java小程序：画板的实现

 面板类：

为什么要继承JFrame类？

答：考虑到重绘图形时需要在JFrame对象上重绘出数组中的图形，所以我们让面板类继承JFrame类成为它的一个子类，以便之后重写paint()方法。

package drawingboard;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.util.ArrayList;

import javax.swing.JButton;
import javax.swing.JFrame;
/*
 * 画板类
 */
public class Drawing extends JFrame {
	private String str[] = new String[] { "直线", "多边形", "图片", "文字" };
	private Color c[] = new Color[] { Color.black, Color.green, Color.yellow, Color.blue };
	private Graphics g;
	private ArrayList<Shape> listshape=new ArrayList();
	/*
	 * 初始化界面函数
	 */
	public void initUI() {
		this.setTitle("画板");   								//标题
		this.setSize(700, 600);									//大小
		this.setLocationRelativeTo(null);						//居中
		this.setDefaultCloseOperation(3);						//关闭
		this.setLayout(new FlowLayout());						//流式布局

		DrawingListener dl = new DrawingListener();				//实例化接口对象
		this.addMouseListener(dl);								//给面板对象加监听方法addMouseListener，指定监听接口对象为dl

		for (int i = 0; i < str.length; i++) {					//循环添加按钮
			JButton jb = new JButton(str[i]);
			this.add(jb);
			jb.addActionListener(dl);
		}
		for (int i = 0; i < c.length; i++) {
			JButton jb1 = new JButton();
			jb1.setBackground(c[i]);
			jb1.setPreferredSize(new Dimension(30, 30));
			this.add(jb1);
			jb1.addActionListener(dl);
		}
		this.setVisible(true);									//可见
		g = this.getGraphics();									//取画笔，注意只能在窗体可见之后
		dl.setg(g);												//传画笔
		dl.setlist(listshape);									//传数组
	}

	public void paint(Graphics g) {
		super.paint(g);											//调用父类paint方法
		for (int i = 0; i < listshape.size(); i++) {			//取出listshape数组中的元素进行重绘
			Shape shape = listshape.get(i);
			if (shape != null) {
				shape.drawshape(g);
			} else {
				break;
			}
		}
	}

	/*
	 * 主函数
	 */
	public static void main(String[] args) {
		Drawing dr = new Drawing();								//实例化窗体类对象dr
		dr.initUI();											//调用dr对象的初始化窗体的方法
	}

}

监听接口类：

为什么用MouseAdapter类而不用MouseListener接口？

答：使用MouseAdapter类不需要重写其中所有的方法，而使用接口要重写其中所有的抽象方法。

package drawingboard;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;

/*
 * 监听类
 */
public class DrawingListener extends MouseAdapter implements ActionListener {
	private int x1, y1, x2, y2, x3, y3; 		// 用于存储坐标值，注意窗体界面y往下为大，往上为小
	private String xingzhuang;
	private JFrame jf;
	private Graphics g;
	private Color color;
	private boolean flag = true; 				// 标志位，注意方法执行顺序，按下-释放-点击
	private ArrayList<Shape> listshape;
	/*
	 * 监听类构造函数，传递参数的作用
	 */
	public void setg(Graphics g) {				//传画笔
		this.g = g;
	}

	public void setlist(ArrayList<Shape> listshape) {//传数组
		this.listshape = listshape;
	}

	@Override
	public void actionPerformed(ActionEvent e) {
		String s = e.getActionCommand();
		if (s.equals("")) {								//颜色按钮
			JButton jb = (JButton) e.getSource();
			color = jb.getBackground();
			g.setColor(color);							//给画笔上色
		} else {
			xingzhuang = s;								//形状按钮
		}

	}

	@Override
	public void mouseClicked(MouseEvent e) {
		x3 = e.getX();
		y3 = e.getY();
		if (xingzhuang.equals("多边形")) {
			g.drawLine(x2, y2, x3, y3);
			x2 = x3;
			y2 = y3;
			if (e.getClickCount() == 2) {
				g.drawLine(x1, y1, x3, y3);
			}
		} else if (xingzhuang.equals("文字")) {
			g.setFont(new Font("宋体", Font.BOLD, 50));
			String ss = "熊哥！";
			g.drawString(ss, x3, y3);
			Shape shape = new Shape();					//实例化Shape类的对象
			shape.setShape(x1, y1, x2, y2, x3, y3, xingzhuang, color);//给Shape类的对象传参
			listshape.add(shape);						//把对象添加到数组当中
		}

	}

	@Override
	public void mousePressed(MouseEvent e) {
		if (flag) {
			x1 = e.getX();
			y1 = e.getY();
		}
	}

	@Override
	public void mouseReleased(MouseEvent e) {
		if (flag) {
			x2 = e.getX();
			y2 = e.getY();
		}
		if (xingzhuang.equals("直线") && flag) {
			g.drawLine(x1, y1, x2, y2);
			Shape shape = new Shape();
			shape.setShape(x1, y1, x2, y2, x3, y3, xingzhuang, color);
			listshape.add(shape);
		} else if (xingzhuang.equals("多边形") && flag) {
			g.drawLine(x1, y1, x2, y2);
			flag = !flag;
		} else if (xingzhuang.equals("图片")) {
			ImageIcon t = new ImageIcon("C:\\Users\\lcz\\Desktop\\***\\IMG_0456.JPG");
			g.drawImage(t.getImage(), Math.min(x1, x2), Math.min(y1, y2), Math.abs(x1 - x2), Math.abs(y1 - y2), null);
			Shape shape = new Shape();
			shape.setShape(x1, y1, x2, y2, x3, y3, xingzhuang, color);
			listshape.add(shape);
		}
	}

}

形状类：

为什么我们要定义一个形状类？

答：方便我们把形状对象添加到ArrayList数组当中，而且进行实例化的时候也非常的方便。

package drawingboard;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;

import javax.swing.ImageIcon;

public class Shape {
	private int x1, x2, y1, y2, x3, y3;
	private Color color;
	private String xingzhuang;
	private ImageIcon t = new ImageIcon("C:\\Users\\lcz\\Desktop\\***\\IMG_0456.JPG");

	public void setShape(int x1, int y1, int x2, int y2, int x3, int y3, String xingzhuang, Color color) {
		this.x1 = x1;
		this.x2 = x2;
		this.y1 = y1;
		this.y2 = y2;
		this.x3 = x3;
		this.y3 = y3;
		this.xingzhuang = xingzhuang;
		this.color = color;
	}

	public void drawshape(Graphics g) {
		switch (xingzhuang) {
		case "直线":
			g.setColor(color);
			g.drawLine(x1, y1, x2, y2);
			break;
		case "图片":
			g.setColor(color);
			g.drawImage(t.getImage(), Math.min(x1, x2), Math.min(y1, y2), Math.abs(x1 - x2), Math.abs(y1 - y2), null);
		case "文字":
			g.setColor(color);
			g.setFont(new Font("宋体", Font.BOLD, 50));
			String ss = "熊哥！";
			g.drawString(ss, x3, y3);
		}
	}
}

多边形的重绘方法我没有写呀，感兴趣的小伙伴可以自己去考虑。