无法在Java中创建简单的绘图应用程序？

Question

我正在尝试为我的作业创建一个简单的绘图应用程序，用户可以选择三种颜色中的一种来绘制。要激活＆＃34;笔＆＃34;，用户需要单击窗口，然后拖动鼠标（未按下）在屏幕上绘制线条。我有一些事情很难。其中之一是在更改颜色时，我想改变该点的颜色（在用户选择不同颜色之后），而不是用户制作的所有先前点（绘图）。我也很难正确实现鼠标监听器。我不确定如何在用户点击屏幕（mousePressed？）后激活mouseMoved函数，并在用户第二次单击时停用mouseMoved。 以下是我到目前为止的情况：

package project.pkg6;

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

public class DrawingTester extends JPanel{
    //linked list holds points
    private LinkedList<Point> pointList;
    //default color
    private Color selectedColor = Color.red;
    //colors
    private final Color red;
    private final Color blue;
    private final Color yellow;
    //eraser
    private final Color white;
    //radio buttons
    private final JRadioButton redButton;
    private final JRadioButton blueButton;
    private final JRadioButton yellowButton;
    private final JRadioButton eraserButton;
    //group radio buttons
    private final ButtonGroup colorRadioGroup;
    //size of dot 
    private final static int DIAMETER = 10;

    public DrawingTester(){
        setLayout(new FlowLayout());
        setBackground(Color.WHITE);
        pointList = new LinkedList<Point>();

        this.addMouseMotionListener(new MouseDrawListener());
        this.addMouseListener(new MouseDrawListener());

        //create radio buttons
        redButton = new JRadioButton("Red", true);
        blueButton = new JRadioButton("Blue", false);
        yellowButton = new JRadioButton("Yellow", false);
        eraserButton = new JRadioButton("Eraser", false);
        add(redButton);
        add(blueButton);
        add(yellowButton);
        add(eraserButton);

        //add radio buttons to group
        colorRadioGroup = new ButtonGroup();
        colorRadioGroup.add(redButton);
        colorRadioGroup.add(blueButton);
        colorRadioGroup.add(yellowButton);
        colorRadioGroup.add(eraserButton);

        //create color objects
        red = Color.RED;
        blue = Color.BLUE;
        yellow = Color.YELLOW;
        white = Color.WHITE;

        //register events
        redButton.addItemListener(new RadioButtonHandler(red));
        blueButton.addItemListener(new RadioButtonHandler(blue));
        yellowButton.addItemListener(new RadioButtonHandler(yellow));
        eraserButton.addItemListener(new RadioButtonHandler(white));
    }

    @Override
    public void paintComponent(Graphics pen){
        super.paintComponent(pen);

        //draw points
        for(Point point : pointList){
            int x = (int) point.getX();
            int y = (int) point.getY();
            pen.setColor(selectedColor);
            pen.fillOval(x, y, DIAMETER, DIAMETER);

        }
    }
    //event handlers
    private class RadioButtonHandler implements ItemListener{
        private Color color;

        public RadioButtonHandler(Color c){
            color = c;
        }

        @Override
        public void itemStateChanged(ItemEvent e){
            if (e.getSource() == redButton){
                selectedColor = Color.RED;
            }
            else if(e.getSource() == blueButton){
                selectedColor = Color.BLUE;

            }
            else if(e.getSource() == yellowButton){
                selectedColor = Color.YELLOW;

            }
            else if (e.getSource() == eraserButton){
                selectedColor = Color.WHITE;
            }
        }
    }
    private class MouseDrawListener extends MouseAdapter{
        @Override
        public void mousePressed(MouseEvent e){
            //repaint();
            draw(e);
        }
        @Override
        public void mouseMoved(MouseEvent e){
            /*Point point = e.getPoint();

            pointList.add(point);
            repaint();
           draw(e);*/
        }
        void draw(MouseEvent e){
            Point point = e.getPoint();

            pointList.add(point);
            repaint();
        }
    }

    public static void main(String[] args){
       EventQueue.invokeLater(new Runnable(){
           public void run(){
               JFrame drawPanel = new JFrame("Drawing");
               drawPanel.setSize(400,400);
               DrawingTester drawing = new DrawingTester();
               drawPanel.getContentPane().add(drawing);
               drawPanel.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
               drawPanel.setVisible(true);
           }
       });
    }
}