Java在调用paint方法时清除屏幕 - 如何避免这种情况？

Question

我试图在Java画布中画两条线，分别调用两个方法，但是当我绘制第二条线时，第一条线消失（Java清除屏幕）。我怎样才能避免这种情况？我想看到两条线。我见过绘画教程（如何制作像Windows上的Paint一样的程序），用户使用鼠标绘制线条，当绘制一条线时，另一条线不会消失。他们只是调用paint方法，并没有清除屏幕。

如果有人能帮助我，我将不胜感激。 谢谢。

视图类

import java.awt.BorderLayout; 
import java.awt.Canvas; 
import java.awt.Color; 
import java.awt.Graphics; 

import javax.swing.JFrame; 

public class CircuitTracePlotView extends JFrame { 


    private CircuitTracePlot circuitTracePlot; 

    public CircuitTracePlotView() { 


     this.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE); 
     this.getContentPane().add(circuitTracePlot = new CircuitTracePlot(), BorderLayout.CENTER); 
     this.pack(); 
     this.setSize(250,250); 
     this.setLocationRelativeTo(null); 

     this.setVisible(true); 
     circuitTracePlot.drawLine(); 
     circuitTracePlot.drawOval(); 
    } 


} 

class CircuitTracePlot extends Canvas { 

    private final static short LINE = 1; 
    private final static short OVAL = 2; 
    private int paintType; 

    private int x1; 
    private int y1; 
    private int x2; 
    private int y2; 

    public CircuitTracePlot() { 
     this.setSize(250,250); 
     this.setBackground(Color.WHITE); 

    } 

    private void setPaintType(int paintType) { 
     this.paintType = paintType; 
    } 

    private int getPaintType() { 
     return this.paintType; 
    } 

    public void drawLine() { 
     this.setPaintType(LINE); 
     this.paint(this.getGraphics()); 
    } 

    public void drawOval() { 
     this.setPaintType(OVAL); 
     this.paint(this.getGraphics()); 
    } 

    public void repaint() { 
     this.update(this.getGraphics()); 
    } 

    public void update(Graphics g) { 
     this.paint(g); 
    } 

    public void paint(Graphics g) { 
     switch (paintType) { 
     case LINE: 
      this.getGraphics().drawLine(10, 10, 30, 30);    
     case OVAL: 
      this.getGraphics().drawLine(10, 20, 30, 30); 
     } 


    } 


} 

主要类

import javax.swing.SwingUtilities; 

import view.CircuitTracePlotView; 

public class Main { 

    public static void main(String[] args) { 
     SwingUtilities.invokeLater(new Runnable() { 
      public void run() { 
       CircuitTracePlotView cr = new CircuitTracePlotView(); 
      } 
     }); 
    } 
} 

Answer 1

• 你几乎永远不应该直接调用paint(...)。我一方面可以计算出我需要这样做的时间。
• 不要通过在组件上调用getGraphics()来获得Graphics对象，因为这将返回非持久Graphics对象。相反，可以绘制一个BufferedImage并在绘制方法中显示或者绘制绘制方法（如果是AWT）。
• 由于这是一个Swing GUI，请不要使用AWT组件来绘制。使用JPanel并覆盖paintComponent(...)方法，而不是paint(...)方法。否则，您将失去Swing图形的所有优点，包括自动双缓冲。
• super.paintComponent(g)方法应在paintComponent(Graphics g)重写中调用，通常作为此方法内的第一个方法调用。这让组件可以做自己的家务绘画，包括擦除需要擦除的图纸。
• 阅读有关Swing图形的教程，因为大部分内容在这里都有很好的解释。对于例如，请看看这里：
  • Lesson: Performing Custom Painting
  • Painting in AWT and Swing

编辑

• 为了使您的图片坚持，我建议你绘制到一个BufferedImage然后在您的JPanel的paintComponent(...)方法中显示该图像。
• 或者另一个选项是创建一个Shape对象集合，也许是一个ArrayList<Shape>并用它填充想要绘制的图形，然后在paintComponent(...)方法中将Graphics对象转换为Graphics2D对象并迭代Shape在迭代过程中收集每个形状的图形g2d.draw(shape)。

由于垃圾张贴了他的代码，...

import java.awt.Dimension; 
import java.awt.Graphics; 
import java.awt.event.ActionEvent; 
import java.awt.event.ActionListener; 

import javax.swing.*; 

public class CircuitTracePlot2 extends JPanel { 

    private static final int PREF_W = 250; 
    private static final int PREF_H = PREF_W; 

    private int drawWidth = 160; 
    private int drawHeight = drawWidth; 
    private int drawX = 10; 
    private int drawY = 10; 
    private PaintType paintType = PaintType.LINE; 

    public CircuitTracePlot2() { 

    } 

    @Override 
    public Dimension getPreferredSize() { 
     return new Dimension(PREF_W, PREF_H); 
    } 

    public void setPaintType(PaintType paintType) { 
     this.paintType = paintType; 
     repaint(); 
    } 

    @Override 
    protected void paintComponent(Graphics g) { 
     super.paintComponent(g); 
     if (paintType == null) { 
     return; 
     } 
     switch (paintType) { 
     case LINE: 
     g.drawLine(drawX, drawY, drawWidth, drawHeight); 
     break; 
     case OVAL: 
     g.drawOval(drawX, drawY, drawWidth, drawHeight); 
     break; 
     case SQUARE: 
     g.drawRect(drawX, drawY, drawWidth, drawHeight); 

     default: 
     break; 
     } 
    } 

    private static void createAndShowGui() { 
     final CircuitTracePlot2 circuitTracePlot = new CircuitTracePlot2(); 

     JFrame frame = new JFrame("CircuitTracePlot2"); 
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     frame.getContentPane().add(circuitTracePlot); 
     frame.pack(); 
     frame.setLocationByPlatform(true); 
     frame.setVisible(true); 

     int timerDelay = 2 * 1000; 
     new Timer(timerDelay , new ActionListener() { 
     private int paintTypeIndex = 0; 

     @Override 
     public void actionPerformed(ActionEvent arg0) { 
      paintTypeIndex++; 
      paintTypeIndex %= PaintType.values().length; 
      circuitTracePlot.setPaintType(PaintType.values()[paintTypeIndex]); 
     } 
     }).start(); 
    } 

    public static void main(String[] args) { 
     SwingUtilities.invokeLater(new Runnable() { 
     public void run() { 
      createAndShowGui(); 
     } 
     }); 
    } 
} 

enum PaintType { 
    LINE, OVAL, SQUARE; 
} 

Answer 2

下面是实现大部分@气垫船的有益的建议对你的程序的变化。尝试注释拨打setPaintType()的电话以查看效果。

import java.awt.BorderLayout; 
import java.awt.Color; 
import java.awt.Dimension; 
import java.awt.Graphics; 
import javax.swing.JFrame; 
import javax.swing.JPanel; 
import javax.swing.SwingUtilities; 

/** @see http://stackoverflow.com/a/15854246/230513 */ 
public class Main { 

    public static void main(String[] args) { 
     SwingUtilities.invokeLater(new Runnable() { 
      @Override 
      public void run() { 
       CircuitTracePlotView cr = new CircuitTracePlotView(); 
      } 
     }); 
    } 

    private static class CircuitTracePlotView extends JFrame { 

     private CircuitTracePlot plot = new CircuitTracePlot(); 

     public CircuitTracePlotView() { 
      this.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE); 
      plot.setPaintType(CircuitTracePlot.OVAL); 
      this.add(plot, BorderLayout.CENTER); 
      this.pack(); 
      this.setLocationRelativeTo(null); 
      this.setVisible(true); 
     } 
    } 

    private static class CircuitTracePlot extends JPanel { 

     public final static short LINE = 1; 
     public final static short OVAL = 2; 
     private int paintType; 

     public CircuitTracePlot() { 
      this.setBackground(Color.WHITE); 
     } 

     public void setPaintType(int paintType) { 
      this.paintType = paintType; 
     } 

     @Override 
     protected void paintComponent(Graphics g) { 
      super.paintComponent(g); 
      switch (paintType) { 
       case LINE: 
        g.drawLine(10, 10, 30, 30); 
       case OVAL: 
        g.drawOval(10, 20, 30, 30); 
       default: 
        g.drawString("Huh?", 5, 16); 
      } 
     } 

     @Override 
     public Dimension getPreferredSize() { 
      return new Dimension(250, 250); 
     } 
    } 
}