Chapter 2
Primitve Data Types, Input,
Output, and Assignment
Go
Sect 1 - Steps for Creating and Running a Java Program
Go
Sect 2 - Framework of a Simple Java Program
Go
Sect 3 - Three Types of Java Programs
Go
Sect 4 - Data Types and Basic Input and Output
Go
Sect 5 - Calling Methods
Go
Sect 6 - Syntax and Compile-Time Errors
Go
Sect 7 - Graphics and GUI Programs
Section 1
The Steps for Creating &
Running a Java Program
A program is a sequence of instructions
that perform a task.
2
Fastest Growing Programming Language
Here are some reasons that Java is the fastest growing programming
language:
•
It is a modern object-oriented language.
•
It is secure. Viruses can’t be spread with Java programs.
•
It is robust. This means that Java programs can respond efficiently
to a variety of situations while a program is running if the correct
precautions are taken when writing the code.
•
It supports threads. This means two or more parts (threads) of a
Java program can be running at the same time.
•
It supports the development of programs that do not overwrite
memory. In other words, if one program takes up a portion of RAM
memory while it is running and a second program is started. The
second program cannot be loaded into the memory taken up by the
first program if there is not enough RAM memory.
3
Steps for Creating & Running a Program
1. Write the source code in a .java file
2. Compile the source code into byte code creating a .class file
3. JVM interprets the byte code into machine language 1s and 0s and
the computer then executes the instructions
4
Source Code is Written in a .java File
Step1. When a programmer writes Java code, it is stored in a source
code file that has a .java extension like RobotMan.java.
The source code file is what the programmer compiles and runs to see
the output of a program. We will write our Java progams in the IDE
software called Eclipse. IDE stands for Integrated Development
Environment. An IDE lets you write, compile, and execute
programs.
Source code can be typed in any simple text editor, but we will use
Eclipse, which is both a text editor and a compiler.
Eclipse is a nice Java editor and compiler because it will point out
syntax errors when java code is typed incorrectly. A program will
not run until all syntax errors are removed.
5
Source Code is Compiled to Byte Code
Step 2. An IDE (Integrated Development Environment) like Eclipse,
has a built in compiler that translates the Java source code into
Java byte code.
When source code is compiled into byte code, the byte code is stored
in a file that has the same name as the source code file but with a
.class extension. It is not a text file and you cannot open it!
For example, the source code file RobotMan.java will be compiled
into a byte code file named RobotMan.class.
The file is created during the compiling process, however, Eclipse
sometimes translates .java files into .class files behind the scenes
before you ever compile them. You can see these files in your
package folders if you open your workspace folder on your hard
drive.
6
Byte Code is Interpreted to Machine Code
Step 3. The JVM (Java Virtual Machine) must interpret the byte code
into machine language. Interpret basically means to translate. The
JVM is software that acts like hardware. That is why it is called a
virtual machine. Java bytecodes are translated on the fly to 1s and
0s instead of being stored in an executable file, which some
languages do. The main advantage of an interpreter is that it will
run on any computer and there are JVMs for Apple and Windows
machines. This makes the code portable from one platform to
another.
Interpreting can be slow, but software improvements are always being
made to speed up this process.
Every kind of computer, whether it is an Apple, Windows, or Unix
machine, has an operating system that contains a JVM software
component. The JVM was automatically installed when your
computer’s system software was installed and updates to the JVM
may be contained in any system software updates you install now
or in the future.
7
What are Applets?
Applets are small Java programs that are embedded in web pages.
When you load a web page that has a game in it, the game is an
applet.
More specifically, the applet is usually a jar file that contains the byte
code for all the .java files that are needed for the game. A jar file is
like a zip file, where a number of files are compressed into one file.
An HTML web page can contain a special kind of tag called an Applet
tag. That tag tells the web page that the byte code is stored in a jar
file with a specific name.
The browser software, like FireFox, Chrome, Camino, or Safari will
then use its built-in JVM interpreter to translate the byte code into
machine language 1s and 0s.
8
Just-in-Time Compilation by the JVM
Some JVMs support just-in-time compilation (JIT). JITs will
translate byte code instructions the first time they are encountered
into machine language code and that code is saved so that the
next time that code is encountered it can be executed without
being re-interpreted. This can speed up the interpreting of the byte
code of a program so it will run faster.
9
Machine Language Code is Executed
Step 4. Finally, the machine language code (patterns of 1s and 0s),
those instructions that the computer understands at the lowest
level, are executed. This allows the program to appear in its
runnable form, either in a console window, an applet window, or a
GUI (Graphical User Interface) window. Again the overall process
is ….
10
Section 2
Framework of a Simple Java Program
1.
2.
3.
4.
package declaration
import statements
class declaration line
main method
11
Framework of a Simple Java Program
The Framework (in order) of a simple console or standalone program:
1.
package declaration for the file listed first (where it is stored)
2.
followed by import statements (if any)
3.
class declaration line
4.
main method
package ch03;
import java.util.Scanner;
public class DistanceCalculator
{ // opening curly brace for the DistanceCalculator class
public static void main(String args[ ])
{ // opening curly brace for main method
// code for program goes inside these curly braces
} // ending curly brace for main method
Sample
Skeleton
Program
Note: curly braces
always appear in
pairs.
} // ending curly brace for the DistanceCalculator class
All Java console and JFrame (standalone) programs have this framework. The
framework for applets is different and you will become familiar with it later.
12
Classes in Java
A Java program can use more than one class file (a .java file). In fact, it may
be made up of many .java files.
And most of the time, every .java source code file in a Java program contains
only one class.
However, it is possible to have more than one class in a .java file, but this is
usually done in more advanced programming projects where the code is
more complicated and there is a reason for doing it.
So assume that every .java file contains only one class, but a java program
can use one or more .java files.
A class is a module of code that can stand alone by itself and if made “public”
other files can “see it” and “access parts of it”. Eclipse is a nice IDE for
organizing Java files so that classes are accessible to each other if they
are in the same package folder or if an appropriate import statement is
used to direct Eclipse to where a package is located. Eclipse manages
this behind the scenes automatically and lets you know if something is
“unorganized”, so you can fix it.
13
Declaring Classes in a File
A class is declared with a line like:
public class RobotMan
and has a set of curly braces that go with it { and }.
In Java, a class like RobotMan must be stored in a file named
RobotMan.java. If they don’t match, then Eclipse or any other IDE
will flag it with an error and it must be corrected before the file is
used in a program. Eclipse has short cuts to renaming the file
(compilation unit) or the class if necessary. You’ll learn about that
soon.
Simple Java programs can be designed so that they contain just one
class. We will now look at some examples.
14
Section 3
Three Types of
Java Programs
15
Three Types of Java Programs
There are basically three types of Java programs that a
programmer can make:
1.
A console text output program.
2.
A standalone graphics or GUI program that can be displayed
in a standard window frame.
3.
An applet graphics or GUI program that is embedded in a
web page.
16
Console Text Output Programs
Some Java programs, like RobotMan, are displayed only in a console window,
because they show only text output. You will learn how to do more than
draw things with text characters in a console window. Some programs
may make mathematical calculations and display them.
When there is only one file
in a Java Program, we
sometimes refer to that file
as a “Driver file”, because
it is the file that makes the
program run. If a program
has more than one file,
then one of the files is a
driver file. The driver file’s
job is always to initiate the
program to begin the
compiling and running
process.
17
Graphics & GUI Programs
Besides console text programs, you can have applet or standalone
programs that contain graphics or GUI components:
1.
GUI components are elements in a program like text fields,
buttons, labels, text areas, or menus.
2.
Graphics are lines, and curves, and pretty much anything that can
be drawn.
So you can have:
1.
A web applet program with GUI components.
2.
A web applet program with graphics.
3.
A standalone program with GUI components.
4.
A standalone program with graphics.
5.
A web applet or standalone program that has both GUI
components and graphics.
18
Applet GUI Output Programs
Java applets are small Java programs downloaded from web pages that run
in a Web browser. A JVM is incorporated into every browser, so the applet
can be interpreted, executed and displayed inside the browser. Any game
you have played online is more than likely a Java applet. Both applet and
standalone programs can have GUI components.
Because Java programs run
inside a Virtual Machine, it is
possible to limit their
capabilities. What this means
is that some other
programmer can’t inject a
virus into the code you have
written for an applet program.
Therefore, everybody doesn’t
have to worry about a Java
applet infecting their
computers with a virus that
will erase files on their hard
drive or steal sensitive
information.
A GUI program with fields, buttons, labels, and a text area.
19
Applet or Standalone Graphics Output Programs
Graphics program can contain lines, arcs, ovals, rounded-rectangles, etc. and
they can be animated. GUI or graphics programs pretty much look the
same whether they are in an applet or standalone program.
20
Section 4
Data Types and the Basics
of Input and Output
In this section, you will learn some very important information
about:
1.
print and println Statements
2.
Variables
Simple data types
The assignment operator =
How to declare and construct a Scanner object so you can
read input from the keyboard
The new operator
3.
4.
5.
6.
21
Java Is Hot Source Code
package ch01;
public class JavaIsHot
{
public static void main(String args[ ])
{
System.out.println( "
d
");
System.out.println( "
o l
");
System.out.println( "
l r
");
System.out.println( "
l o
");
System.out.println( "
e w
");
System.out.println( "
H
");
System.out.println( " xxxxxxxxxxxxxxxxx ");
System.out.println( " x
x x ");
System.out.println( " x Java x x ");
System.out.println( " x
xxxx ");
System.out.println( " x is Hot! x
");
System.out.println( " x
x
");
System.out.println( " xxxxxxxxxxxxxx ");
} // end of main method
} // end of JavaIsHot class
System.out.println statements are used to print text to the console output window.
22
print and println Statements
Output to a console text window in Java is accomplished through the
use of print and println (pronounced print-line) statements.
print and println are two operations (or as we say in Java) … methods
that can be called. These operations output text information to the
console window in Eclipse.
We call methods when we want to accomplish some operation or task.
23
print and println Statements
Here are lines of code that call the two methods print and
println:
1.
System.out.print(“Hello World”);
2.
System.out.println(“Hello World”);
Think of methods as operations, but we always refer to them
as methods in Java. Some other languages refer to
them as functions or procedures.
When we say we are “calling a method”, what we mean is
that we are executing an operation!
24
print and println Statements
You may be wondering what the difference between the two methods
print and println are. print will display everything on one line but
not start a new line for anything else that will be printed by the next
print or println statement. println will display everything on one line
and then start a new line of output for the output of any other print
or println statement. Here is an example:
System.out.print(“Hello World! ”);
System.out.print(“How are you doing? ”);
System.out.print(“I am doing fine.”);
(Note the extra spaces at the end of the first two print statements or the
sentences would jam up together in output)
The output in the console window is all on one line:
Hello World! How are you doing? I am doing fine.
25
print and println Statements
Again … print will display everything on one line but not start a new
line of output. println will display everything on one line and then
start a new line of output.
Consider these three lines of code:
System.out. print(“Hello World! ”);
System.out. println(“How are you doing?”);
System.out. println(“I am doing fine.”);
The output is on two lines in the console window:
Hello World! How are you doing?
I am doing fine.
26
print Statements and Syntax Errors
So when we use the line of code:
System.out.print(“Hello World”);
We are calling the print method. Printing to a console window requires
that we place System.out. prior to the word print.
Please notice the periods “.” (method selector operators) that separate
the words System, out, and print.
Also, notice that the first S of System is capitalized (upper case). The
reason is System is the name of a class.
Also, notice that after the word print there are parentheses that include
in double quotes the information that we want to display to the
screen and the entire line of code ends in a semicolon.
All of this is required and must be typed correctly or you will get a
syntax error. Programs with syntax errors won’t run!
27
Semicolons End Most Java Statements
•
A Semicolon (;) marks the end of most Java statements. A
statement is a “sentence” in a program. Examples:
–
System.out.println(“Hello World!”);
–
num1 = reader.nextInt();
–
sum = num1 + num2;
Some Java lines do not end in a semicolon. You will learn which ones
don’t as you are introduced to other coding structures.
28
Using Variables to Store Values
A variable names a memory address (location) in RAM memory where
a value can be stored.
The variable is considered to be a nickname for the memory address.
The memory address of a variable may be some unusual combination
of 1s or 0s or a hexadecimal number, so its great that we can
decide on the name of variables. We do this when we declare
them.
We can think of a variable as a small box that holds a value.
X
Y
name
an int variable
a double variable
a string variable
29
Simple Java Data Types
In Java, we have different kinds of data types. Here you will learn three of
them. We can store each of the three types of data values in different
kinds of variables.
We can have …
1.
int variables that can hold integers (type int)
2.
double variables that can hold real numbers or what we refer to also as
floating-point numbers (type double)
3.
String variables that refer to an object that can hold a string of
characters, in other words, a bunch of characters that make up a word
(type String)
The data types int and double are simple numeric data types and we store
those kind of values in simple variables that are not considered object
variables.
However, Strings are objects and a String value must be stored in an object
variable. Let’s look at how we do that with the assignment operator.
30
The Assignment Operator
The assignment operator is the = character and you can initialize
variables to literal numeric or string values in one line of code.
So to store a value in a variable, we use the assignment operator.
Here is how to declare the different kinds of variables and store values
in them or as we like to say initialize them:
int x = 129;
// x is the int variable and 129 is being stored.
double y = 3.14159; // y is the double variable and π is stored.
String name = “Java”;
In the last line of code, name is the String variable and the word “Java”
is being stored. Notice the double quotes around Java since we
are storing a literal String value Don’t place double quotes when
you store numbers.
31
Visualizing Simple and Object Variables
simple variables
object variables
object variables, like
String or Scanner
variables, can send
messages to
objects, but simple
variables can’t.
32
Numeric Variables
A numeric variable names a memory address (location) in RAM
memory where a number can be stored. It is considered to be a
nickname for the memory address. Numeric variables are of type int
or double.
When we use the line of code:
double celsius;
then enough RAM memory is allocated so that a floating-point value
can be stored in celsius. Java does this automatically for you! A
double variable gets twice as much memory as an int variable.
Thus, the data type was named double.
33
Important Facts About Variables
Important points to remember about variables:
•
A variable’s value may change during a program, but its name
remains constant.
•
A variable’s type defines what kind of values can be stored in RAM.
The type of the variable cannot be changed while a program is
running. Remember when we say type we are referring to int,
double, boolean, String, or other data types.
34
Java’s Mathematical Operators
Java basically has 5 mathematical operators.
Addition is represented by the + sign.
Subtraction is represented by the - sign.
Multiplication is represented by the * sign.
Division is represented by the / sign.
Mod is represented by the % sign. Mod gives the remainder of int
division. (You’ll be amazed at how much you will use mod)
In the Convert.java program, the line of code:
celsius = ( fahrenheit - 32.0 ) * 5.0 / 9.0;
uses three of the above mathematical operators.
You’ll see the code for the program in a few of slides.
35
Declaring and Using Scanner Objects
We also use the assignment operator when creating objects
(constructing objects) so an object variable can refer to them.
If we want to receive input from the keyboard during a program, then
we need to construct a Scanner object first. This allows the user to
enter numbers or string values into a program.
To do this we need to import Java’s Scanner class with the line:
import java.util.Scanner; // goes above the class declaration line
Next, inside the main method, we need to construct the Scanner
object with the line:
Scanner reader = new Scanner(System.in);
We need to use System.in as the parameter because this indicates
the keyboard, which is the default input device for Java. Note:
Scanner is a class so the S is capitalized.
36
General Form for Constructing Objects
•
•
In programming, the process of constructing an object is called
instantiation.
In general, constructing or instantiating and object takes the general
form:
<Name of Class> <variable> = new <Name of Class> (<parameters>);
You can see that the line of code below follows this form:
Scanner reader = new Scanner (System.in);
You can think of reader as something that is “scanning the keyboard
waiting for input”.
37
Constructing a Scanner Object with new
In the line of code :
Scanner reader = new Scanner(System.in);
the name of the class Scanner is used twice.
The first part of the line:
Scanner reader
declares reader to be an object variable of type Scanner. We need to
do this or reader can’t refer to the Scanner object we construct.
The second part of the line:
new Scanner(System.in);
constructs the Scanner object and “attaches it” to the keyboard. The
word new is the new operator that we use to construct things. The
assignment operator makes reader refer to the Scanner object.
38
More About Reading Input from the Keyboard
Assume the following lines of code appear in the main method of a program.
Scanner reader = new Scanner (System.in);
System.out.print(“Enter your name and press return: ”);
String name = reader.nextLine();
System.out.print(“Enter your age and press return: ”);
int age = reader.nextInt();
System.out.print(“Enter your gpa and press return: ”);
Note: the print
statements
“prompt” the
user to enter
data, otherwise
he or she
wouldn’t know
the computer is
waiting for
input!
double gpa = reader.nextDouble();
Notice when we prompt the user, we use a print statement NOT a println
statement. We can receive different kinds of input using reader. We use
the object variable reader to “call the method nextLine() to receive a String
value from the keyboard. We use reader to “call the method nextInt() to
receive an integer and we use reader to “call the method nextDouble() to
receive a floating-point value from the keyboard.
39
Echoing the Input from the Keyboard
We can now “echo the input” (print the information back to the screen
that was entered) by using some println statements. In each line,
we will print a literal string (something in double quotes) and the
value contained in a variable. We use a plus sign to concatenate
the literal string value and the value stored in the variable together
to make a larger string that is then printed.
System.out.println(“Your name is: ” + name);
System.out.println(“Your age is: ” + age);
System.out.println(“Your gpa is: ” + gpa);
Notice that there are no double quotes around the variables name,
age, and gpa.
40
Algorithm for Converting Temperatures
If we wanted to write the code for a program that would convert
Fahrenheit temperatures to Celsius, it would be good to stop and
develop an algorithm. An algorithm is a step by step procedure
for solving a problem. This helps our code to be more efficient
and we save a lot of time, because we consider all that needs to
be done … NOT just the mathematical formula needed to
convert the temperature.
Any suggestions?
41
Algorithm for Converting Temperatures
Consider this algorithm:
1.
Prompt the user to enter a Fahrenheit temperature.
2.
Read the value from the keyboard and store it in a variable.
3.
Use the value stored in the variable in an assignment statement
that applies the correct mathematical formula in code form.
4.
Store the value calculated in a second variable.
5.
Output the converted value to the screen.
42
The Convert.java Program
package ch02;
import java.util.Scanner;
public class Convert
{
public static void main(String args[])
{
Scanner reader = new Scanner(System.in);
double fahrenheit;
double celsius;
// prompt the user to enter a value from the keyboard when the program runs.
System.out.print("Enter degrees Fahrenheit and press return: ");
// read the value from the keybaord and store it in the variable fahrenheit.
fahrenheit = reader.nextDouble();
// calculate the equivalent celsius value and store it in the variable celsius
celsius = (fahrenheit - 32.0) * 5.0 / 9.0;
// print out the following string of characters to the terminal window.
System.out.print("The equivalent in Celsius is ");
// print out the value stored in the variable celsius to the terminal window.
System.out.println(celsius);
}
}
Note the key Scanner lines identified by the red arrows.
43
More about Import Statements
In the Convert.java code you saw that the first line after the package
declaration was an import statement:
import java.util.Scanner;
This tells the compiler where to find a class that will be used during
the program. The class may be either in a Java library file or
another file you have in a folder. The import statement contains
the path name of where to find the class.
The import statement tells us that the Scanner class is found in a
sub-package folder named util that is in the java package folder.
Now you know enough that you can finish the second half of the
Convert.java program!
44
Section 5
Calling Methods
In this section, you will learn some very
important information about:
1.
2.
3.
Method Calls and Parameters
Concatenation
Readability of Code
45
Method Calls with Parameters
Consider the following line of code:
System.out.println(“Hello World!”);
•
System is a class and out is an object of that class that
knows how to display or print characters in a console or terminal
window.
–
println is the name of the method (operation) being
executed. (Another way to say it is we are “sending the
message println to the object represented by System.out”.)
–
The item inside the parentheses “Hello World” is the
parameter of what needs to be printed. Here the parameter is
a string (string of characters) that make up the words “Hello
World”. Notice they appear in quotation marks. This tells Java
to print to the screen the literal string value “Hello World”. The
parameter could be a variable that contains a value instead of
a literal string value in double quotes.
46
General Rule of Method Calls
The general form for calling methods is:
<name of object> . <name of method> (<parameters>)
A message may require zero, one, or multiple parameters. Here
are some examples:
•
To print a blank line, we can use System.out.println(); without
any parameters in the ( ).
•
To print “Hello World”, we need only one parameter the literal
string value “Hello World” in the parenthesis …
System.out.println(“Hello World!”);
•
To print “Hello World. Java Rules”, we still need only one
parameter but the parameter may be the concatenation of two
literal string values as in …
System.out.println(“Hello World!” + “Java Rules!”); Notice that we
concatenate two literal strings together using a + symbol.
•
To call a method named calculateArea with two parameters
length and width, we use the code ...
calculateArea(length, width);
Notice the two parameters are separated by a comma.
47
The Method Selector is the Period
The Method selector operator is the period . and is always placed between
the object’s name and the method’s name. It is also placed between the
name of a class and the name of an object when needed as in System.out.
Scanner reader = new Scanner(System.in);
System.out.println(“Enter an integer: ”);
int num1 = reader.nextInt();
System.out.println(“Enter an integer: ”);
int num2 = reader.nextInt();
System.out.println(“Enter a floating-point number: ”);
double num3 = reader.nextDouble();
Above, reader has been declared to be a Scanner class variable that
represents a new Scanner object. That Scanner object can read data
from the input of the keyboard. Reader can call either of the methods
nextInt() or nextDouble() to read integers or floating-point numbers from
the keyboard. The variable reader is also referred to specifically as an
object variable.
48
Declaring int and double Variables
TIME OUT!!! What are we doing with the lines?
int num1 = reader.nextInt();
double num3 = reader.nextDouble();
In the first line, we declare a variable named num1, but we have to tell what
type of data we intend for it to hold. Since we want it to hold an integer
(whole number), we use Java’s int data type. The type of data must be
listed before the name of the variable!
The assignment operator = means we want to store something in num1. What
we want to store is what is on the right side of the = sign. It could be an
arithmetic expression or a reader line.
When a program runs and it encounters a line like reader.nextInt(), the
program will pause and wait for something to be entered from the
keyboard. Once the user enters an integer and presses return, then the
number is read by reader’s nextInt() method, and then the value is stored
in num1.
With the second line of code, we declare a variable num3 that can hold a
floating-point number (one that contains a decimal point like 4.237). The
method nextDouble() is used to read the value from the keyboard and
49
store it in the double (floating-point) variable num3.
Concatenating Output Items
Consider this segment of code:
System.out.println(“Enter an integer: ”);
int num1 = reader.nextInt();
// a whole number is stored in num1
System.out.println(“Enter an integer: ”);
int num2 = reader.nextInt();
// a whole number is stored in num2
int sum = num1 + num2;
// a whole number is stored in sum
System.out.println(“The sum of ” + num1 + “ and ” + num2 + “
is ” + sum);
In the last output line, we concatenate 6 things together to be written to output. Three
of the items are literal strings and three are variables of type int that hold integers.
Notice there are NO double quotes around the variables. You never place double
quotes around any variable … only literal string values. Also, notice the blank
spaces at the beginning and end of some literal string values. This keeps the
numbers from being jammed up against the words.
50
Review of Simple and Object Variables
In the Convert.java program, simple variables like fahrenheit and
celsius each hold a single floating-point number.
Object variables like reader and System.out hold references to
objects.
Object variables are used to send messages to objects.
reader.nextDouble() sends the message “get me the next floating point
number” from the keyboard.
In summary, to write effective Java programs, a programmer does not
need to have detailed knowledge of the inner workings of any
object, he or she just needs to know how to construct objects and
how to send messages to the object by calling methods.
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The Readability of Code
It is important for your code to be readable by others. In the real world,
programmers are on software teams as they develop and maintain
software. So everyone must be able to read your code! Programmers
have developed a standard for how code should be formatted.
•
The main factors that determine whether code is readable or not are
– Spacing (referring to extra blank lines that space things out)
– Indentation (referring to tabs or indentions on a specific line)
•
Spacing and Indentation are just for programmer readability. The compiler
ignores any kind of spacing and indentation. It just checks to make sure
that everything is syntactically correct (spelled correctly)!
•
Eclipse assists you with indenting segments of code as you type by
automatically properly indenting your next line depending on the kind of
Java code you are writing. But if you mess up the indenting then all you
have to do is select all code by typing Control “a” on a Windows machine
or Apple “a” on a Mac, then type either Control “i” for Windows or Apple “i”
for a Mac and everything will get indented properly.
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Section 6
Syntax and Compile-Time
Errors
53
Syntax Errors
Syntax errors keep a program from compiling and running until the
errors are corrected. Examples are:
•
forgetting to place a semicolon after Java lines that need it
•
forgetting one of the two ( ) when calling a method like println.
•
forgetting to place double quotes where they are needed.
•
misspelling Java key words like print, println, System, public, and
class.
Syntax errors are a form of compile-time errors.
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Compile-Time Errors
Some errors are not syntax errors. They are just compile-time errors.
Here are some examples:
•
not storing the class in a file with the same name.
•
trying to store a data value of one type in a variable of another
type.
•
leaving out the opening or ending curly brace for a method.
If you try to run a program that has syntax or compile-time errors, a
compiler like Eclipse will halt the process and display error
messages in the console window that …
•
indicates the type of error detected
•
indicates the file and line number where the error was detected55
How Eclipse Points Out Errors
•
Eclipse will …
1.
2.
3.
•
describe the error in the source code window
tell the line of code where the error is
suggest a solution to the error when you mouse over the bad code.
Eclipse displays a red circle with an X in it to the left of any line that
has a syntax error.
•
Eclipse will display a red box with an X in it for any line that has a
compile-time error.
•
You can …
–
–
mouse over the red circle or red box and a message will pop-up
that describes the error.
click on the red circle or red box and then choose an option of
how to correct it. This saves time if you choose the correct fix,
then the error will be corrected automatically for you.
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Computer Hacking
Originally, a hacker was a programmer who exhibited
rare problem solving ability. This person was an
asset to any company. Especially, when there were
difficult problems to solve. In fact, a “hack” refers to
a programming solution to a difficult problem that
some might think is unsolvable.
Currently, hacker has a negative connotation, referring
to someone who breaks into computer systems or
circumvents a system that is intended to be used.
They may do this just to impress their peers or to
cause actual harm.
In addition, anyone who lacks a disciplined approach
to programming is many times called a hacker.
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Section 7
Graphics and GUI Programs
•
Graphics and GUI programs in Java can run either as a standalone application or as an applet.
•
We will study how to use applets in upcoming chapters.
•
Standalone GUI applications run in windows.
•
A window is a container for graphical components to be
displayed to the user.
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JApplet Windows
Windows have numerous properties.
•
Width and height
•
Ability to be dragged or resized by the user
•
Ability to be painted or drawn on.
The code for applet windows is located in the class JApplet, which is
imported from the package javax.swing. To use this class you
need to import it with the statement:
import javax.swing.JApplet;
or
import javax.swing.*;
The last import statement that uses the * imports all the classes in
the swing package not just JApplet.
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AppletWindow1.java Code
This code produces an empty applet window.
public class AppletWindow1 extends JApplet
{
// applet programs like to have this line of code, but the applet will still
// run without it. However, it will show a warning. This warning is NOT an error!
private static final long serialVersionUID = 1L;
// This method initializes the applet and loads it. It creates an applet
// window that is 300 pixels wide and 200 pixels high.
public void init ()
{
resize(300, 200);
}
// This method is needed to paint things in the applet window. However,
// nothing is painted because we have no drawing or painting commands in it.
// The line of code super.paint(g); serves to erase the applet window before
// anything is painted.
public void paint(Graphics g)
{
super.paint(g);
// missing painting and drawing code would go below.
}
}
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AppletWindow1.java Output
The output will look something like this.
An applet window is really just an empty container called a
pane that we can fill with other objects. One object we can
place in the window is a panel.
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Panels and Colors
A panel is a flat, rectangular area suitable for displaying other objects.
It may contain geometric shapes and images.
To use a panel you must import the JPanel class with:
import javax.swing.JPanel;
You can use Java’s default colors or create your own RGB colors by
importing the Color class with:
import java.awt.Color;
To change the current drawing and painting color to red, use the paint
brush g to call the setColor() method as follows:
g.setColor(Color.red);
Note the dot “.” between g (an object) and setColor (a method) and in
parenthesis since red is a color constant of the Color class we must
use Color with a capital C dot red.
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2.7 Java’s Default Color Constants
Color Constant
RGB Value Construction
Color.red
new Color (255, 0, 0)
Color.green
new Color (0, 255, 0)
Color.blue
new Color (0, 0, 255)
Color.yellow
new Color (255, 255, 0)
Color.cyan
new Color (0, 255, 255)
Color.magenta
new Color (255, 0, 255)
Color.orange
new Color (255, 200, 0)
Color.pink
new Color (255, 175, 175)
Color.black
new Color (0, 0, 0)
Color.white
new Color (255, 255, 255)
Color.gray
new Color (128, 128, 128)
Color.lightGray
new Color (192, 192, 192)
Color.darkGray
new Color (64, 64, 64)
If you were going to construct these colors by yourself, you would
use the 3 integer values in the parenthesis.
63
Customized Colors
You can construct a customized color if the color you need is not
one of the Color constants seen on the previous slide.
You can construct a new Color object by using three int values
between 0 and 255 with:
Color aColor = new Color(redValue, greenValue, blueValue);
In this code, redValue, greenValue, blueValue must be integer values.
You would then use the code:
g.setColor(aColor);
( don’t use Color.aColor in () )
Here is an actual example:
Color brown = new Color(164, 84, 30);
g.setColor(brown );
(don’t use Color.brown in () )
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AppletWindow2.java Code
// This code produces an empty, pink panel.
// Note if you try to add a second panel to this JApplet, then you won't see them both unless you
// specify what region of the default BorderLayout you are adding the panel to.
public class AppletWindow2 extends JApplet
{
// other code …
public void init()
{
resize(300, 200);
JPanel panel = new JPanel();
// The following line of code sets the background color of the panel
panel.setBackground(Color.pink);
Container pane = getContentPane();
pane.add(panel);
} // end of init method
// other code …
65
AppletWindow2.java Code Modified
Now let’s create a customized color to set the background to. Let’s say with a dark blue.
public class AppletWindow2 extends JApplet
{
// other code …
public void init()
{
resize(300, 200);
JPanel panel = new JPanel();
Color myColor = new Color (51, 0, 153); // define the color midnight blue
panel.setBackground(myColor);
Container pane = getContentPane();
pane.add(panel);
} // end of init method
// other code …
66
Layout Managers and Multiple Panels
Every container object (frame or panel) uses a layout manager
object to organize and lay out the graphical components contained
in it.
The default layout manager for frames is an object of the class
BorderLayout.
You can arrange up to five graphical objects in a container in the
following positions:
NORTH, SOUTH, EAST, WEST, and CENTER
If we add fewer than 5 objects to a BorderLayout, the layout manager
stretches some of them to fill the unoccupied areas.
An object of the GridLayout class divides a container into rows and
columns with equal size cells.
67
BorderLayout on AppletWindow3.java Output
If you look at the code in AppletWindow3.java you can see how to set
up a BorderLayout Applet Window. The code is too lengthy to show
here. We will go over it in class. Here is the output of
AppletWindow3.java
north region
center region
east region
west region
south region
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AppletWindow4.java Code and Output
This code produces an applet window with a grid containing 2 rows and 2 columns of colored panels.
public class AppletWindow4
{
public void init()
{
resize(300, 200);
JPanel panel1 = new JPanel();
panel1.setBackground(Color.white);
JPanel panel2 = new JPanel();
panel2.setBackground(Color.black);
JPanel panel3 = new JPanel();
panel3.setBackground(Color.gray);
JPanel panel4 = new JPanel();
panel4.setBackground(Color.white);
Container pane = getContentPane();
pane.setLayout(new GridLayout(2, 2)); // (rows, columns)
pane.add(panel1);
pane.add(panel2);
pane.add(panel3);
pane.add(panel4);
} // end of init method
}
69