Java画图板的简单实现。（重绘，多边形，曲线画笔，画图片）

思路：
①画图板的界面
②创建监听器类
③给按钮加上鼠标监听
③画图版的重绘

第一部分：界面类
①画图板的界面 ③给按钮加上鼠标监听

public class SampleDraw extends JFrame {
    public static void main(String[] args) {
        SampleDraw sam = new SampleDraw();//创建该类对象
        sam.showUI();//调用showUI方法
    }
    public void showUI() {
        this.setSize(800, 600);//设置尺寸
        this.setTitle("画板v1.0");//设置标题
        this.setLocationRelativeTo(null);//设置居中

        this.setDefaultCloseOperation(3);//设置关闭进程
        FlowLayout flow = new FlowLayout();//布局方式为流式布局
        this.setLayout(flow);
        Color[] color = { Color.GREEN, Color.RED, Color.BLUE, Color.YELLOW };//创建一个数组保存颜色
        DrawListener drawListener = new DrawListener();//新建监听器对象
        for (int i = 0; i < color.length; i++) {//利用循环将颜色依次将赋给按钮的背景色
            JButton jbu = new JButton();
            jbu.setPreferredSize(new Dimension(30, 30));//设置按钮尺寸
            jbu.setBackground(color[i]);//设置按钮的背景色
            this.add(jbu);//把按钮加给窗体
            jbu.addActionListener(drawListener);//给按钮添加监听器

        }

        String[] name = { "直线", "三角形", "正方形","矩形", "椭圆","棱形", "五边形", "图片1", "图片2", "画笔", "特殊三角形", "多边形" };////创建一个数组保存按钮名称

        for (int i = 0; i < name.length; i++) {//利用循环将名字赋给按钮
            JButton jbu1 = new JButton(name[i]);//创建按钮对象
            this.add(jbu1);//把按钮对象加给窗体
            jbu1.addActionListener(drawListener);//给按钮添加监听器
        }

        this.setVisible(true);  //设置窗体可见
        Graphics g = this.getGraphics();//创建画笔

        this.addMouseListener(drawListener);//添加鼠标监听器
        this.addMouseMotionListener(drawListener);//添加鼠标拖动监听器
        drawListener.setGr(g);//调用监听器的set方法
        drawListener.setShape(listshape);//调用监听器的setShape方法
    }
}

第二部分：监听器类
②创建监听器
该类应该继承MouseListener, ActionListener, MouseMotionListener的接口

public class DrawListener implements MouseListener, ActionListener, MouseMotionListener {

    private Graphics gr;//声明对象
    private Color color;
    private int flag = 1;
    private String name;
    private int x1, y1, x2, y2, x3, y3,x4,y4;
    private ImageIcon icon = new ImageIcon("C:\\Users\\Administrator\\Pictures\\lu.jpg");//添加图片。注意路径为绝对路径，单斜杠变成双斜杠，加文件名和后缀格式
    private ImageIcon icon2 = new ImageIcon("C:\\Users\\Administrator\\Pictures\\hua.jpg");
    private ArrayList listshape;

    public void setGr(Graphics g) {//传画笔
        gr = g;
    }

    public void setShape(ArrayList listshape) {//传数组
        this.listshape = listshape;
    }

    public void actionPerformed(ActionEvent e) {//监听方法

        if ("".equals(e.getActionCommand())) {//如果按钮的文本内容为""
            JButton jbu = (JButton) e.getSource();
            color = jbu.getBackground();//获取按钮的背景色
            gr.setColor(color);
        } else {//按钮文本内容不为空
            name = e.getActionCommand();//将按钮的文本内容赋给name
        }

    }

    public void mouseDragged(MouseEvent e) {
        x3 = e.getX();//x3  y3 为鼠标按压拖动时的坐标
        y3 = e.getY();

        if ("画笔".equals(name)) {//如果点击是“画笔”按钮

            gr.drawLine(x1, y1, x3, y3);//画线
            x1 = x3;
            y1 = y3;
//          更新x1,y1的值，把每一个点依次相连。
        }
    }
    public void mouseMoved(MouseEvent e) {
    }
    public void mouseClicked(MouseEvent e) {
        x4 = e.getX();//将鼠标点击时的坐标赋给x4,y4
        y4 = e.getY();
        if ("特殊三角形".equals(name) && flag == 2) {//此时flag为2时，才能将点击的点和直线的两端链接起来
            gr.drawLine(x1, y1, x4, y4);//画线
            gr.drawLine(x2, y2, x4, y4);
            flag = 1;//画完线后，将flag的值赋为1，这样能接着画第二个三角形
        }

        if ("多边形".equals(name) && flag == 2) {
            gr.drawLine(x2, y2, x4, y4);
            x2 = x4;
            y2 = y4;
            if (e.getClickCount() == 2) {//如果为双击
                gr.drawLine(x1, y1, x4, y4);
                flag = 1;
            }
        }

    }
    public void mousePressed(MouseEvent e) {
        if (flag == 1) {
            x1 = e.getX();
            y1 = e.getY();
        }
    }
    public void mouseReleased(MouseEvent e) {
        if (flag == 1) {
            x2 = e.getX();
            y2 = e.getY();
        }
        if ("直线".equals(name)) {
            gr.drawLine(x1, y1, x2, y2);
        }
        if ("矩形".equals(name)) {
            gr.drawRect(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x1 - x2), Math.abs(y1 - y2));
        }
        if ("椭圆".equals(name)) {
            gr.drawOval(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x1 - x2), Math.abs(y1 - y2));
        }
        if ("三角形".equals(name)) {
            int[] xPoints = new int[] { x1, Math.abs((x1 + x2) / 2), x2 };
            int[] yPoints = new int[] { y2, y1, y2 };
            int nPoints = 3;
            gr.drawPolygon(xPoints, yPoints, nPoints);
        }
        if ("图片1".equals(name)) {
            gr.drawImage(icon.getImage(), Math.min(x1, x2), Math.min(y1, y2), Math.abs(x1 - x2), Math.abs(y1 - y2),
                    null);
        }
        if ("图片2".equals(name)) {
            gr.drawImage(icon2.getImage(), Math.min(x1, x2), Math.min(y1, y2), Math.abs(x1 - x2), Math.abs(y1 - y2),
                    null);
        }
        if ("正方形".equals(name)) {
            gr.drawRect(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x1 - x2), Math.abs(x1 - x2));
        }
        if ("棱形".equals(name)) {
            int[] xPoints = new int[] { (x1 + x2) / 2, x2, (x1 + x2) / 2, x1 };
            int[] yPoints = new int[] { y2, (y1 + y2) / 2, y1, (y1 + y2) / 2 };
            int nPoints = 4;
            gr.drawPolygon(xPoints, yPoints, nPoints);
        }
        if ("五边形".equals(name)) {
            int[] xPoints = new int[] { (x1 + x2) / 2, x2, x2 - (x2 - x1) / 4, x1 + (x2 - x1) / 4, x1 };
            int[] yPoints = new int[] { y1, (y1 + y2) / 2, y2, y2, (y1 + y2) / 2 };
            int nPoints = 5;
            gr.drawPolygon(xPoints, yPoints, nPoints);
        }
        if ("特殊三角形".equals(name) && flag == 1) {
            gr.drawLine(x1, y1, x2, y2);
            flag++;
        }
        if ("多边形".equals(name) && flag == 1) {
            gr.drawLine(x1, y1, x2, y2);
            flag++;
        }
    }
    public void mouseEntered(MouseEvent e) {
    }
    public void mouseExited(MouseEvent e) {
    }
}

现在就实现了简单的画图版了，但是会有一个问题，当我们改变窗体的大小或者最小化窗体再打开来会发现，之前画的图都不见了。这是因为当窗体发生改变时，程序会重新从内存中获取更新后的数据绘制。我们没有将画过的形状存放到内存中，当窗体发生重绘的时候，就不会绘制了

第三步：画图板的重绘
定义一个Shape类。
其中定义一个队列，用于存放图形对象。（队列的大小随对象的增加而增加）

public class Shape {
    private String name;
    private Color color; 
    private int x1, x2, y1, y2 ;
    private ImageIcon icon = new ImageIcon("C:\\Users\\Administrator\\Pictures\\lu.jpg");
    private ImageIcon icon2 = new ImageIcon("C:\\Users\\Administrator\\Pictures\\hua.jpg");

    public void setShape(int x1, int y1, int x2, int y2, String name, Color color) {
        this.x1 = x1;
        this.x2 = x2;
        this.y1 = y1;
        this.y2 = y2;
        this.name = name;
        this.color = color;
    }

    public void drawShape(Graphics g) {
        switch (name) {
        case "直线":
            g.setColor(color);
            g.drawLine(x1, y1, x2, y2);

            break;

        case "矩形":
            g.setColor(color);
            g.drawRect(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x1 - x2), Math.abs(y1 - y2));
            break;
        case "椭圆":
            g.setColor(color);
            g.drawOval(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x1 - x2), Math.abs(y1 - y2));
            break;
        case "三角形":
            g.setColor(color);
            int[] xPoints = new int[] { x1, Math.abs((x1 + x2) / 2), x2 };
            int[] yPoints = new int[] { y2, y1, y2 };
            int nPoints = 3;
            g.drawPolygon(xPoints, yPoints, nPoints);
            break;
        case "画笔":
            g.setColor(color);
            g.drawLine(x1, y1, x2, y2);

            break;
        case "正方形":
            g.setColor(color);
            g.drawRect(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x1 - x2), Math.abs(x1 - x2));
            break;
        case "五边形":
            g.setColor(color);
            int[] xPoints1 = new int[] { (x1 + x2) / 2, x2, x2 - (x2 - x1) / 4, x1 + (x2 - x1) / 4, x1 };
            int[] yPoints1 = new int[] { y1, (y1 + y2) / 2, y2, y2, (y1 + y2) / 2 };
            int nPoints1 = 5;
            g.drawPolygon(xPoints1, yPoints1, nPoints1);
            break;
        case "棱形":
            g.setColor(color);
            int[] xPoints2 = new int[] { (x1 + x2) / 2, x2, (x1 + x2) / 2, x1 };
            int[] yPoints2 = new int[] { y2, (y1 + y2) / 2, y1, (y1 + y2) / 2 };
            int nPoints2 = 4;
            g.drawPolygon(xPoints2, yPoints2, nPoints2);
            break;
        case "特殊三角形":
            g.setColor(color);
            g.drawLine(x1, y1, x2, y2);
            break;
        case "多边形":
            g.setColor(color);
            g.drawLine(x1, y1, x2, y2);
            break;
        case"图片1":
            g.setColor(color);
            g.drawImage(icon.getImage(), x1,y1,x2,y2,null);
            break;
        case"图片2":
            g.setColor(color);
            g.drawImage(icon2.getImage(), x1,y1,x2,y2,null);
            break;
        }
    }
}

再在监听器类中，画一个图形就将图形存入Shape类中的队列中去。例如：

if ("直线".equals(name)) {

            gr.drawLine(x1, y1, x2, y2);
            Shape shape = new Shape();
            shape.setShape(x1, y1, x2, y2, name, color);
            listshape.add(shape) ;

        }

最后在主界面的类中去定义重绘的方法和调用。在重绘方法中将队列中的对象取出来，调用画的方法

public void paint(Graphics g) {//重绘方法
        super.paint(g);
        //进行重绘
        for (int i = 0; i < listshape.size(); i++) {
            Shape shape = listshape.get(i);//依次取出对象
            if (shape != null) {
                shape.drawShape(g);//调用对象的画的方法
            } else {
                break;
            }
        }
    }

图形的重绘就到这里啦！