java - 绘制一个大小可变的填充矩形

Question

我正在尝试编写一段代码，在拖动鼠标时绘制一个大小可变的填充矩形。当我运行这段代码时，它给我一个空白窗口。每当我按下并拖动时它都不会绘制任何东西。这段代码有什么问题？

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

从显而易见的事情开始

• JFrame 没有 paintComponent 方法(您正在调用 super.paintComponents <- 请注意“s”，这会通常是个坏主意
• 您直接向框架添加一个 MouseListener，如果其上方的子组件也注册了一个 MouseListener，则该框架可能不会收到鼠标事件通知，此外，您还想添加一个 MouseMotionListener

建议:

• 从自定义组件开始，从 JPanel 等扩展，覆盖它的 paintComponent 方法，确保添加 @Override 注释(如果您尝试重写父类未实现的方法并调用 super.paintComponent
，则会引发编译器错误
• 向其添加 MouseListener 和 MouseMotionListener。
• 将自定义组件的实例添加到 JFrame 的实例