scanner and computes their average.
Write a Java program which reads unknown number of
integers using a scanner and counts the number of odd
numbers and the number of even numbers. Assume the input
integers are all positive. Use a negative number as a sentinel.
ICS102: while & do-while
202
Q1 Solution
Write a Java program which computes the sum of all the odd numbers
between 0 and 100.
int sum = 0;
for( int n = 1; n <= 100; n = n + 2) {
sum += n;
}
System.out.println(“The sum is “ + sum);
July 17, 2016
ICS102: while & do-while
203
Q2 Solution
Write a Java program which reads 20 numbers using a scanner and
computes their average.
Scanner kb = new Scanner(System.in);
double x;
double sum = 0;
While (int cnt = 0; cnt < 20; cnt++) {
System.out.println(“Enter a number”);
x = kb.nextDouble();
sum += x;
}
System.out.println(“The Average is “ + sum/cnt);
July 17, 2016
ICS102: while & do-while
204
Q3 Solution
Write a Java program which reads unknown number of integers using a scanner and counts
the number of odd numbers and the count of even numbers. Assume the input integers are
all positive. Use any negative number as a sentinel.
Scanner kb = new Scanner(System.in);
int even_cnt = 0;
int odd_cnt = 0;
int n;
For(;;) {
n = kb.nextInt();
if (n < 0)
break;
else if ( mod(n,2) == 0)
even_cnt++;
else
odd_cnt++;
}
System.out.println(“Even = “ + even_count + “ odd = “ odd_cnt);
July 17, 2016
ICS102: while & do-while
205
Nested-loop examples
Questions
1. Write a java program which gives the following output
1
22
333
4444
55555
2.
Write a java program which prints all the prime numbers less
than 1000.
Q1 Solution
Write a java program which gives the following output
1
22
333
4444
55555
for(int k = 1; k <= 5; k++) {
For ( int j = 1; j <=k; j++)
System.out.print(k);
System.out.println();
}
Q2 solution
Write a java program which prints all the prime numbers less than 1000.
int n, j;
for(int k = 2; k < 100; k++) {
n = 0;
j = 2;
while(n == 0 && j < k/2) {
if (mod(k,j) == 0) n++;
j++;
}
if( n ==0) System.out.println(k);
}
The end
Important to do at home :
- read section 3.3 (pages 166-177)
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 11 : Arrays
Arrays

An array is an ordered list of values
Entire array has
A single name
scores
Each value has
A numeric index
[0]
79
[1]
[2]
[3]
62
98
57

An array of size N is indexed from zero to N-1

This array holds 4 values that are indexed from 0 to 3
Processing Array Elements


Access individual elements of an array using:
 the name of the array
 a number (index or subscript) that tells which of the
element of the array
What value is stored in scores[2]?
scores
[0]
79
[1]
[2]
[3]
62
98
57
Arrays

For example, an array element can be assigned a value,
printed, or used in a calculation:
scores[2] = 87;
scores[first] = scores[first] + 2;
Avg = (scores[0] + scores[1])/2;
System.out.println (“Max = " + scores[3]);
Arrays

The values held in an array are called array elements

Array elements are of the same type

The element type can be a primitive type or an object
reference.


Therefore, we can create an array of integers, or an array
of characters, or an array of String objects, etc.
In Java, the array itself is an object

Therefore the name of the array is a object reference
variable, and the array itself must be instantiated
Declaring an Array

Defining an array
Type [] name

Where:
•This declares the handle only
•Initialized to null
•Stores an address when arrays are created

Type specifies the kind of values the array stores

the brackets [ ] indicate this is an array

name is the handle to access the array
Declaring an Array

The scores array could be declared as follows:
double [] scores = new double [4];

The above statement will:

allocate block of memory to hold 4 doubles

initialize block with zeros

create a handle or a pointer or a reference called scores

store address of the block in scores
scores
Handle
[0]
[1]
[2]
0.0
0.0
0.0
Block
[3]
0.0
Syntax

Forms
ElementType [] arrayName;
ElementType [] arrayName = new ElementType [size];
ElementType [] arrayName = array-literal;

Where:

ElementType is any type

arrayName is the handle for the array

array-literal is a list of literals enclosed in curly braces {
}
Declaring Arrays

Some examples of array declarations:
float[] prices = new float[500];
boolean[] flags;
flags = new boolean[20];
char[] codes = new char[1750];
Bounds Checking

Once an array is created, it has a fixed size

An index used in an array reference must specify a valid element

That is, the index value must be in bounds (0 to N-1)


The Java interpreter throws an ArrayIndexOutOfBoundsException
if an array index is out of bounds
This is called automatic bounds checking
Bounds Checking


For example, if the array scores can hold 100 values, it can be
indexed using only the numbers 0 to 99
If scores has the value 100, then the following reference will
cause an exception to be thrown:
System.out.println (scores[count]);

Often for loop is used to process array. It’s common to introduce
off-by-one errors when using arrays
problem
for (int index=0; index <= 100; index++)
scores[index] = index*50 + epsilon;
Bounds Checking


Each array object has a public constant called length that
stores the size of the array
It is referenced using the array name:
scores.length

Note that length holds the number of elements, not the
largest index
Initializer Lists

An initializer list can be used to instantiate and initialize an array in
one step

The values are delimited by braces and separated by commas

Examples:
int[] scores = {98, 76, 54, 83, 87, 65, 99, 66};
char[] letterGrades = {'A', 'B', 'C', 'D', ’F'};
Initializer Lists: Example 1

Used when exact size and initial values of an array are known
in advance
int [] scores = {98, 76, 54, 83}
Visualize the results of the above command
scores
[0]
[1]
[2]
[3]
98
76
54
83
Initializer Lists: Example 2

Consider the following 4 student names
String [] STUDENTS = { “Aref", “Ali“, “Emad”, "Sami"};

Note results:
STUDENTS
[0]
Aref
[1]
Ali
[2]
Emad
[3]
Sami
Array elements are handles for String values
Initializer Lists



Note that when an initializer list is used:

the new operator is not used

no size value is specified
The size of the array is determined by the number of items in
the initializer list
An initializer list can only be used only in the array declaration
The Assignment Operation
Java provides a few operations to use with arrays, including assignment

Consider:
int [] alist = { 1, 12, 15, 7};
int [] blist;
blist = alist;

Recall that alist and blist are handles



alist contains an address of where the numbers are
blist now contains that same address
blist does not have a copy of alist
Array Cloning

To actually create another array with its own values, Java
provides the .clone() method
int [] alist = {1, 12, 15, 7};
int [] blist;
blist = alist.clone();

Now there are two separate lists of numbers, one with handle
alist, the other with handle blist
Array Cloning with Reference Types

Recall previous declaration:
STUDENTS
[0]
Aref

[1]
Ali
[2]
Emad
[3]
Sami
Consider:
Sometimes called a
"shallow copy"
operation
String[] s_list = STUDENTS.clone():

This will create another list of handles, also pointing to the names
Array Cloning with Reference Types

We can write our own "deep copy"
String [] original = {“Aref”,“Ali”, “Emad”, “Sami”};
String [] result = new String(original.length);
for (i = 0; i < original.length; i++)
result[i] = original[i].clone();
Array Equality

Java has an equals() method for classes
if (a1.equals(a2)) …


If a1 and a2 are arrays, the equals() method just looks at
the addresses the handles point to
They could be pointing to different addresses but the contents
of the array still be equal

It is the contents that we really wish to compare
Array Equality

We must write our own method to compare the arrays


they both must have the same length
then use a for( ) loop to compare element by element for
equality
Boolean areEqual
= true;
As soon
as one pair of elements is found
if (list1.length != list2.length)
equal, the arrays are not equal
areEqual =not
false;
else {
for (int i = 0; i < list1.length; i++)
if (list1[i] != list2[i]) areEqual = false;
}
if(areEqual) System.out.println(“They are equal”);
else System.out.println(“They are NOT equal”);
Array exercises


Write an application that inputs 10 numbers, each
between 10 and 100. As each number is read,
display it only if it is not a duplicate of a number
already read.
Given an array of integers, write a java code that
allows to check if the array is “palindromic”. A
palindromic array is a symmetric one:

For example the arrays 1 6 4 6 1 and 258852 are both
palindromic, but the array 3753 is not.
Array exercises

Write a program that reads a sequence of 10 integers
into an array and then computes the alternating sum of
all elements in the array.

For example if the array is : 1 4 9 16 9 7 4 9 11
then it computes : 1 – 4 + 9 – 16 + 9 – 7 + 4 – 9 + 11 = -2
The end
Important to do at home :
- read section 6.1 (pages 372-379)
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 12 : 2-D Arrays
Two-Dimensional Arrays

Two-dimensional (2-D) Arrays.

2-D Array Declaration.

Initializer Lists.

Differing Numbers of Elements in each Row.

Implementation of a 2-D Array

Printing a 2-D Array.
2-D Arrays: Why?



Often data come in a 2-D form. For example:
 The layout of a printed page is 2-D
 Computer screen is 2-D
 The spread sheet (table) is 2-D
For these situations we need a 2-D array.
A 2-D array can be thought of as a collection of "slots" laid out in
a 2-D grid.
 each slot can hold a value
 two indexes are needed to specify a slot.
Example: Table of Student Grades


Imagine a class of 7 students that have a quiz grades for 5
weeks. These grades can be stored in a table form.
A particular cell of the table is identified by student number and
week number. For example:

The grade for student 0 in week 1 is 42
Student

The grade for student 3 in week 4 is 93

The grade for student 6 in week 2 is 78
Week
0
1
2
3
4
0
99
42
74
83
100
1
90
91
72
88
95
2
88
61
74
89
96
3
61
89
82
98
93
4
93
73
75
78
99
5
50
65
92
87
94
6
43
98
78
56
99
2-D Arrays in Java





In Java, a table may be implemented as a 2-D array.
As with 1-D arrays, every slot in a 2-D array is of the same type
which can be a primitive type or an object reference type.
Each slot of the array is specified with a row and column number.
Suppose that gradeTable is a 2-D array then the syntax to specify a
particular slot should be gradeTable[row][col]
For example
Student
Week
0
1
2
3
4

gradeTable[0][1] is 42
0
99
42
74
83
100
1
90
91
72
88
95

gradeTable[3][4] is 93
2
88
61
74
89
96
3
61
89
82
98
93
4
93
73
75
78
99
5
50
65
92
87
94
6
43
98
78
56
99

gradeTable[6][2] is 78
2-D Arrays in Java

The subscripted variables of a 2-D array can be used in assignment
statements and arithmetic expressions just like any variable:
gradeTable[ 0 ][ 1 ] = 33 ; // puts a 33 into row 0 column 1.
gradeTable[ 3 ][ 4 ]++; //increments the value at row 3 column 4.
int value = (gradeTable[ 6 ][ 2 ] + 2)/2;//puts 40 into value
Student

Write a Java statement that puts a
zero into row 5 column 3.
gradeTable[ 5 ][ 3 ] = 0
Week
0
1
2
3
4
0
99
33
74
83
100
1
90
91
72
88
95
2
88
61
74
89
96
3
61
89
82
98
94
4
93
73
75
78
99
5
50
65
92
87
94
6
43
98
78
56
99
2-D Array Declaration



In Java, a 2-D array is an object.
To declare a reference variable myArray to a 2-D array of int:
int[][] myArray;
To create an array object of 3 rows and 5 columns, and put the
reference in myArray, we write
int[][] myArray = new int[3][5];

All the elements of myArray are initialized to zero.

We can create the above array by using the initializer list:
int[][] myArray = {{0,0,0,0,0}, {0,0,0,0,0}, {0,0,0,0,0}};
Different Numbers of Elements per Row

In Java, each row of a 2-D array may have a different number of
elements. In the following example, the array A has

3 elements in its first row,

2 in its second row,

and 5 in its last row.
int[][] A = {{ 1, 9, 4 }, { 0, 2}, { 0, 1, 2, 3, 4 }};
Length of a 2-D Array

The length of a 2-D array is the number of rows it has.




So the row index (number) is from 0 to length-1.
As each row in a 2-D array is like a 1-D array, we can refer to an entire row by
specifying just the row index, e.g., A[0] means row # 0
The length of a row is the number of columns in the row.
As each row of a 2-D array can have a different number of columns, so each
row has its own length.
int[][] A = { { 1, 9, 4 },
System.out.println("Length
System.out.println("Length
System.out.println("Length
System.out.println("Length
{ 0, 2}, { 0, 1, 2, 3, 4 } };
of array is: " + A.length );
of row[0] is: " + A[0].length );
of row[1] is: " + A[1].length );
of row[2] is: " + A[2].length );
Implementation of 2-D Array


A 2-D array is implemented as an array of 1-D arrays.
To understand the idea, we construct the 2-D array stepby-step.
int[][] myArray;

declares a variable myArray
myArray = new int[3][] ;

// 2
Creates an array object. The array object has 3 slots.
Each slot may refer (in the future) to an array of int
(i.e., a row)
myArray[0] = new int[3] ;

// 1
// 3
Creates a 1-D array object and puts its reference in
slot 0 of myArray.
Implementation of 2-D Array

A previously constructed 1-D array can be
assigned to a row:
int[] x = {0, 2};
int[] y = {0, 1, 2, 3, 4};
myArray[1] = x ;
myArray[2] = y ;
//4

The rows do not need to have the same
number of elements
Printing a 2-D Array

Suppose we want to print every element of a 2-D array A

It can be done by using nested loops.

The expression A[row].length gives a different number for
each row of the array.
int[][] A = {{ 1, 9, 4 }, { 0, 2}, { 0, 1, 2, 3, 4 }};
for ( int row=0; row < A.length; row++ )
{
System.out.print("Row " + row + ": ");
for ( int col=0; col < A[row].length; col++ )
System.out.print( A[row][col] + " ");
System.out.println();
}
The end
Important to do at home :
- read section 6.4 (pages 425-436)
Exercises


Write a program that creates a two dimensional
array, fills it using Scanner, and then prints the sum
of every column.
Given a two-dimensional array, dataTable, of type
double such that the rows can have different
lengths. Write a code fragment that computes the
average of each row and saves it in a singledimension array of size corresponding to the rows
of dataTable.
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 13 : Methods
A Java Program
A Java Program



A Java program consists of one
or more classes
A Java class consists of one or
more methods
A Java method consists of one
or more statements
Java
classes
Java
Methods
- What is a method …

A Java application program consists of one or more classes.

Each class has one or more methods

Each method consists of one or more statements

Each method has a name.

One of the methods must be called main


When a Java application program is run, the run-time system
automatically invokes the method named main
All Java application programs start with the main method
… - What is a method …
public class class-name {
method1
method 2
method 3
…
…
method n
}
Example of a Java Program
Example of a Java Program
Class name
Main method
Class body
Instruction
Exercise

Write a program that computes the factorial (n!) of 6
then the factorial of 3, then the factorial of 10.
Exercise

Write a program that computes the factorial (n!) of 6
then the factorial of 3, then the factorial of 10.
Method
invocation
Return
type
Method
parameter
Method
definition
Return
instruction
-- Method Structure
If method doesn’t return value
Method name
public or private<static> <void or typeReturned>myMethod(<parameters>)
{
statement
Type of the return value
Variable list
statement
statement
Method body
…
…
statement
}
- Invoking a Methods



The statements inside a method body are executed when the
corresponding method is called from another method.
Calling a method is also called invoking a method
Each time the method is invoked, its corresponding body is
executed
- return Statements …

The body of a method that returns a value must also
contain one or more return statements

A return statement specifies the value returned and ends
the method invocation.
… - return Statements


A void method need not contain a return statement,
unless there is a situation that requires the method to
end before all its code is executed
Example : write a method that prints all the numbers
between 10 and 30
- Local Variables

A variable declared within a method definition is called a local
variable

All variables declared inside the method are local variables

All method parameters are local variables
- Local Variables

If two methods each have a local variable of the same name,
they are still two entirely different variables. Example:
Local variable in main method
Local variable in factorial method
Local variable in addition method
- Method Parameters …

A parameter list provides a description of the data required by a
method

It indicates the number and types of data pieces needed, the
order in which they must be given, and the local name for
these pieces as used in the method
public double myMethod(int p1, int p2, double p3)

Example: What is the parameter list of a method division that
divides two integers and returns the result (double)?
Parameters list
… - Method Parameters …


When a method is invoked, the appropriate values must be passed
to the method in the form of arguments
The number, order, and types of the arguments must exactly
match that of the parameter list
Error: type mismatch
… - Method Parameters …

Is this program correct?
Yes it is! .. Details in the next slide ..
… - Method Parameters …

If argument and parameter types do not match exactly, Java
will attempt to make an automatic type conversion


In the preceding example, the int value of argument a
would be cast to a double
A primitive argument can be automatically type cast from
any of the following types, to any of the types that
appear to its right:
byteshortintlongfloatdouble
char
… - Method Parameters …

A parameters is often thought of as a blank or placeholder
that is filled in by the value of its corresponding argument

However, a parameter is more than that: it is actually a local
variable


When a method is invoked, the value of its argument is
computed, and the corresponding parameter (i.e., local
variable) is initialized to this value
Even if the value of a formal parameter is changed within a
method (i.e., it is used as a local variable) the value of the
argument cannot be changed
Call-by-Value Example
Prints 10
Prints 10
- Method Parameters: Array Parameters


Methods can have array
parameters.
Example: Write a program
that prints the values of an
array of integers, then
increments each value of the
array by two and then prints
the new values of the same
array.
A parameter of type
array
- Details


Array arguments are always passed by reference. This means any changes
done to an array element by the invoked method, will also change the
corresponding array element in the caller method.
Example:
Public static void m1()
{ int [] A = { 1, 2, 3}; m2(A); }
public static void m2(int [] A)
{ A[1] = 5};

Method m2 changes the value of a[1] to 5; as a result the value of a[1] in
m1 will also change to 5.
- Method Overloading …




In java the same class can have methods with the same name.
Such methods are called overloaded methods.
Overloaded methods must differ in the number or type of their
arguments.
The name of a method together with the number, order, and types
of its arguments are called the method signature. No two methods
of the same class must have the same signature.
The compiler treats overloaded methods as completely different
methods. It knows which one to call by using method signatures.
- Method Overloading …
These 3 methods
have the same
name but different
signatures
The end
Important to do at home :
- read pages 206-212
Exercises


Write a method that takes as input an integer value
and returns the sum of all integers less than that
value. For example, if the input is 6, the output is
5+4+3+2+1 = 15. If the input is negative, the
output should be -1.
Write a method that takes as input an array of
integers and returns the average of the values in the
array.
Exercises


















Write a method public void printTriangleNumbers(int n) such that:
The call: printTriangleNumbers(5) prints the following on the screen:
12345
1234
123
12
1
The call: printTriangleNumbers(6) prints the following on the screen:
123456
12345
1234
123
12
1
The call: printTriangleNumbers(3) prints the following on the screen:
123
12
1
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 14 : Classes
Outline

Introduction

Class Definitions

A Class Is a Type

The new Operator

Instance Variables and Methods

Information Hiding

public and private Modifiers

Accessor and Mutator Methods

Encapsulation
- Introduction


Classes are the most important language feature
that make object-oriented programming (OOP)
possible
Programming in Java consists of defining a number
of classes




Every program is a class
All helping software consists of classes
All programmer-defined types are classes
Classes are central to Java
Example


Suppose you are asked to write a program for a
commercial company:
The company has several:





Employees
Clients
Products
Stores
If you use an object-oriented language such as Java,
you will have to define:




Class
Class
Class
Class
Employee :
Client:
Product:
Store:
public
public
public
public
class
class
class
class
Employee{ ..}
Client{..}
Product{..}
Store{..}
- Class Definitions

You already know how to use classes and the
objects created from them, and how to invoke their
methods


For example, you have already been using the predefined
String and Scanner classes
Now you will learn how to define your own classes
and their methods, and how to create your own
objects from them …
next slide
Class Definition

A class definition is composed of two parts:



Data members (Data part)
Methods (Operations part)
Example: define a class Employee
- Details




A class definition specifies the data items and methods
that all of its objects will have
These data items and methods are sometimes called
members of the object
Data items are called fields or instance variables
Instance variable declarations and method definitions can
be placed in any order within the class definition
Object Declaration

It is possible to declare several objects from a class:
e1
e2
Class Employee
e3
e4
s1
The same principle as:
s2
String
s3
s4
Note: classes Employee and TestClass should be
saved in the same directory
- A Class Is a Type

A class is a special kind of programmer-defined type,
and variables can be declared of a class type

A value of a class type is called an object or an
instance of the class

e1
The following phrases are equivalent:



“e1 is of type Employee,"
“e1 is an object of the class Employee," and
“e1 is an instance of the class Employee"
e2
Employee
e3
e4
- Object Creation

The instruction:
Employee e1;
only declares e1. But the object is still not created.

To create the object the operator new must be used:
e1 = new Employee();

These can be combined as follows:
Employee e1 = new Employee();
- Object Creation
Declaration
Creation
Question: What is the name of Employee e1?
How to change the name of Employee e1? … next slide ..
- Instance Variables and Methods …

In order to refer to a particular instance variable, preface it
with its object name as follows:
objectName.instanceVar1
objectName.instanceVar2

Example:




e1.name
e1.age
e1.salary
To change the name of e1:

e1.name = “Mohamed”;
- Instance Variables and Methods …

In order to invoke a method of a class, you need an object
of that class:
objectName.method1()
objectName.method2(argument1)

Example:


e1.outputDetails();
e1.outputDetails();
- Instance Variables and Methods …
Modifying the name
Invoking a method
Calling object
Accessing the name
- Information Hiding and Encapsulation

Information hiding is the practice of separating how to use a class
from the details of its implementation


Abstraction is another term used to express the concept of
discarding details in order to avoid information overload
Encapsulation means that the data and methods of a class are
combined into a single unit (i.e., a class object), which hides the
implementation details


Knowing the details is unnecessary because interaction with
the object occurs via a well-defined and simple interface
In Java, hiding details is done by marking them private
- public and private Modifiers …

Instance variables and methods of a class can be declared



Example:






public, or
private
public String name;
private int age;
public void outputDetails{..}
The modifier public means that there are no restrictions on
where an instance variable or method can be used
The modifier private means that an instance variable or method
cannot be accessed by name outside of the class
It is considered good programming practice to make all instance
variables private
- public and private Modifiers …
Illegal because we try to access a private member (age)
from outside the class Employee
… - public and private Modifiers

Within the definition of a class, private members of
any object of the class can be accessed, not just
public members of the calling object
Problem ..

It is considered good programming practice to make all
instance variables private
Question: how to access and modify the
instance variables of Employee objects e1,
e2, e3 and e4? .. answer .. Use accessor
and mutaor methods …. next slide ..
- Accessor and Mutator Methods …

Accessor methods allow the programmer to obtain the value of an object's
instance variables




The data can be accessed but not changed
The name of an accessor method typically starts with the word get
Example: public String getName()
{ return name;}
Mutator methods allow the programmer to change the value of an object's
instance variables in a controlled manner



Incoming data is typically tested and/or filtered
The name of a mutator method typically starts with the word set
Example: public void setName(String n)
{ name = n;}
- Accessor and Mutator Methods (Example)
Accessor method for instance variable name
Mutator method for instance variable name
Modifying the name of e1 using a mutator method
The end
Important to do at home :
- read sections 4.1 and 4.2 (pages
200-243)
Exercise

Define a class Date with three instance variable:





month of type String
day of type integer
year of type integer
Provide a method WriteOutput to print the details of
the Date
Provide accessor and mutator methods for all
instance variables.
Solution
Solution
Solution
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 15 : Classes II
Constructors

A constructor is a special kind of method that is designed to
initialize the instance variables for an object:
public ClassName(ParametersList){…}

A constructor must have the same name as the class

A constructor has no type returned, not even void

Constructors are typically overloaded
Constructor Example
Constructor
How Constructors are called

A constructor is called when an object of the class is created using
new
ClassName objectName = new ClassName(anyArgs);

The name of the constructor and its parenthesized list of
arguments (if any) must follow the new operator

A constructor cannot be invoked like an ordinary method

Example:
Employee e1 = new Employee(“Mohamed”, 20, 5000);
Constructors
Constructor
Calling the constructor
Constructors

In the previous lecture we saw this example:
We did not define any
constructor,
but
we created objects using
new !!
Explanation .. Next slide ..
Include a No-Argument Constructor



If you do not include any constructors in your class, Java
will automatically create a default or no-argument
constructor that takes no arguments, performs no
initializations, but allows the object to be created
If you include even one constructor in your class, Java
will not provide this default constructor
If you include any constructors in your class, be sure to
provide your own no-argument constructor as well
No-argument constructor
No-argument
constructor
Copy Constructor

A copy constructor is a constructor with a single argument of the
same type as the class.
It creates an object which is an exact copy of the argument object

Example:

How to invoke a copy constructor:

- The methods equals and toString


Java expects certain methods, such as equals and toString, to
be in all, or almost all, classes
The purpose of equals, a boolean valued method, is to compare
two objects of the class to see if they satisfy the notion of "being
equal“

Note: You cannot use == to compare objects
public boolean equals(ClassName objectName)

The purpose of the toString method is to return a String
value that represents the data in the object
public String toString()
equals example
equals example

Invoking equals method:
equals invocation
toString example
toString invocation
The end
Important to do at home :
- read sections 4.3 and 4.4 (pages
244-273)
Exercise: Temperature Class (1/2)


Write a Temperature class that has two instance variables: a
temperature value (a floating-point number) and a character
for the scale, either 'C' for Celsius or 'F' for Fahrenheit.
The class should have four constructor methods:




one for each instance variable (assume zero degrees if no value is
specified and Celsius if no scale is specified),
one with two parameters for the two instance variables, and
a no-argument constructor (set to zero degrees Celsius).
Include two accessor methods to return the temperature:
 getTempCelsius: to return the degrees Celsius,
 getTempFahrenheit: to return the degrees Fahrenheit
IMPORTANT: use the following formulas :


degreesC = 5(degreesF - 32)/9
degreesF = (9(degreesC)/5) + 32
Exercise: Temperature Class (2/2)

Include three mutator methods,






setValue to set the value,
setScale to set the scale ('F' or 'C'), and
setValueAndScale to set both;
Include a suitable toString method.
Include a equals method
Then write a test class called TestTemperature that tests all the
methods. Be sure to use each of the constructors.
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 16 : Classes III
Class variables Vs Primitive type variables
int i = 15;
Employee e1 = new Employee(“Mohamed”, 20, 5000);


i is a variable of type integer that contain the value 15.
e1 is a variable of type Employee that contains the address
where the object is located.

The object named by the variable is stored in some other
location in memory
i
e1
15

“Mohamed”
20
5000
July 17, 2016
ICS102: Classes 3/5
320
References

What happens if we do:
Employee e1 = new Employee(“Mohamed”, 20, 5000);
Employee e2 = e1;

e1 and e2 will refer to the same object!


July 17, 2016
The assignment operator sets the reference (memory address)
of one class type variable equal to that of another
Any change to the object named by one of theses variables will
produce a change to the object named by the other variable,
since they are the same object !!
ICS102: Classes 3/5
321
Example
What will be printed here?
- Class Parameters …


All parameters in Java are call-by-value parameters

A parameter is a local variable that is set equal to the
value of its argument

Therefore, any change to the value of the parameter
cannot change the value of its argument
Class type parameters appear to behave differently from
primitive type parameters

July 17, 2016
They appear to behave in a way similar to parameters in
languages that have the call-by-reference parameter
passing mechanism
ICS102: Classes 3/5
323
Call-by-Value
Call-by-Reference
What salary will printed here?
… - Class Parameters

The value plugged into a class type parameter is a
reference (memory address)


July 17, 2016
Therefore, the parameter becomes another name for the
argument
Any change made to the object named by the parameter
(i.e., changes made to the values of its instance variables)
will be made to the object named by the argument, because
they are the same object
ICS102: Classes 3/5
326
- Differences Between Primitive and Class-Type
Parameters


July 17, 2016
A method cannot change the value of a variable of a
primitive type that is an argument to the method
In contrast, a method can change the values of the
instance variables of a class type that is an argument to
the method
ICS102: Classes 3/5
327
Pitfall: Use of == with Class type variables

The test for equality (==) also behaves differently for class
type variables




The == operator only checks that two class type variables have
the same memory address
Unlike the equals method, it does not check that their
instance variables have the same values
Two objects in two different locations whose instance variables
have exactly the same values would still test as being "not
equal“
Use equals method instead:
July 17, 2016
ICS102: Classes 3/5
328
- The Constant null

null is a special constant that may be assigned to a variable of
any class type
YourClass yourObject = null;

It is used to indicate that the variable has no "real value“
July 17, 2016
ICS102: Classes 3/5
329
The end
Important to do at home :
- read section 5.2
(pages 310-324)
Advanced Example

An advanced example of classes is the Class Person
defined in Display 5.19 of Chapter 5 of the textbook
(Pages 296-304 of the 4th edition)
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS102
Lecture 18 : Static Members
Static Methods



A static method is one that can be used without a calling
object.
A static method still belongs to a class, and its definition is
given inside the class definition.
When a static method is defined, the keyword static is
placed in the method header
public static returnedType myMethod(parameters)
{ . . . }

Static methods are invoked using the class name in place of a
calling object
returnedValue = MyClass.myMethod(arguments);
Static Variables

Static variables can be declared and initialized at the same time
private static int myStaticVariable = 0;

If not explicitly initialized, a static variable will be automatically
initialized to a default value




boolean static variables are initialized to false
Other primitive types static variables are initialized to the zero of their
type
Class type static variables are initialized to null
It is always preferable to explicitly initialize static variables rather
than rely on the default initialization
Static Variables

A static variable should always be defined private, unless it is also
a defined constant



The value of a static defined constant cannot be altered, therefore it
is safe to make it public
In addition to static, the declaration for a static defined constant
must include the modifier final, which indicates that its value
cannot be changed
public static final int BIRTH_YEAR = 1954;
When referring to such a defined constant outside its class, use
the name of its class in place of a calling object
int year = MyClass.BIRTH_YEAR;
Pitfall: Invoking a Nonstatic Method Within a
Static Method

A static method cannot refer to an instance variable of the
class, and it cannot invoke a nonstatic method of the class


A static method has no this, so it cannot use an instance
variable or method that has an implicit or explicit this for a
calling object
A static method can invoke another static method, however
Simple Example
static field
static method
Example (Part 1 of 4)
Example (Part 2 of 4)
Example (Part 3 of 4)
Example (Part 4 of 4)