如何完全重新绘制 Java 框架的画布？

Question

I am using AWT in Java to create animated frame. It is composed of a Frame and a Canvas , as well as some other components. The code basically is:

public class MyFrame extends Frame implements Runnable{

     private Canvas c = new Canvas();
     public  MyFrame(){

        c.setSize(100, 100);
        this.setSize(100, 100);
        this.add(c);
     }

}

Now I want to draw a part of an image on the canvas. I tried using the frame's paint method:

public void paint(Graphics g){

     Graphics canvasG = c.getGraphics();
     canvasG.drawImage(img, x1, y1, w1, h1, x2, y2, w2, h2, null);

{

This draws the part of the picture, over the part that has been drawn previously. I, however, want to repaint the canvas completely.

Answer 1

Wow. To answer your question:

• Don't draw in the class that extends Frame but rather in the paint method of a class that extends Canvas. In other words, create a class called MyCanvas that extends Canvas and give it a paint method override.
• Create an object of this class and add it to the Frame so it can be displayed.
• The first call in your Canvas class's paint method override should be the super method, super.paint(g) . This will redraw the Canvas erasing things that need to be erased.
• Don't use getGraphics to get a Graphics object from a component since the object obtained will not persist causing bad side effects (try this and then minimize and restore your GUI to see what I mean).
• Instead use the Graphics object obtained from the JVM and passed into the paint method.
• The exception to this is when working with BufferedImages. For them, yeah, go ahead and get the graphics object via getGraphics() when you need, it, but just don't forget to dispose of it when you're done so as not to run out of system resources.

Have fun.