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Tutorials : Java by Example : Section 1 :
Section One Contents
Basic Graphics Functions
Simple Methods And Basic Data Types
If, Else And Switch: Basic Control Structures

Java Basics

If, Else And Switch: Basic Control Structures

To check for an expression or condition being true or false, you usually use the IF statement:
if(test-expression)
   body
That means: if the test expression results to TRUE, the body is executed, otherwise the whole statement is skipped and the sourcecode below it is executed, if any. Test expressions can be a boolean variable, or the result of a function or calculation: if(3*3==9) is also true.
In many cases you want to execute a certain piece of code if a condition or expression is true, and a different piece, if that same expression does not result to true. To accomplish this, you can use the following statement:
if(test-expression)
   body1
else
   body2
The applet below should make this clear, it tests for mouseclicks being in the left or in the right half and displays the result in a text string.

//Sourcecode

import java.awt.*;
import java.applet.*;

public class Project5A extends Applet
{
        int mouseX, mouseY;
        boolean clicked=false;

        public boolean mouseDown(Event evt,int x,int y)
        {
                clicked=true;

                mouseX=x;
                mouseY=y;

                repaint();

                return true;
        }

        public void paint(Graphics g)
        {
                g.setFont(new Font("Helvetica",Font.PLAIN,15));

                g.drawString("Click inside the applet!", 80,50);

                //we only draw this if the mouse has been clicked once
                if(clicked)
                {
                        if(mouseX < size().width/2)
                                g.drawString("You clicked on the left side!",60,100);

                        else
                                g.drawString("You clicked on the right side!",60,100);
                }
        }
}

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