JPanel重绘问题

Question

我有一个JFrame在BorderLayout中包含2个JPanel子类和2个JLabel。其中一个JPanel包含JButton，另一个用于显示图形。 JLabels位于北部和南部，西部的按钮JPanel和中央的显示器JPanel。

显示器JPanel需要不断刷新，所以我通过swing定时器生成的动作事件调用repaint（）方法。我也重写它的paintComponent（）方法来完成我的绘图。

而不是显示我画的内容，“JFrame的内容”正被绘制到显示器JPanel上。我知道在执行绘图之前，我可以通过使用g.fillRect（）或super.paintComponent（）来简单地“清除”显示JPanel。

我只是好奇为什么会发生这种情况。

我使用jdk 1.6u27。下面是我的代码：

package test; 

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

public class Main { 

public static void main(String[] args) { 
    Simulation sim = new Simulation(); 

    } 
} 

class Simulation extends JFrame { 

    public JLabel state; 
    private JLabel id; 
    private ButtonPanel control; 
    private Display display; 

    public Simulation() { 
     id = new JLabel("Test"); 
     state = new JLabel("Test"); 
     control = new ButtonPanel(); 
     display = new Display(this); 

     this.setLayout(new BorderLayout()); 
     this.add(id, BorderLayout.NORTH); 
     this.add(control, BorderLayout.WEST); 
     this.add(display, BorderLayout.CENTER); 
     this.add(state, BorderLayout.SOUTH); 

     this.setSize(500, 600); 
     this.setVisible(true); 
     this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 

    } 

    public ButtonPanel getControl() { 
     return this.control; 
    } 
} 

class ButtonPanel extends JPanel implements ActionListener { 

    public JButton b[] = new JButton[8]; 
    public boolean bp[] = new boolean[8]; 

    public ButtonPanel() { 
     this.setLayout(new GridLayout(8, 1)); 

     for (int i = 0; i < b.length; i++) { 
      b[i] = new JButton(""+i); 
      b[i].addActionListener(this); 
      bp[i] = false; 
      this.add(b[i]); 
     } 
    } 

    public void actionPerformed(ActionEvent e) { 
     //do something 
    } 
} 

class Display extends JPanel implements ActionListener { 

    private Timer tm; 
    private int yco; 
    private Simulation sim; 

    public Display(Simulation sim) { 
     tm = new Timer(100, this); 
     tm.start(); 

     yco = 0; 

     this.sim = sim; 
    } 

    @Override 
    public void paintComponent(Graphics g) { 
     //draw something 
     g.drawLine(0, yco, 100, 100); 
    } 

    public void actionPerformed(ActionEvent e) { 
     yco ++; 
     this.repaint(); 
    } 
} 

Answer 1

没有super.paintComponent(g)，结果取决于你的平台默认为JPanel UI委托，PanelUI的opacity财产。我碰巧是true，但您可以在您的平台上进行试验，如下所示。

附录：“如果您不尊重opaque property，您可能会看到视觉artifacts。” - paintComponent()。您观察到的artifact将因平台而异，但它不是非典型的。实际上，您违背了绘制每个像素的承诺，并且您会看到某些缓冲区中剩下的内容。

import java.awt.*; 
import java.awt.event.*; 
import java.util.ArrayList; 
import java.util.List; 
import javax.swing.*; 

public class Main { 

    public static void main(String[] args) { 
     EventQueue.invokeLater(new Runnable() { 

      @Override 
      public void run() { 
       Simulation sim = new Simulation(); 
      } 
     }); 
    } 
} 

class Simulation extends JFrame { 

    public JCheckBox state; 
    private JLabel id; 
    private ButtonPanel control; 
    private Display display; 

    public Simulation() { 
     id = new JLabel("Test"); 
     state = new JCheckBox("Opaque"); 
     control = new ButtonPanel(); 
     display = new Display(this); 

     this.setLayout(new BorderLayout()); 
     this.add(id, BorderLayout.NORTH); 
     this.add(control, BorderLayout.WEST); 
     this.add(display, BorderLayout.CENTER); 
     this.add(state, BorderLayout.SOUTH); 
     state.addItemListener(new ItemListener() { 

      @Override 
      public void itemStateChanged(ItemEvent e) { 
       display.setOpaque(e.getStateChange() == ItemEvent.SELECTED); 
      } 
     }); 
     state.setSelected(true); 

     this.pack(); 
     this.setVisible(true); 
     this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    } 

    public ButtonPanel getControl() { 
     return this.control; 
    } 
} 

class ButtonPanel extends JPanel { 

    private static final int N = 8; 
    private List<JToggleButton> list = new ArrayList<JToggleButton>(N); 

    public ButtonPanel() { 
     this.setLayout(new GridLayout(0, 1)); 
     for (int i = 0; i < N; i++) { 
      final JToggleButton b = new JToggleButton(String.valueOf(i)); 
      b.addActionListener(new ActionListener() { 

       @Override 
       public void actionPerformed(ActionEvent e) { 
        //System.out.println(b.isSelected()); 
       } 
      }); 
      list.add(b); 
      this.add(b); 
     } 
    } 
} 

class Display extends JPanel { 

    private Simulation sim; 
    private Timer tm; 
    private int yco; 

    public Display(Simulation sim) { 
     this.setPreferredSize(new Dimension(320, 320)); 
     this.setOpaque(true); 
     this.sim = sim; 
     tm = new Timer(100, new ActionListener() { 

      @Override 
      public void actionPerformed(ActionEvent e) { 
       yco++; 
       repaint(); 
      } 
     }); 
     tm.start(); 
    } 

    @Override 
    public void paintComponent(Graphics g) { 
     //super.paintComponent(g); 
     g.drawLine(0, yco, getWidth()/2, getHeight()/2); 
    } 
}