繪制一個可變大小的填充矩形

Question

我正在嘗試編寫一個代碼，該代碼在拖動鼠標時繪制大小可變的填充矩形。 當我運行此代碼時，它會給我一個空白窗口。 每當我按下並拖動時，它都不會繪制任何內容。 這段代碼有什么問題？

import java.awt.Color;
import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.Point;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.awt.geom.Rectangle2D;

public class pr2 extends JFrame implements MouseListener, MouseMotionListener 
{
    Container cp;

    Point p1;
    Point p2;

    Rectangle2D rectangle;

    public pr2 (String Name)
    {
        super (Name);
        setLayout(new FlowLayout ());
        setBackground(Color.LIGHT_GRAY);
        setSize(500, 500);
        setVisible(true);
        setResizable(false);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        cp = getContentPane ();

        addMouseListener(this);
     }

    public boolean isPointTwoInQuadOne(Point p1, Point p2) 
    {
       return p1.x >= p2.x && p1.y >= p2.y;
    }

    public boolean isPointTwoInQuadTwo (Point p1, Point p2) 
    {
        return p1.x <= p2.x && p1.y >= p2.y;
    }

    public boolean isPointTwoInQuadThree(Point p1, Point p2) 
    {
        return p1.x <= p2.x && p1.y <= p2.y;
    }

    public boolean isPointTwoInQuadFour (Point p1, Point p2) 
    {
        return p1.x >= p2.x && p1.y <= p2.y;
    }

    public void paintComponent(Graphics g) 
    {
         g.setColor(Color.BLACK);
         super.paintComponents(g);
         Graphics2D g2 = (Graphics2D)g;
         if (rectangle != null) 
         {
             g2.fill(rectangle);
         }
     }

     @Override
     public void mousePressed(MouseEvent e) 
     {
         p1 = e.getPoint();
         rectangle = new Rectangle2D.Double(p1.x, p1.y, p1.x - p1.x, p1.y - p1.y);
     }

    @Override
    public void mouseDragged(MouseEvent e) 
    {
          p2 = e.getPoint();
          if (isPointTwoInQuadOne(p1, p2)) 
          { 
                rectangle.setRect(p2.x, p2.y, p2.x, p1.y);
                repaint();
          } 
          else if (isPointTwoInQuadTwo(p1, p2))
          {
                rectangle.setRect(p1.x, p2.y, p2.x - p1.x, p1.y - p2.y);
                repaint();
          }
          else if (isPointTwoInQuadThree(p1, p2))
          {
                rectangle.setRect(p1.x, p1.y, p2.x - p1.x, p2.y - p1.y);
                repaint();
          }
          else if (isPointTwoInQuadFour(p1, p2))
          {
                rectangle.setRect(p2.x, p1.y, p1.x, p2.y);
                repaint();
          }   
      }

      @Override
      public void mouseClicked(MouseEvent e) { }

      @Override
      public void mouseReleased(MouseEvent e) { }

      @Override
      public void mouseEntered(MouseEvent e) { }

      @Override
      public void mouseExited(MouseEvent e) { }

      @Override
      public void mouseMoved(MouseEvent e) { }
}

Answer 1

從顯而易見的開始

• JFrame沒有paintComponent方法（您正在調用super.paintComponents <-注意's'，這通常是一個壞主意
• 您直接將MouseListener添加到框架中，如果其上方的子組件也已注冊有MouseListener ，則可能不會收到鼠標事件的通知，此外，您還想添加MouseMotionListener

意見建議：

• 從一個自定義組件開始，從諸如JPanel擴展，覆蓋它的paintComponent方法，確保添加了@Override注解（如果您嘗試覆蓋一個未由父類實現的方法，則會引發編譯器錯誤）並調用super.paintComponent
• 向其添加一個MouseListener和MouseMotionListener 。
• 將自定義組件的實例添加到JFrame的實例