Fonts and Graphics
Flicker Free Graphics, .GIF and .JPEG Display
You will have observed the flickering whenever we have changed the content of the applet before. To avoid this, you need to implement a technique called "double buffering". We have to create a Graphics object inside the memory to which we do all our drawing now. Inside the paint()-method we copy the memory bitmap to the screen then. Just follow the code of this applet, you don't have to understand everything yet to use this technique.
This applet uses a boolean variable (moveRight) to keep track of the direction, the pink square is going. With if and else it is determined whether the condition is true to continue movement in the previous direction, or whether it is time to change the direction.
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//Sourcecode
import java.awt.*;
import java.applet.*;
//a flicker free animation!
public class Project9 extends Applet implements Runnable
{
Thread runner;
private Image Buffer;
private Graphics gBuffer;
int x, x1, x2=100;
boolean moveRight=true;
//Init is called first, do any initialisation here
public void init()
{
//create graphics buffer, the size of the applet
Buffer=createImage(size().width,size().height);
gBuffer=Buffer.getGraphics();
}
public void start()
{
if (runner == null)
{
runner = new Thread (this);
runner.start();
}
}
public void stop()
{
if (runner != null)
{
runner.stop();
runner = null;
}
}
public void run()
{
while(true)
{
//Thread sleeps for 15 milliseconds here
try {runner.sleep(15);}
catch (Exception e) { }
//paint background blue
gBuffer.setColor(Color.blue);
gBuffer.fillRect(0,0,size().width,size().height);
//move circle (slowly) right until it leaves applet
//then let it appear again on the left side
x++;
if(x>size().width)
x=-50;
//move square left (with double speed) until it
//leaves applet then let it reappear right side
x1-=2;
if(x1<-50)
x1=size().width;
//move the pink square right until it touches
//border, then set flag "moveRight" to false
if(moveRight==true)
{
if(x2<size().width-50)
x2+=2;
else
moveRight=false;
}
//if flag "moveRight" is false, the pink square moves left
//until it touches left border, then switch direction
if(moveRight==false)
{
if(x2>0)
x2-=2;
else
moveRight=true;
}
//the red circle
gBuffer.setColor(Color.red);
gBuffer.fillOval(x,30,50,50);
//the green square
gBuffer.setColor(Color.green);
gBuffer.fillRect(x1,110,50,50);
//the pink square
gBuffer.setColor(Color.pink);
gBuffer.fillRoundRect(x2,200,50,50,16,16);
repaint();
}
}
//is needed to avoid erasing the background by Java
public void update(Graphics g)
{
paint(g);
}
public void paint(Graphics g)
{
g.drawImage (Buffer,0,0, this);
}
}
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