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.
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//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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