解决Java Swing绘图应用仅显示最后一个图形的问题：理解引用传递与对象拷贝

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

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

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

        if (object == 0) {
            // 将startPoint和endPoint的引用传递给Line构造器
            canvas.addPrimitive(new Line(startPoint, endPoint, temp));
        }
        // ...
    }

public class Painter implements ActionListener, MouseListener, MouseMotionListener {
    // ...
    // 可以将startPoint和endPoint初始化为null或在mousePressed中直接赋值
    Point startPoint; 
    Point endPoint;
    // ...

    @Override
    public void mousePressed(MouseEvent e) {
        // 直接创建一个新的Point对象，而不是修改现有对象的location
        startPoint = new Point(e.getPoint()); 
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        // 同样，创建一个新的Point对象
        endPoint = new Point(e.getPoint()); 

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

        if (object == 1){
            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; // 无需初始化，将在构造器中赋值
    Point endPoint;   // 无需初始化，将在构造器中赋值

    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实例持有独立的Point对象
        this.startPoint = new Point(start); 
        this.endPoint = new Point(end);
    }

    public void drawGeometry(Graphics g) {
        System.out.println("draw geo called"); // 注意：在实际应用中应避免在paintComponent中进行System.out.println，会影响性能
        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(); // 添加新图形后立即请求重绘
    }

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

        // 遍历并绘制所有存储的图形基元
        for (PaintingPrimitive shape : primitives) {
            // 移除不必要的固定线条绘制
            // g.drawLine(0,0,100,100); 
            shape.draw(g);
        }
    }
}