Java Swing绘图应用中图形重叠问题的根源与解决方案

public class Painter implements ActionListener, MouseListener, MouseMotionListener {
    // ...
    Point startPoint = new Point(); // 初始化的Point对象
    Point endPoint = new Point();   // 初始化的Point对象
    // ...
}

@Override
public void mousePressed(MouseEvent e) {
    startPoint.setLocation(e.getPoint()); // 更新现有startPoint对象的坐标
}

@Override
public void mouseReleased(MouseEvent e) {
    endPoint.setLocation(e.getPoint());   // 更新现有endPoint对象的坐标

    if (object == 0) {
        canvas.addPrimitive(new Line(startPoint, endPoint, temp)); // 将startPoint和endPoint传递给Line对象
    }
    // ...
}

public class Painter implements ActionListener, MouseListener, MouseMotionListener {
    // ...
    // startPoint 和 endPoint 仍然可以是成员变量，但现在它们将在每次事件中被重新赋值为新对象
    Point startPoint; 
    Point endPoint;
    // ...

    @Override
    public void mousePressed(MouseEvent e) {
        // 创建一个新的 Point 实例来存储鼠标按下的位置
        startPoint = new Point(e.getPoint()); 
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        // 创建一个新的 Point 实例来存储鼠标释放的位置
        endPoint = new Point(e.getPoint()); 

        if (object == 0) {
            // 现在传递给 Line 构造器的是新创建的、独立的 Point 对象
            canvas.addPrimitive(new Line(startPoint, endPoint, temp));          
        }

        if (object == 1){
            // 同样，Circle 也会接收到独立的 Point 对象
            canvas.addPrimitive(new Circle(startPoint, endPoint, temp));            
        }

        canvas.repaint();
    }
    // ...
}

import java.awt.Graphics;
import java.awt.Point;
import java.awt.Color;

public class Line extends PaintingPrimitive{
    Point startPoint; // 声明为成员变量，但不在声明时初始化，而是在构造器中初始化
    Point endPoint;

    public Line(Point start, Point end, Color c) {
        super(c);
        // 对传入的 Point 对象进行防御性拷贝，确保 Line 实例拥有独立的 Point 数据
        this.startPoint = new Point(start); 
        this.endPoint = new Point(end);
    }

    public void drawGeometry(Graphics g) {
        System.out.println("draw geo called");
        g.drawLine(startPoint.x, startPoint.y, endPoint.x, endPoint.y);
    }

    @Override
    public String toString() {
        return "Line";
    }
}

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.ArrayList; // 假设 PaintingPanel 在同一文件或可访问

public class Painter implements ActionListener, MouseListener, MouseMotionListener {
    // ... 其他成员变量
    Color temp = Color.RED;
    int object = 0; // 0 = line, 1 = circle
    PaintingPanel canvas;

    // 不再在声明时初始化，而是在 mousePressed/Released 中赋值新对象
    Point startPoint; 
    Point endPoint;

    Painter() {
        // ... 构造器中的其他UI初始化代码
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(500, 500);
        // ... 其他面板和按钮设置

        canvas = new PaintingPanel();
        // ... 将 canvas 添加到 holder

        // 注册监听器
        // ... 按钮监听器
        canvas.addMouseListener(this); // 将鼠标监听器添加到 canvas 上
        // ...

        frame.setContentPane(holder);
        frame.setVisible(true);
    }

    // ... actionPerformed, mouseDragged, mouseMoved, mouseClicked, mouseEntered, mouseExited 方法

    @Override
    public void mousePressed(MouseEvent e) {
        // 关键修改：每次按下鼠标时创建新的 Point 对象
        startPoint = new Point(e.getPoint()); 
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        // 关键修改：每次释放鼠标时创建新的 Point 对象
        endPoint = new Point(e.getPoint()); 

        if (object == 0) {
            canvas.addPrimitive(new Line(startPoint, endPoint, temp));          
        }

        if (object == 1){
            canvas.addPrimitive(new Circle(startPoint, endPoint, temp));            
        }

        canvas.repaint(); // 通知 PaintingPanel 重新绘制
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new Painter()); // 推荐在事件分发线程中创建UI
    }
}

import java.awt.Graphics;
import java.awt.Point;
import java.awt.Color;

public class Line extends PaintingPrimitive{
    Point startPoint; // 不再在声明时初始化
    Point endPoint;   // 不再在声明时初始化

    public Line(Point start, Point end, Color c) {
        super(c);
        // 关键修改：在构造器中进行防御性拷贝
        this.startPoint = new Point(start); 
        this.endPoint = new Point(end);
    }

    public void drawGeometry(Graphics g) {
        // System.out.println("draw geo called"); // 调试信息可以移除
        g.drawLine(startPoint.x, startPoint.y, endPoint.x, endPoint.y);
    }

    @Override
    public String toString() {
        return "Line";
    }
}

import java.util.ArrayList;
import javax.swing.JPanel;
import java.awt.Graphics;
import java.awt.Color;

public class PaintingPanel extends JPanel {

    ArrayList<PaintingPrimitive> primitives = new ArrayList<PaintingPrimitive>();

    PaintingPanel() {
        setBackground(Color.WHITE);
    }

    public void addPrimitive(PaintingPrimitive obj) {
        primitives.add(obj);
        // this.repaint(); // 可以在 Painter 中统一调用，或者在这里调用，确保UI更新
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g); // 始终调用父类的 paintComponent 来清空背景等

        for (PaintingPrimitive shape : primitives) {
            // g.drawLine(0,0,100,100); // 这行调试代码可以移除
            shape.draw(g); // 调用每个图形的 draw 方法
        }
    }
}