为什么我的 Java repaint() 方法不起作用？

Question

Heyo, for my college homework I need to draw some images and race them, but I am stuck even at drawing the images (or in this case, icons). I want to draw ANYTHING (that's why I have the drawLine method, just to test it out) as of right now on one of many JPanels, but my repaint() method does not call my paintComponent method, why?

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


public class Races{
   private int numberOfRacers;

   public Races(int numberOfRacers){
      this.numberOfRacers = numberOfRacers;
      JFrame frame = new JFrame("Races - Name Surname");

      Icon icon = new ImageIcon("races.jpg");
      frame.setLayout(new GridLayout(numberOfRacers, 1));
      frame.setSize(icon.getIconWidth()*20, (icon.getIconWidth()*2)*numberOfRacers);
      frame.setVisible(true);
      frame.setLocationRelativeTo(null);
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

      try{
         Thread.sleep(1000);  // Sleeping 1 sec 
         System.out.println("Sleeping one second for the user!");
      }catch(InterruptedException ie){
         System.out.println(ie);
      }


      for(int i = 0; i < numberOfRacers; i++){
         innerRacer racer = new innerRacer();
         frame.add(racer.panel);
         //JLabel iconLabel = new JLabel(icon);
         //iconLabel.setHorizontalAlignment(JLabel.LEFT);
         //panel.add(iconLabel);
         Thread t = new Thread(racer);
         t.start();

      }
   }

   public static void main(String args[]){

      if(args.length > 0 && Integer.parseInt(args[0]) > 0 && Integer.parseInt(args[0]) < 100){
         Races races = new Races(Integer.parseInt(args[0]));   // From command line number of racers
         System.out.println("Number of racers: " + args[0]);
      }else{
         Races races = new Races(5);      // Default number of racers
         System.out.println("Number of racers: 5");
      }
   }

   public class innerRacer extends JPanel implements Runnable{ 
      JPanel panel;
      Icon icon;
      public innerRacer(){
         panel = new JPanel();
         panel.setBackground(Color.WHITE);

         icon = new ImageIcon("races.jpg");
      } //end of innerRacer constructor

      @Override
         public void run(){
            repaint();     
            System.out.println("TEST");  
         }
     @Override
     public void paintComponent(Graphics g){
         super.paintComponent(g);
         g.drawLine(10, 20, 30, 40);
         System.out.println("Why is this one not called?");
         icon.paintIcon(panel, g, 0, 0);
     } 

   } // end of innerRacer class
} //end of Races class

Thanks in advance.

Answer 1

You've added the JPanel that innerRacer contains rather than the JPanel that innerRacer is.

Change:

     frame.add(racer.panel);

to:

     frame.add(racer);

Also I see you are adding components to the frame after setting it to visible. When you do this in AWT/Swing they wont get automatically laid out or painted. You will need to follow with the line:

  frame.revalidate(); 

Alternatively, the setVisible line could be moved down.

As general notes:

• It's a good idea to stick to Java naming conventions - always initial caps for types.
• JPanel panel; in innerRacer can be deleted.
• Swing components should always be accessed from the AWT Event Dispatch Thread (EDT). Use java.awt.EventQueue.invokeLater .
• Lots of tutorials suggest overriding JPanel without adding any components if you want to paint anything. This appears to be because it is opaque by default. Neither the API docs for JPanel nor even the OpenJDK source code mention opaqueness. Indeed, it isn't guaranteed. It's just a hack. Prefer setOpaque(true) to make a component opaque.
• Having a class extend another and implement an interface (or implement multiple interfaces (not markers, Comparable , etc)) isn't great. Use a lambda method, method reference, inner class or just another outer class as appropriate.