JFrame绘图不正确

Question

我想要一个简单的程序，用滑块改变矩形的宽度。 当我运行这个jpanel不能正常工作，因为它只适合panel.width宽度，它不能正确重绘。

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

public class panel extends JPanel { 

    private int width = 50; 

    public void paintComponent(Graphics g) { 
     super.paintComponent(g); 
     g.setColor(Color.GRAY); 
     g.fillRect(20, 20, width, 25); 
    } 

    public void setWidth(int width) { 
     this.width = (width > 0) ? width : 0; 
     repaint(); 
    } 

    public int getWidth() { 
     return width; 
    } 
} 

GUI：

import java.awt.*; 

import javax.swing.*; 
import javax.swing.event.*; 

public class Gui extends JFrame { 

    private panel p; 
    private JSlider slider; 

    Gui() { 
     super("Draw program"); 
     setDefaultCloseOperation(EXIT_ON_CLOSE); 
     p = new panel(); 
     p.setBackground(Color.RED); 
     slider = new JSlider(SwingConstants.HORIZONTAL, 0, 300, p.getWidth()); 
     slider.setMajorTickSpacing(50); 
     slider.setPaintTicks(true); 
     add(p); 
     add(slider, BorderLayout.SOUTH); 
     slider.addChangeListener(new ChangeListener() { 
      public void stateChanged(ChangeEvent e) { 
       p.setWidth(slider.getValue()); 
      } 
     }); 
    } 
} 

Answer 1

你的面板类覆盖Component类（其的JPanel从继承）的一个密钥的方法：的getWidth。

问题在于，Swing布局管理器可能会使用此方法中的值来调整组件大小。

解决方案：更改方法名称。

例如，

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

public class Gui extends JFrame { 

    private MyPanel p; 
    private JSlider slider; 

    Gui() { 
     super("Draw program"); 
     setDefaultCloseOperation(EXIT_ON_CLOSE); 
     p = new MyPanel(); 
     p.setBackground(Color.RED); 
     slider = new JSlider(SwingConstants.HORIZONTAL, 0, MyPanel.GRAY_MAX_LENGTH, p.getGrayWidth()); 
     slider.setMajorTickSpacing(50); 
     slider.setPaintTicks(true); 
     add(p); 
     add(slider, BorderLayout.SOUTH); 
     slider.addChangeListener(new ChangeListener() { 
     public void stateChanged(ChangeEvent e) { 
      p.setGrayWidth(slider.getValue()); 
     } 
     }); 
    } 

    // so we can test our code 
    public static void main(String[] args) { 
     SwingUtilities.invokeLater(new Runnable() { 
     public void run() { 
      Gui gui = new Gui(); 
      gui.pack(); 
      gui.setLocationRelativeTo(null); 
      gui.setVisible(true); 
     } 
     }); 
    } 
} 

// note that this class should not be named 
// panel as class names should start with an upper-case letter 
class MyPanel extends JPanel { 
    // constants to avoid use of "magic" numbers 
    public static final int GRAY_MAX_LENGTH = 300; 
    private static final int RECT_HEIGHT = 25; 
    private static final int GAP = 20; 
    private static final int PREF_W = GRAY_MAX_LENGTH + 2 * GAP; 
    private static final int PREF_H = RECT_HEIGHT + 2 * GAP; 
    private int grayWidth = 50; 

    @Override 
    protected void paintComponent(Graphics g) { 
     super.paintComponent(g); 
     g.setColor(Color.GRAY); 
     g.fillRect(GAP, GAP, grayWidth, RECT_HEIGHT); 
    } 

    @Override // so our GUI is big enough 
    public Dimension getPreferredSize() { 
     if (isPreferredSizeSet()) { 
     return super.getPreferredSize(); 
     } 
     return new Dimension(PREF_W, PREF_H); 
    } 

    public void setGrayWidth(int width) { 
     this.grayWidth = (width > 0) ? width : 0; 
     repaint(); 
    } 

    // ***** key is to change this method's name so you don't 
    // override JPanel's getWidth() method 
    public int getGrayWidth() { 
     return grayWidth; 
    } 
}