java/Swing問題與paintComponent

Question

我遇到的問題是問題是我想讓paintComponent只有當鼠標被點擊，拖動，然後鬆開才能繪製圓。然而，在我的paintPanel類中，我必須初始化我創建的對象（例如movedCircle myCircle = new movedCircle（0,0,0,0）;）只是創建對象movedCircle myCircle;直到我實際上用一個值完全初始化對象時纔給出錯誤。

我在尋找什麼： 什麼被認爲是這個問題的最佳實踐。我不想在需要之前畫出任何不必要的東西。

我知道如何解決這個問題的方法： paintComponent內部的布爾值，這樣它就不會畫到實際存在的東西。

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

public class drawCircle extends JFrame{ 
    private JPanel myPanel = new paintPanel(); 

    public drawCircle(){ 
     add(myPanel); 
    } 

    private class paintPanel extends JPanel{ 
     private int x1, y1, x2, y2; 
     movedText myText = new movedText(0,0,0,0); 
     movedCircle myCircle = new movedCircle(0,0,0,0); 

     public paintPanel(){ 
      addMouseListener(new MouseAdapter(){ 
       public void mousePressed(MouseEvent e){ 
        x1 = e.getX(); 
        y1 = e.getY(); 
        myCircle = new movedCircle(x1, y1, 0, 0); 
        repaint(); 
       } 
       public void mouseReleased(MouseEvent e){ 
        x2 = e.getX(); 
        y2 = e.getY(); 
        myCircle = new movedCircle(x1, y1, x2, y2); 
        repaint(); 
       } 
      }); 

      addMouseMotionListener(new MouseMotionAdapter(){ 
       public void mouseDragged(MouseEvent e){ 
        x2 = e.getX(); 
        y2 = e.getY(); 
        myText = new movedText(x1, y1, x2, y2); 
        myCircle = new movedCircle(x1, y1, x2, y2); 
        repaint(); 
       } 
       public void mouseMoved(MouseEvent e){ 
        x1 = e.getX(); 
        y1 = e.getY(); 
        x2 = 0; 
        y2 = 0; 

        myText = new movedText(x1, y1, x2, y2); 
        repaint(); 
       } 
      }); 
     } 

     protected void paintComponent(Graphics g){ 
      super.paintComponent(g); 
      //draw oval after mouse released 
      myText.paintText(g); 
      myCircle.paintCircle(g); 
     } 
    } 

    class movedCircle{ 
     private int x1, y1, x2, y2; 

     public movedCircle(int x1, int y1, int x2, int y2){ 
      this.x1 = x1; 
      this.y1 = y1; 
      this.x2 = x2; 
      this.y2 = y2; 
     } 

     public void paintCircle(Graphics g){ 
      g.drawOval(x1, y1, x2 - x1, y2 - y1); 
     } 
    } 
    class movedText{ 
     private int x1, y1, x2, y2; 

     public movedText(int x1, int y1, int x2, int y2){ 
      this.x1 = x1; 
      this.y1 = y1; 
      this.x2 = x2; 
      this.y2 = y2; 
     } 

     public void paintText(Graphics g){ 
      g.drawString("x1: "+x1+" y1: "+y1+" x2: "+x2+" y2: "+y2, x1, y1); 
     } 
    } 

    class RedSquare{ 
     private int xPos = 50; 
     private int yPos = 50; 
     private int width = 20; 
     private int height = 20; 

     public void setX(int xPos){ 
      this.xPos = xPos; 
     } 

     public int getX(){ 
      return xPos; 
     } 

     public void setY(int yPos){ 
      this.yPos = yPos; 
     } 

     public int getY(){ 
      return yPos; 
     } 

     public int getWidth(){ 
      return width; 
     } 

     public int getHeight(){ 
      return height; 
     } 

     public void paintSquare(Graphics g){ 
      g.setColor(Color.RED); 
      g.fillRect(xPos,yPos,width,height); 
      g.setColor(Color.BLACK); 
      g.drawRect(xPos,yPos,width,height); 
     } 
    } 

    public static void main(String[] args){ 
     JFrame frame = new drawCircle(); 

     frame.setTitle("Is in ellipse? Demo"); 
     frame.setSize(400, 400); 
     frame.setLocationRelativeTo(null); 
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     frame.setVisible(true); 
    } 
} 

Answer 1

我會去與if s，但問題是在哪裏。

1. 你可以將它們添加到paintPanel.paintComponent()
2. 你可以將它們添加到movedCircle.paint()和不畫任何東西，如果它的座標是假的（例如< 0）。與movedText相同

或者，將您的數字放在合理的起始位置。

（還有一個解決辦法：子類movedCircle與nullMovedCircle不畫任何東西，創造，起初在paintPanel然而，對於行爲的這樣一個小的改變創造新的類對我來說似乎矯枉過正。）

Answer 2

不知道我是否錯過了這個觀點，但如果你不想畫出一些東西，直到它被初始化，那麼不要！你能不能僅僅檢查它是否已經被創建，而不是在構造器完成之後創建一個不處於可用狀態的新實例？例如

super.paintComponent(g); 
myText.paintText(g); 
if (myCircle != null) { 
    myCircle.paintCircle(g); 
}