The Java Boutique: Java Tutorial: Java by Example: Java Basics

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Section One Contents

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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