Chapter 2
Java Fundamentals
1
Contents
1. The Parts of a Java Program
2. The print and println Methods, and
the Java API
3. Variables and Literals
4. Primitive Data Types
5. Arithmetic Operators
6. Combined Assignment Operators
7. Conversion between Primitive Types
8. Creating Named Constraints with final
2
Contents
9. The String Class
10.Scope
11.Comments
12.Programming Style
13.Reading Keyboard Input
14.Dialog Box
3
1. The Parts of a Java
Program
 Problem

Write a Java program to display a message
(Programming is great fun!) on the screen.
4
1. The Parts of a Java
Program (Cont’d)
 Run eclipse
 Create a new Java project

Enter the project name: MyFirstProject
 Create a new Java class:

Enter the class name: Simple

This will create a source code file Simple.java
 Enter the code
5
1. The Parts of a Java
Program (Cont’d)
6
1. The Parts of a Java
Program (Cont’d)
 Run the program
7
1. The Parts of a Java
Program (Cont’d)
 Line 1:
//This is a simple Java program

// :



marks the beginning of a comment.
The compiler ignores everything from the
double-slash to the end of the line. You can
type anything you want.
Comments help explain what’s going on.
8
1. The Parts of a Java
Program (Cont’d)
 Line 2:


A blank line
Programmers often insert blank lines in
programs to make them easier to read.
 Line 3:
public class Simple

This is a class header, and it marks the
beginning of a class definition.
9
1. The Parts of a Java
Program (Cont’d)


A Java program must have at least one
class definition.
public:



public is a Java keyword
must be written in all lowercase letters
public is an access specifier, and it controls
where the class may be accessed from. The
public specifier means access to the class is
unrestricted (the class is “open to the public”).
10
1. The Parts of a Java
Program (Cont’d)

class:



a Java keyword, must be written in lowercase
letter.
indicates the beginning of a class definition.
Simple:

Simple is the class name which is made up
by the programmer. Programmer-defined
names may be written in lowercase letters,
uppercase letters, or a mixture of both.
11
1. The Parts of a Java
Program (Cont’d)
public class Simple




tell the compiler that a public accessible class named
Simple is being defined.
We can create more than one class in a file, but
we may only have one public class per Java file.
When a Java file has a public class, the name of
the public class must be the same as the name
of the file (without the .java extension).
Java is a case-sensitive language.
Public ≠ public ≠ puBlic ≠ pUblic …
12
1. The Parts of a Java
Program (Cont’d)

Line 4:
{



is a left brace, or an opening brace, and is
associated with the beginning of the class
definition.
All programming statements that are parts of
class are enclosed in a set of braces. In line 9,
we will have the closing brace.
Everything between the two braces is the
body of the class named Simple.
13
1. The Parts of a Java
Program (Cont’d)
14
1. The Parts of a Java
Program (Cont’d)
 Line 5:
Name of
the method
public static void main(String[] args)


a method header, the beginning of a
method.
main is a name of the method. Every Java
program must have a method named
main. The main method is the starting
point of an application.
15
1. The Parts of a Java
Program (Cont’d)
 Line 6:
{



This opening brace belongs to the main
method. Every opening brace must have a
accompanying closing brace.
We will have a closing brace in line 8 that
corresponds with this opening brace.
Everything between these braces is the
body of the main method.
16
1. The Parts of a Java
Program (Cont’d)

Line 7:
System.out.println("Programming is great fun!");



This line is a statement. It displays a message
on the screen. The message, “Programming is
great fun!”, is printed without the quotation
marks.
The group of characters inside the quotation
marks is called a string literal.
There is a semicolon at the end of this line.
Semicolon marks end of a statement in Java.
17
1. The Parts of a Java
Program (Cont’d)

Not every line of code ends with a semicolon:




Comments do not have to end with a semicolon
Class headers and method headers do not end with
a semicolon.
The brace characters, { and }, are not statements,
so we do not place a semicolon after them.
Lines 8 and 9 contain the closing braces
for main method and the class definition.
18
1. The Parts of a Java
Program (Cont’d)





Java is a case-sensitive language.
All Java program must be stored in a file
with a name that ends with .java
Comments are ignored by the compiler.
A .java file may contain many classes,
but may only have one public class.
If a .java file has a public class, the
class must have the same name as the
file.
19
1. The Parts of a Java
Program (Cont’d)



Every Java application program must
have a method named main.
For every brace, or opening brace, there
must be a corresponding right brace, or
closing brace.
Statements are terminated with
semicolons. This does not include
comments, class headers, method
headers, or braces.
20
2. The print and println
Methods, and Java API

The print and println methods




are used to display text output.
are parts of the Java API (Application
Programmer Interface).
API is a collection of prewritten classes and
methods for performing specific operations.
The console


Standard output: the monitor
Standard input: the keyboard
21
2. The print and println
Methods, and Java API
System.out.println("Programming is great fun!");
Hierarchical Relationship
among the System class,
the out object, and the
print and println
methods
22
2. The print and println
Methods, and Java API




The System class is part of the java API. It has member
objects and methods for performing system-level
operations, such as sending output to the console.
The out object is a member of the System class. It
provides methods for sending output to the screen.
The print and println methods are members of the
out object. They actually perform the work of writing
characters on the screen.
We use the period to separate System, out and print
or println. The period is pronounced “dot”.
23
2. The print and println
Methods, and Java API

The value that is to be displayed on the
screen is places inside the parentheses.
This value is known as an argument.
System.out.println(“King Arthur”);

The println method is that after it
displays its message, it advances the
cursor to the beginning of the next line.
24
2. The print and println
Methods, and Java API
25
The print Method
 The print method



part of the System.out object.
serves a purpose to display output on the
screen.
does not advance the cursor to the next
line after its message is displayed.
26
The print Method
27
The print Method
28
The print Method

There are two ways to fix the Unruly
program:



use println method ?
use escape sequences to separate the output into
different lines
An escape sequence starts with the blackslash
character (\), and is followed by one or more
control characters.


The escape sequence that causes the output
cursor to go to the next line is \n
\n is called the newline escape sequence.
29
The print Method
30
The print Method
31
Common Escape Sequences
 See table 2-2







\n
\t
\b
\r
\\
\’
\”
New line
Horizontal Tab
backspace
Return
Backslash
Single quote
Double quote
32
3. Variables and Literals
 A variable is a named storage location
in the computer’s memory.
 A literal is a value that is written into
the code of a program.
33
3. Variables and Literals
(Cont’d)
34
3. Variables and Literals
(Cont’d)
 Line 7:
variable’s name
int value;
data type



Variable declaration
Variables must be declared before they can
be used.
A variable declaration tells the compiler


the variable’ name
the type of data the variable will hold
35
3. Variables and Literals
(Cont’d)
 Line 9:
value=5;



Assignment statement
The equal sign = is an operator that stores
the value on its right (5) into the variable
named on its left (value).
After this line executes, the value variable
will contain the value 5.
36
3. Variables and Literals
(Cont’d)
37
3. Variables and Literals
(Cont’d)
 Line 11:
System.out.println(value);
 The method println will display the
variable’s contents on the console.
 There are no quotation marks around
variable value

Compare with
System.out.println(“value”);
38
Display Multiple Items with
the + Operator

The + operator is used with strings:

String concatenation operator
System.out.println(“This is ” + “one string.”);
This is one string.

The + operator can be used to
concatenate the contents of a variable to
a string
number = 5;
System.out.println(“The value is ” + number);
The value is 5
39
Display Multiple Items with
the + Operator (Cont’d)
 A string literal cannot begin on one
line and end on another
Error
System.out.println(“Enter a value that is
greater than zero and less than 10.”);
 Breaking the argument up into smaller
string literals, and use the + operator
System.out.println(“Enter a value that” +
“ is greater than zero and less “ +
“than 10.”);
40
Display Multiple Items with
the + Operator (Cont’d)
41
Identifiers
 Identifier




A programmer-defined name.
Do not use any of the Java keywords for
identifiers.
Represents some element of a program.
Variable names, class names, name of
methods, …
42
Identifiers (Cont’d)

Should choose names that give an
indication of what they are used for, what
the purpose is

For example

Number of ordered items
int x;
int itemsOrdered;
not good
43
Identifiers (Cont’d)
 There are some rules with all
identifiers:




The first character must be one of the
letters a-z, A-Z, _, $
After the first character, we can use letters
a-z, A-Z, digits 0-9, _, $
Uppercase and lowercase characters are
distinct.
Identifiers cannot include spaces
44
Identifiers (Cont’d)
Variable Name
Legal or Illegal ?
dayOfWeek
Legal
3dGraph
Illegal
june1997
Legal
mixture#3
Illegal
week day
Illegal
45
Identifiers (Cont’d)
 Variable Names


It is standard practice to begin variable
names with a lowercase letter
Capitalize the first the first letter of each
subsequent word.
int itemOdered;
46
Identifiers (Cont’d)
 Class Names


It is standard practice to begin variable
names with a uppercase letter
Capitalize the first letter of each
subsequent word.
public class PayRoll
47
4. Primitive Data Types
 Each variable has a data type, which is
the type of data that the variable can
hold.
 Selecting the proper type for variables
is important


amount of memory
the way the variable formats and stores
data
48
4. Primitive Data Types
(Cont’d)
 Primitive data types for numeric data
Integer
Data
Types
FloatingPoint
Data
Types
Data Type Size
Range
byte
1 byte
Integers in the range of -128 to +127
short
2 bytes Integers in the range of -32,768 to +32,767
int
4 bytes Integers in the range of -2,147,483,648 to
+2,147,483,647
long
8 bytes Integers in the range of -9,223,372,036,854,775,808
to +9,223,372,036,854,775,807
float
4 bytes Floating-point numbers in the range of ±3.4×10-38 to
±3.4×1038 , with 7 digits of accuracy
double
8 bytes Floating-point numbers in the range of ±1.7×10-308 to
±1.7×10308 , with 15 digits of accuracy
49
4. Primitive Data Types
(Cont’d)

General format of a variable declaration
DataType VariableName;



DataType : name of the data type
VariableName : name of the variable
Examples:
byte inches;
int speed;
short month;
float salesComission;
double distances;
50
4. Primitive Data Types
(Cont’d)
 Combining variables of the same data
type
int length;
int width;
int area;
int length, width, area;
51
Integer Literals

Do not embed commas in numeric
literals.
int number;
number = 1,257,649;
number = 1257649;

//ERROR !
We can force an integer literal to be
treated as long


57
57L, 57l
be treated as int value
be treated as long values
52
Floating-Point Literals

Java assumes floating-point literals such
as 29.75, 1.76, … to be of double data
type.
float number;
number = 25.5;
//Error !
float number;
number = 25.5F;
number = 25.5f;
53
Scientific and E Notation
 Floating-point literals can be
represented in scientific notation.

47,281.97
4.728197×104
 Java uses E notation to represent
values in scientific notation.
Decimal Notation
Scientific Notation
E Notation
247.91
2.4791×102
2.4791E2
0.00072
7.2×10-4
7.2E-4
2,900,000
2.9×106
2.9E6
54
The boolean Data Type
 The boolean data type allows us to
create variables that hold one of two
possible values:
true
false
 Variables of boolean data type are
useful for evaluation conditions that
are either true or false.
55
The boolean Data Type
(Cont’d)
56
The char Data Type



The char data type is used to store
characters.
A variable of the char data type can
hold one character at a time.
Character literals


‘A’, ‘B’, ‘a’, ‘s’, …
Characters are internally represented by
numbers (Unicode)

‘A’ : 65
‘B’: 66
…
57
The char Data Type (Cont’d)
 Characters and how they are stored in
memory
58
The char Data Type (Cont’d)
59
The char Data Type (Cont’d)
60
Variable Assignment and
Initialization
 Variable assignment



Using = assignment operator
The operand on the left side of the =
operator must be a variable.
The value in right side of the = operator is
assigned to the variable in left side.
int unitSold;
unitSold = 12;
12 = unitSold; //ERROR !
61
Variable Assignment and
Initialization (Cont’d)
 Initialization

We may also assign values to variables as
part of the declaration statement
int month = 2, days = 28;
int flightNum = 89, travelTime, departure = 10;
62
Variables Hold Only One Value
at a Time
 A variable can hold only one value at a
time.
 When we assign a new value to a
variable, the new value replaces the
variable’s previous contents.
63
5. Arithmetic Operator
 There are three types of operators:

Unary

Requires only a single operand
-5

(negative operator)
Binary


-number
Requires two operands
Ternary

Requires three operands
64
5. Arithmetic Operator
(Cont’d)
 Common operators
Operator Meaning
Type
Example
+
Addition
Binary
total = cost + tax;
-
Subtraction
Binary
cost = total – tax;
*
Multiplication
Binary
tax = cost * rate;
/
Division
Binary
salePrice = original / 2;
%
Modulus
Binary
remainder = value % 3;
65
Integer Division

When both operands of a division
statement are integers, the statement
will result in integer division.
The result of the integer division will be an
integer.
double parts;
double parts;
parts = 17 / 3;
parts = 17.0 / 3;

parts is assigned
the value 5.0
parts is assigned the
value 5.666666666667
66
Operator Precedence
 Precedence of arithmetic operators
(highest to lowest)
- (unary negation)
 * / %
 + outcome = 12 + 6 / 3
= 12 + 2
= 14

67
Operator Precedence (Cont’d)
 Associativity of arithmetic operators
Operator
Associativity
- (unary negation)
Right to left
*/%
Left to right
+-
Left to right
Expression
Value
5+2*4
10 / 2 – 3
8 + 12 * 2 – 4
4 + 17 % 2 – 1
6–3*2+7–1
68
Grouping with Parentheses
 To force some operations to be
performed before others
Expression
Value
(5 + 2) * 4
10 / (5 – 3)
8 + 12 * (6 – 2)
(4 + 17) % 2 – 1
69
The Math class
 Math class is a member of Java API.
 The Math.pow method
Rising a number to a power
result = Math.pow(4.0, 2.0); // 42

 The Math.sqrt method

accepts a double value as its argument and
returns the square root of the value.
result = Math.sqrt(9.0); //
9.0
70
6. Combined Assignment
Operators
 The combined assignment operators
combine the assignment operator with
the arithmetic operators.
x += 5;
equivalent to
x = x + 5;
1. Add 5 to x
2. The resut is then
assigned to x
71
6. Combined Assignment
Operators (Cont’d)
 Combined assignment operators
Operator
Example Usage
Equivalent To
+=
x += 5;
x = x + 5;
-=
y -= 2;
y = y – 2;
*=
z *= 10;
z = z * 10;
/=
a /= b;
a = a / b;
%=
c %= 3;
c = c % 3;
72
7. Conversion between
Primitive Data Types
 Before a value can be stored in a
variable, the value’s data type must be
compatible with the variable’s data
type.
 Java performs some conversions
between data types automatically, but
does not perform any conversion that
can result in the loss of data;
73
7. Conversion between
Primitive Data Types (Cont’d)
int x;
double y = 2.5;
x = y; //ERROR ! Possible loss
precision
int x;
short y = 2;
x = y; // OK
74
7. Conversion between
Primitive Data Types (Cont’d)


In assignment statements where values of lowerranked data types are stored in variables of
hight-ranked data types, Java automatically
converts the lower-ranked value to the higherranked type. (widening conversion)
Primitive data type ranking






double
float
long
int
short
byte
Highest Rank
double x;
int y = 10;
x = y;
//widening conversion
Lowest Rank
75
Cast Operators



A narrowing conversion is the
conversion of a value to a lower-ranked
type.
Narrowing conversions can cause a loss
of data, Java does not automatically
perform them.
The cast operator lets you manually
convert a value, even if it is a narrowing
conversion.
76
Cast Operators (Cont’d)
 Cast operators are unary operators
that appear as a data type name
enclosed in a set of parentheses.
int x;
double y = 2.5;
x = (int)y;
// x = 2;
cast operator
77
Cast Operators (Cont’d)
int pie = 10, people = 4;
double piesPerPerson;
piesPerPerson = pie / people;
2.0
piesPerPerson = (double)pie / people;
piesPerPerson = pie / (double)people;
2.5
2.5
piesPerPerson = (double)(pie / people);
2.0
78
Mixed Integer Operations
 With arithmetic operations on int,
byte, and short variables:


Values of the byte or short data types
are temporarily converted to int value.
The result will always be an int.
short x = 10, y = 20, z;
z = x + y;
//causes an error
x + y results an
int number
79
Other Mixed Mathematical
Expressions
1. If one of an operator’s operands is a double,
the value of the other operand will be converted
to a double. The result of the expression will be
a double.
2. If one of an operator’s operands is a float, the
value of the other operand will be converted to a
float. The result of the expression will be a
float.
3. If one of an operator’s operands is a long, the
value of the other operand will be converted to a
long. The result of the expression will be a
long.
80
8. Creating Named Constants
with final
 The final keyword can be used in a
variable declaration to make the
variable a named constant.
 Named constants are initialized with a
value, and that value cannot change
during the execution of the program
81
8. Creating Named Constants
with final (Cont’d)
amount = balance * 0.069;
Interest rate
final double INTEREST_RATE = 0.069;
amount = balance * INTEREST_RATE;
 The Math.PI Named Constant
is assigned the value 3.1415926535897323845
 is an approximation of the mathematical value pi.
area = Math.PI * radius * radius;

82
9. The String Class

String literals are enclosed in double
quotation marks.
“Hello World” “Joe Mahoney”


Java does not have a primitive data type
for storing strings in memory.
The String class


Allows you to create objects for holding
strings.
Has various methods that allow you to work
with strings.
83
9. The String Class (Cont’d)


Objects are created from classes.
Declare a variable of the String class
String name;
 A class type variable does not hold the actual data
item. It holds the memory address of the data item.



name is a class type variable, holds the memory address of
a String object.
When a class type variable holds the address of an
object, it is said that the variable references the object.
Class type variables are known as reference variables.
84
9. The String Class (Cont’d)

Creating a String object
String name;
name = “Joe Mahoney”;
 String object is created in memory with the
value “Joe Mahoney” stored in it.
 The address of that object is stored in the
name variable.
85
9. The String Class (Cont’d)

Class type variables
String name;
name = “Joe Mahoney”;
Primitive type variables
int number;
number = 25;

86
9. The String Class (Cont’d)
87
9. The String Class (Cont’d)
 String provides numerous methods
for working with strings.
 To call a method means to execute it.
ReferenceVariable.method(arguments…)
String name;
name = “Tony Gaddis”;
int stringSize;
stringSize = name.length();
length method of
String is called
88
A few String class methods
 charAt(index)


index is an int value and specifies a
character position in the string. The first
character in the string is at position 0.
The method return the character at the
index position.
 length()

Returns the number of characters in the
string.
89
A few String class methods
 toLowerCase()

Returns a new string that is the lowercase
equivalent of the string contained in the
calling method.
 toUpperCase()

Returns a new string that is the uppercase
equivalent of the string contained in the
calling method.
90
A few String class methods
91
10. Scope
 Every variable has a scope.
 A variable’s scope is the part of the
program that has access to the
variable.

Variables can not be accessed by
statements that are outside the scope.
 Variables that are declared inside a
method are called local variables.
92
10. Scope (Cont’d)

A local variable’s scope begins at the
variable’s declaration and ends at the end of
the method in which the variable is declared.
93
10. Scope (Cont’d)

We can not have two local variables with
the same name in the same scope.

94
11. Comments
 Comments are notes of explanation.


Comments are parts of the program, but
the compiler ignores them.
They are intended for people who may be
reading the source code
 Documented programs will make your
life easier.
95
11. Comments (Cont’d)
 Three ways to comment in Java

Single-line comments



Using two forward slashes //
Everything from // to the end of the line is
ignored.
Multi-line comments


Multi-line comments start with /* and end with
*/.
Everything between /* and */ is ignored.
96
11. Comments (Cont’d)

Documentation Comments



Documentation comments can be read and
processed by a program named javadoc.
The javadoc program reads Java source code
files and generates HTML files that document
the source code.
If the source code files contain any
documentation comments, the information in
the comments becomes part of the HTML
documentation.
97
11. Comments (Cont’d)


Any comment that starts with /** and ends
with */ is considered a documentation
comments.
Normally we write a documentation comment



before a class header, giving brief description of the
class
before a method header, giving brief description of
the method.
Run javadoc
javadoc SourceFile.java
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11. Comments (Cont’d)
 Generating javadoc in eclipse

Project \Generate Javadoc …

Enter Javadoc command


C:\Program Files\Java\jdk1.6.0_10\bin\javadoc.exe
Use standard doclet
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12. Programming Style
 Programming Style refers to the way a
programmer uses Spaces,
Indentations, Blank lines, Punctuations
to arrange a program’s source code.
 This program is syntactically correct
and is very difficult to read:
100
12. Programming Style
(Cont’d)
101
12. Programming Style
(Cont’d)
 A common programming style is to
indent all lines inside a set of braces
 To handle statements that are too
long to fit in one line:

Extra spaces are inserted at the beginning
of statement’s second, third … lines, which
indicate that they are continuations.
102
12. Programming Style
(Cont’d)
Extra spaces
Extra spaces
103
12. Programming Style
(Cont’d)
 Declaration of multiple variables of the
same type with a single statement

To write each variable name on a separate
line with a comment explaining the
variable’s purpose.
int fahrenheit,
centigrade,
kelvin;
//Fahrenheit temperature
//Centigrade temperature
//Kelvin temperature
104
13. Reading Keyboard Input

System.in is an object



Standard input device
Is used to read keystrokes
Reads input only as byte values


Is not very useful because programs normally requires
values of other data types
Objects of the Scanner class can be used to
read input from the keyboard.


Reads input from a source such as System.in
Provides methods to retrieve the input formatted
as primitive values or strings.
105
The Scanner object

The Scanner class is not automatically
available to Java programs.
Using import statement to tell the compiler
where in the Java library to find the Scanner
class
 import statement must be placed near the
beginning of the file, before any class
definition
import java.util.Scanner;

106
The Scanner object (Cont’d)
 Create the Scanner object and
connect it to the System.in object.
Scanner keyboard;
keyboard = new Scanner(System.in);

new


a Java keyword
is used to create an object in memory
107
The Scanner object (Cont’d)
This declares a variable named
keyboard . The variable can
reference an object of the
Scanner class.
This creates a Scanner object in
memory. The object will read
input from System.in
Scanner keyboard = new Scanner(System.in);
The = operator assigns the
address of the Scanner object
to the keyboard variable.
108
The Scanner object (Cont’d)
 The Scanner class has methods for
reading strings, bytes, integers, long
integers, short integers, float and
doubles.
int number;
Scanner keyboard;
keyboard = new Scanner(System.in);
System.out.println(“Enter an integer:”);
number = keyboard.nextInt();
109
The Scanner object (Cont’d)
 Some of the Scanner methods

byte nextByte():


double nextDouble():


returns input as a double.
float nextFloat():


returns input as a byte.
returns input as a float.
int nextInt():

returns input as an int.
110
The Scanner object (Cont’d)

String nextLine():


long nextLong():


returns input as a String.
returns input as a long.
short nextShort():

returns input as a short.
111
The Scanner object (Cont’d)
 Problem: User’s gross pay

Write a Java program to calculate the
gross pay of a user. The program allows
user to enter user’s name, hours worked,
pay rate, and display the user’s gross pay
on the console.
112
User’s gross pay








Display “What is your name?”.
Input user’s name. → Store in a variable
Display “How many hours did you work?”.
Input hours. → Store in a variable
Display “How much do you get paid per
hour?”.
Input rate. → Store in a variable
Calculate the user’s gross pay and store in a
variable
Display user’s name and gross pay
113
User’s gross pay
114
User’s gross pay
115
Reading a Character


The Scanner class does not have a
method for reading a single character.
To read a single character using the
Scanner class:
1.
2.
Use the nextLine method of the Scanner
class to read a string from the keyboard.
Use the charAt method of the String
class to extract the first character of the
string.
116
Reading a Character (Cont’d)
String input;
char answer;
//To hold a line of input
// To hold a single character
Scanner keyboard;
keyboard = new Scanner(System.in);
System.out.println(“Are you having fun? (Y/N)”);
input = keyboard.nextLine(); // Get a line of input
answer = input.charAt(0);
// Get the first character
117
Mixing Calls to nextLine with
calls to Other Scanner Methods
 When we call one of the Scanner
class’s methods to read a primitive
value, then call the nextLine
method to read a string → The string
is not read

Because the Scanner class’s methods to
read a primitive value left a newline
character in the keyboard buffer.
118
Mixing Calls to nextLine with
calls to Other Scanner Methods
119
Mixing Calls to nextLine with
calls to Other Scanner Methods
120
Mixing Calls to nextLine with
calls to Other Scanner Methods

To fix the problem

Insert the nextLine method to consume, or
remove, the newline character that remains in the
keyboard buffer.
121
14. Dialog Boxes
 A dialog box is a small graphical
window that displays a message or
requests input.
 The JOptionPane class allows you to
quickly display a dialog box


Display dialog: To display a message
Input dialog: To request input
122
Display Message Dialogs
 The showMessageDialog method is
used to display a message dialog
JOptionPane.showMessageDialog(null, “Hello World”);
123
Display Input Dialog
 The showInputDialog method is
used to display an input dialog
String name;
name = JOptionPane.showInputDialog(“Enter your name”);

The name variable will reference the string
value entered in the text field.
Text field
124
Converting String Input to
Numbers
 Because the showInputDialog
method always returns the user’s input
as a String, even if the user enters
numeric data

We must convert the input string to a
numeric value if the string represents
numeric data.
125
Converting String Input to
Numbers (Cont’d)
 Methods for converting strings to
numbers

Byte.parseByte
Convert a string to a byte
byte num;
Num = Byte.parseByte(str);


Double.parseDouble

Convert a string to a double
126
Converting String Input to
Numbers (Cont’d)

Float.parseFloat


Integer.parseInteger


Convert a string to an int
Long.parseLong


Convert a string to a float
Convert a string to a long
Short.parseShort

Convert a string to a short
127
PayrollDialog
 Write the Payroll program using
dialogs to get user’s input and display
user’s information.
128
PayrollDialog
129
PayrollDialog
130
131