Simple Java Programs - Introduction to Programming in Java - Lecture Slides, Slides of Network security

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• Objectives
  • give some simple examples of Java

applications and one applet

2. Simple Java Programs

Contents

1. Steps in Writing a Java Application

2. Hello.java

3. A Better Programming Environment?

4. Comparison.java

5. Steps in Writing a Java Applet

6. WelcomeApplet.java

2. Hello.java

import java.io.*;

public class Hello

public static void main(String args[])

System.out.println(“Hello Andrew”);

} // end of class

Compile & Run

• The Java file (e.g. Hello.java) must contain a

public class with the file’s name (e.g. Hello

class).

continued

• System.out is the standard output stream
  • like cout (C++) or stdout (C)
• System.out.println() is the main print

function (method) in Java.

Hello

main() calls

System.out.println(…)

writes to screen

via output stream

• Useful Notepad++ features
  • it will format Java code automatically
  • colour-coded display of code
  • it is possible to add calls to javac, java,

appletviewer to the Notepad++ menu

• no need to leave the editor to compile/run
• there is an optional window that show the output

from running Java code

4. Comparison.java

import javax.swing.JOptionPane; // GUI dialogs

public class Comparison

public static void main( String args[] )

{ String firstNumber,secondNumber,result;

int number1,number2;

// read user numbers

firstNumber = JOptionPane. showInputDialog(

"Enter first integer:");

secondNumber = JOptionPane. showInputDialog (

"Enter second integer:" );

// convert numbers

number1 = Integer.parseInt( firstNumber ); number2 = Integer.parseInt( secondNumber );

result = ""; if ( number1 == number2 ) result = number1 + " == " + number2; if ( number1 != number2 ) result = number1 + " != " + number2; if ( number1 < number2 ) result = result + "\n" + number1 + " < " + number2; if ( number1 > number2 ) result = result + "\n" + number1 + " > " + number2; if ( number1 <= number2 ) result = result + "\n" + number1 + " <= " + number :

Compile & Run$ javac Comparison.java

$ java Comparison

Notes

• The Comparison class is just a single

main() function (method)

continued

• Notice the use of familiar C/C++ control

structures (e.g. if, while) and data types

(e.g. int, double)

• “...” + “...” means concatenation

(put strings together)

• String is a pre-defined class for strings.
• int is a built-in type (just like C’s int).

Calling Methods

• Methods are defined in classes.

• Syntax for calling a method:

Class.method-name

or object.method-name

– e.g. JOptionPane.showMessageDialog( ...);

• this calls the showMessageDialog() method in the

class JOptionPane

Classes start

with an upper

case letter.