JPanel上沒有重繪線條

Question

因此，我創建了一個程序，允許用戶在JPanel上繪製多個形狀，類似於繪畫程序。問題是，當我嘗試向存儲所有形狀的ArrayList添加一行，然後在JPanel上重新繪製它們時，該行將被存儲但不會重繪。

因此，例如，假設我從頂部的JMenu中選擇了線工具。然後，我點擊並拖動，並且從我點擊鼠標到當前位置的位置出現該線。我釋放鼠標，線路仍然存在。當我去繪製另一條線時，點擊並拖動，第一條線鬆開鼠標後消失。我怎樣才能得到它，這樣該行將保留在JPanel上？

import ... 

public class GraphicsTest { 

    public GraphicsTest() { 
     JFrame frame = new JFrame(""); 
     Painter painter = new Painter(); 
     frame.setLayout(new BorderLayout()); 
    // frame.setJMenuBar(painter.createMenuBar()); 
     frame.add(painter,BorderLayout.CENTER);   
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     frame.setSize(750,488); 
     frame.setVisible(true); 
    } 

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

    public class Painter extends JPanel implements MouseListener, MouseMotionListener { 
     private java.awt.geom.Line2D.Double line = new java.awt.geom.Line2D.Double(); 

     private List<Shape> shapeList = new ArrayList<Shape>(); 
     private List<Integer> opNumList = new ArrayList<Integer>(); 

     // Initial color 
     private Color color = Color.black; 

     // int variables that determine the position of various on-screen objects 
     private int dragX = 0, dragY = 0, downX = 0, downY = 0, upX = 0, upY = 0; 

     // Fields that determine which tool should be used 
     private int PEN = 0, LINE = 1, RECTANGLE = 2, OVAL = 3; 

     // Initial tool used 
     private int toolNum = LINE; 

     private Point clickPoint, releasePoint; 

     /** 
     * Sole and default constructor of this class 
     **/ 
     public Painter() { 
      // Adding listeners so that the mouse may be used to draw various objects 
      addMouseListener(this); 
      addMouseMotionListener(this); 
     } 

     @Override 
     protected void paintComponent(Graphics g) { 
      super.paintComponent(g); 
      int index = 0; 
      Graphics2D g2 = (Graphics2D) g.create(); 
      g2.setColor(color); 

      if (toolNum == LINE) { 
       g2.draw(new java.awt.geom.Line2D.Double(downX,downY,dragX,dragY)); 
      } 

      System.out.println("Size: " + shapeList.size()); 
      if (!opNumList.isEmpty()) { 
       for (Shape s : shapeList) { 
       // System.out.println("Index: " + index); 
       // System.out.println("Element: " + opNumList.get(index)); 
       // System.out.println(s); 
        System.out.println(); 
        switch (opNumList.get(index)) { 
         case 41: g2.setColor(Color.black); g2.draw(s); break; 
         default: return; 
        } 
        index++; 
       } 
      } 
     } 

     /** 
     * MouseListener interface methods 
     **/ 
     @Override 
     public void mouseClicked(MouseEvent ev) {} 

     @Override 
     public void mouseEntered(MouseEvent ev) {} 

     @Override 
     public void mouseExited(MouseEvent ev) {} 

     @Override 
     public void mousePressed(MouseEvent ev) { 
      clickPoint = new Point(ev.getPoint()); 
      upX = ev.getX(); 
      upY = ev.getY(); 
      downX = ev.getX(); 
      downY = ev.getY(); 
     } 

     @Override 
     public void mouseReleased(MouseEvent ev) { 
      upX = ev.getX(); 
      upY = ev.getY(); 
      releasePoint = new Point(ev.getPoint()); 

      line.x1 = clickPoint.x; 
      line.y1 = clickPoint.y; 
      line.x2 = releasePoint.x; 
      line.y2 = releasePoint.y; 

      if (toolNum == LINE) 
       shapeList.add(line); 

      if (color == Color.black) { 
       if (toolNum == LINE) 
        opNumList.add(41); 
      } 
      repaint(); 
     } 

     /** 
     * MouseMotionListener interface methods 
     **/ 
     @Override 
     public void mouseMoved(MouseEvent ev) {} 

     @Override 
     public void mouseDragged(MouseEvent ev) { 
      dragX = ev.getX(); 
      dragY = ev.getY(); 

      repaint(); 
     } 
    } 
} 

Answer 1

你永遠只能創建line單個實例...

private java.awt.geom.Line2D.Double line = new java.awt.geom.Line2D.Double(); 

你再更新的Line

line.x1 = clickPoint.x; 
line.y1 = clickPoint.y; 
line.x2 = releasePoint.x; 
line.y2 = releasePoint.y; 

這種情況下這意味着，在shapeList每個條目是相同的對象，具有相同的屬性。所以，是的，你的代碼在shapeList中繪製EVERY行，它只是多次繪製同一行...

相反，每次你想添加一個新行時，創建一個新的實例...

Line2D line = new java.awt.geom.Line2D.Double(); 
line.x1 = clickPoint.x; 
line.y1 = clickPoint.y; 
line.x2 = releasePoint.x; 
line.y2 = releasePoint.y; 

然後將其添加到shapeList ...