Repaint Applets in JAVA without losing previous contents

Question

Is it possible to re-paint an applet without losing its previous contents? I was just trying to make a program which allows users to draw lines, Rectangle etc. using mouse.

Answer 1

You need to keep track of everything that has been painted and then repaint everything again.

See Custom Painting Approaches for the two common ways to do this:

Use a ArrayList to keep track of objects painted Use a BufferedImage

here is an exemple of code you can use :

ArrayList<Point> points = new ArrayList<Point>();
private void draw(Graphics g){
    for (Point p: this.points){
            g.fillOval(1, 2, 2, 2);
    }
}

//after defining this function you add this to your paint function :
draw(g)
g.drawRect(x1, y1, x2-x1, y2-y1);
points.add(new Point(x1, y1, x2-x1, y2-y1))
// PS : point is a class I created to refer to a point, but you can use whatever

Answer 2

Two possible solutions:

1. Draw to a BufferedImage using the Graphics object obtained from it via getGraphics(), and then draw the BufferedImage in your JPanel's paintComponent(Graphics g) method. Or
2. Create an ArrayList<Point> place your mouse points into the List, and then in the JPanel's paintComponent(Graphics g) method, iterate through the List with a for loop, drawing all the points, or sometimes better -- lines that connect the contiguous points.

Other important suggestions:

• Be sure that you're using the Swing library (JApplet, JPanel), not AWT (Applet, Panel, Canvas).
• You're better off avoiding applets if at all possible.
• Don't draw in a paint method but rather a JPanel's paintComponent(Graphics g) method.
• Don't forget to call the super's method first thing in your paintComponent(Graphics g) method override.