为什么会出现这种输出？

Question

I am following a tutorial and the output window prints "Hello World!" there. My question is, of the paint() method is not called from anywhere, then how does it print "Hello World!" ?

Here is the code ..

package javagame;

import java.awt.Graphics;

import javax.swing.JFrame;


public class JavaGame extends JFrame{

    public JavaGame(){

        setTitle("sadid java game");    
        setSize(500, 500);
        setResizable(false);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 

    }

    public void paint(Graphics g){
        g.drawString("Hello world !!!!",30,30);
    }

    public static void main(String[] args) {
        new JavaGame();

    }

}

In the tutorial it prints normally the window with "Hello World!" but when I run the same code this horrible output appears. How can this kind of output appear and why? Is there any problem in the code..?

Answer 1

This is a Swing GUI, the posted class extends JFrame and overrides the paint method (something you should not do).

To answer your question, the paint method override of all Swing components are called automatically by the JVM whenever the component is rendered. The component is rendered when the application starts, if the component is visible, when it is resized, and when the operating system feels that the window with the component is "dirty" and needs to be repainted.

The component can also be painted if you suggest that it be repainted by calling repaint() on the component or on any container above it in its hierarchy. Note that I say "suggested" since you can only ask that the repaint manager paint the component but usually don't "demand" that it do so, so you can give the repaint manager the option of not repainting a region if repaint requests stack up.

So this means that paint method is never under your direct control and can be called many times or infrequently.

The reason that this code is not good code to follow is that:

• Their paint example does not call the super.paint(g) method, and so you're overriding a JFrame's paint method without telling it to do its normal painting.
• The paint method is responsible for drawing a component's children and borders, and for a JFrame this is super-important. By not calling the super method, you risk messing up these parts of the JFrame royally.
• The paint method does not do double buffering by default, and if you try to do animation with it, you are guaranteed to have choppy animation unless you double buffer manually.

So in sum, don't override a JFrame's paint method unless you are very sure about what you are doing (which the author of your video was not ) and know the risks involved. To draw correctly, check out the official Painting in Swing Tutorials , and follow their examples. For a more detailed explanation, please read Painting in AWT and Swing .

A better example of what the tutorial is trying to show would be:

import java.awt.*;
import javax.swing.*;

// draw in a JPanel, not in a JFrame
public class JavaGame2 extends JPanel {
   private static final int PREF_W = 500;
   private static final int PREF_H = PREF_W;

   public JavaGame2() {

   }

   // draw within the JPanel's paintComponent method
   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g); // be sure to first call the super's method
      g.drawString("Hello world !!!!", 30, 30);
   };

   // better to set size this way
   @Override
   public Dimension getPreferredSize() {
      return new Dimension(PREF_W, PREF_H);
   }

   private static void createAndShowGui() {
      JavaGame2 mainPanel = new JavaGame2();

      // no need to extend JFrame. Instead just use one when needed
      JFrame frame = new JFrame("sadid java game");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {

      // be sure to start your Swing GUI in a thread-safe way
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

Answer 2

First you have to tell it's parent that you want to draw something (super.paint(g)).I think it will work:

import java.awt.Graphics;

import javax.swing.JFrame;

public class JavaGame extends JFrame{

    public JavaGame(){

        super("sadid java game");   
        setSize(500, 500);
        setResizable(true);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 

    }

    public void paint(Graphics g){
        super.paint(g);
        g.drawString("Hello world !!!!",200,300);


    }

    public static void main(String[] args) {
        new JavaGame();

    }

}