一、创建窗体界面

为了方便，我将所有的功能、按钮都添加到了一个JPanel面板容器上。

JPanel是一个面板容器组件， 可以用来放置其他轻量级的组件，除此之外，还可以在JPanel上绘制图形
JPanel btnPanel= new JPanel();
JPanel backPanel = new JPanel();
最后不要忘记将JPanel添加到窗体上

界面大概就是这个样子，我还在修改中，目前的想法是添加更多的功能，比如清空，撤销，画笔，更多图案等等，然后把所有的按钮都用图片替换。

二、绘制工具

用到的是java.awt中的Graphics类，可以实现在屏幕上绘制图案
通过Graphics g=getGraphics()来获取一个画笔，从而实现图案的绘制
比如 Graphics g=jframe.getGraphics()，g.draeLine(0,0,100,100)这样图案就可以在窗体上显示了

图案	绘制方法
直线	g.drawLine(x1,y1,x2,y2)
椭圆/圆	drawOval (int x, int y, int width, int height)
矩形	drawRect(int x , int y , int width , int height)
圆角矩形	drawRoundRect(int x,int y,int width, int height, int arcWidth, int arcHeight)
多边形	g.drawPolygon(int[] xPoints, int[] yPoints, int nPoints);
实心图形	g.fiiiOval(int x, int y, int width, int height)

三、获取数据

画布、画笔都有了，接下来就是获取数据，实现图案的绘制。因为是利用鼠标来进行绘制，所以需要鼠标监听器（MouseListener）来获取坐标数据，跟踪鼠标的点击和移动。
比如获取鼠标按下和释放的坐标，这样就可以得到两个点，利用drawLine就可以绘制出直线。

	@Override //按下public void mousePressed(MouseEvent e) {x1 = e.getX();y1 = e.getY();System.out.println("按下");}int index = 0;@Override //释放public void mouseReleased(MouseEvent e) {x2 = e.getX();y2 = e.getY();g.drawLine(x1,y1,x2,y2);System.out.println("释放");}

四、重绘

这样一个简单的画图板就设计好了，但是移动窗体就会发现，当窗体移动到电脑屏幕外或者放大缩小窗体后，我们绘制的图案就消失了。这是因为窗体状态发生变化后，会重新绘制窗体，我们之前绘制的图案没有保存，所以刷新后就消失了。
重新绘制窗体时调用了paint方法，为了解决这一问题，可以将我们要绘制的图案保存起来， 写在paint方法里，这样当窗体发生变化后，图形就能重新绘制了，不会消失了。

• 写一个shape类，来保存绘制图案的坐标，颜色，图案类型等
• 继承JFrame，重写paint()方法

五、源代码

DrawPad.java
主类，主要设计了窗体界面

package draw;
import javax.swing.*;
import java.awt.*;
public class DrawPad {//创建一个图形数组Shape[] shapes = new Shape[100];public static void main(String[] args) {DrawPad drawpad = new DrawPad();drawpad.initUI();}public void initUI() {JFrame drawFrame = new JFrame();drawFrame.setTitle("画板");drawFrame.setSize(800,600);drawFrame.setDefaultCloseOperation(drawFrame.EXIT_ON_CLOSE);drawFrame.setLocationRelativeTo(null);drawFrame.getContentPane().setBackground(Color.WHITE);drawFrame.setLayout(null);//菜单栏JMenuBar bar = new JMenuBar();JMenu menu = new JMenu("文件");JMenuItem item = new JMenuItem("新建");menu.add(item);bar.add(menu);bar.setBounds(5, 0, 770, 30);drawFrame.add(bar);//面板按钮容器JPanel  btnPanel= new JPanel();btnPanel.setBounds(5, 30, 100, 515);//画图容器JPanel backPanel = new JPanel();backPanel.setBackground(Color.GRAY);backPanel.setBounds(105, 30, 670, 515);backPanel.setLayout(null);MyJPanel drawPanel = new MyJPanel(shapes);drawPanel.setBounds(5, 5, 660, 505);drawPanel.setBackground(Color.WHITE);backPanel.add(drawPanel);//颜色按钮JButton colorbtn = new JButton();colorbtn.setPreferredSize(new Dimension(30,30));//注册监听器DrawListener dl = new DrawListener(shapes,colorbtn);//功能按钮String[] btnstr = {"圆","矩形","直线","三角形","递归矩形"};for(int i = 0 ; i < btnstr.length ; i++) {JButton btn = new JButton(btnstr[i]);//btn.setPreferredSize(new Dimension(40,30));btnPanel.add(btn);btn.addActionListener(dl);}//功能按钮//String[] btnArray = { "line.png", "circle.png", "rect.png","triangle.png"};  String[] btnArray = { "src/picture/line.png"};  // 循环创建按钮对象  for (int i = 0; i < btnArray.length; i++) {  JButton btn1 = new JButton();btn1.setBackground(Color.WHITE);btn1.setPreferredSize(new Dimension(30,30));ImageIcon icon = new ImageIcon(btnArray[i]);btn1.setIcon(icon);//btnPanel.add(btn1);         } //添加颜色按钮colorbtn.addActionListener(dl);btnPanel.add(colorbtn);//添加面板容器drawFrame.add(btnPanel);drawFrame.add(backPanel);drawFrame.setVisible(true);//监听	Graphics g = drawPanel.getGraphics();dl.setG(g);drawPanel.addMouseListener(dl);}
}

DrawListener.java
鼠标监听器，按钮监听器

package draw;
import java.awt.*;
import javax.swing.*;
public class DrawListener implements MouseListener,ActionListener{int x1,x2,y1,y2;int x3=0,x4=0,x5=0,y3,y4,y5;Graphics g;String shapetype;Color color;Shape[] shapes;JPanel  btnPanel;JButton colorbtn = new JButton();int count = 0;//set传值public void setG(Graphics g) {this.g = g;}public DrawListener(Shape[] shapes,JButton colorbtn) {this.shapes=shapes;this.colorbtn = colorbtn;}@Overridepublic void actionPerformed(ActionEvent e) {String btnstr = e.getActionCommand();	if(btnstr.equals("")) {//设置画笔颜色Color colors = JColorChooser.showDialog(btnPanel, "颜色", null);if (colors == null) {return;}colorbtn.setBackground(colors);color = colors;}else {shapetype = btnstr;}}@Override //点击public void mouseClicked(MouseEvent e) {if (count == 0) {x3 = e.getX();y3 = e.getY();count++;} else if (count == 1) {x4 = e.getX();y4 = e.getY();count++;}else if(count == 2) {x5 =e.getX();y5 = e.getY();count=0;}System.out.println("点击");}@Override //按下public void mousePressed(MouseEvent e) {x1 = e.getX();y1 = e.getY();System.out.println("按下");}int index = 0;@Override //释放public void mouseReleased(MouseEvent e) {x2 = e.getX();y2 = e.getY();Shape shape= new Shape(shapetype, x1, x2, y1, y2, color);shape.drawShape(g);shapes[index]=shape;index++;System.out.println("释放");}@Override //进入public void mouseEntered(MouseEvent e) {System.out.println("进入");}@Override //离开public void mouseExited(MouseEvent e) {System.out.println("离开");}
}

MyJPanel.java
继承JPanel，重写paint方法

package draw;
import java.awt.Graphics;
import javax.swing.JPanel;
public class MyJPanel extends JPanel{Shape[] shapes;public MyJPanel(Shape[] shapes) {this.shapes = shapes;}@Overridepublic void paint(Graphics g) {// TODO Auto-generated method stubsuper.paint(g);for (int i = 0; i < shapes.length; i++) {Shape shape = shapes[i];if(shape!=null) {shape.drawShape(g);}}}
}

Shape.java
保存图形

package draw;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.MouseEvent;
public class Shape {String shapetype;int x1,x2,y1,y2;Color color;//构造方法public Shape(String shapetype, int x1, int x2, int y1, int y2, Color color) {super();this.shapetype = shapetype;this.x1 = x1;this.x2 = x2;this.y1 = y1;this.y2 = y2;this.color = color;}public void drawShape(Graphics g) {g.setColor(color);if(shapetype.equals("直线")) {// 画直线g.drawLine(x1, y1, x2, y2);}else if(shapetype.equals("矩形")) {// 画矩形 左上角的坐标 宽 高g.drawRect(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x2 - x1), Math.abs(y2 - y1));}else if(shapetype.equals("圆")) {// 画圆形g.drawOval(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x2 - x1), Math.abs(y2 - y1));}else if(shapetype.equals("递归矩形")) {MySerbinRect myserbin = new MySerbinRect();myserbin.drawMyrect(g,Math.min(x1, x2),Math.min(y1, y2), Math.abs(x2-x1),Math.abs(y2-y1),3);	}else if(shapetype.equals("三角形")) {int[] x = { Math.min(x1, x2), Math.max(x1, x2), Math.min(x1, x2) + Math.abs(x1 - x2) / 2,Math.min(x1, x2) };int[] y = { Math.max(y1, y2), Math.max(y1, y2), Math.min(y1, y2), Math.max(y1, y2) };//g.fillPolygon(x, y, 4);g.drawPolygon(x, y, 4);}}	
}

MySerbinRect.java
写了两个图形，递归矩形和递归三角形

package draw;
import java.awt.Graphics;
import java.awt.Polygon;
public class MySerbinRect {public void drawMyrect(Graphics g,int x,int y,int width,int height,int n) {if(n<0) {return;}n--;g.drawRect(x, y, width, height);g.fillRect(x+width/3, y+height/3, width/3, height/3);drawMyrect(g, x, y, width/3, height/3, n);//左上drawMyrect(g, x+width/3, y, width/3, height/3, n);//中drawMyrect(g, x+2*(width/3), y, width/3, height/3, n);//右上drawMyrect(g, x, y+height/3, width/3, height/3, n);//左drawMyrect(g, x+2*(width/3), y+height/3, width/3, height/3, n);//右drawMyrect(g, x, y+2*(height/3), width/3, height/3, n);//左下drawMyrect(g, x+width/3, y+2*(height/3), width/3, height/3, n);//下drawMyrect(g, x+2*(width/3), y+2*(height/3), width/3, height/3, n);//右下}public void drawTriangle(Graphics g,int x3,int y3,int x4,int y4,int x5,int y5){Polygon filledPolygon=new Polygon();filledPolygon.addPoint(x3,y3);filledPolygon.addPoint(x4,y4);filledPolygon.addPoint(x5,y5);g.drawPolygon(filledPolygon);}public void drawMytri(Graphics g,int x3,int y3,int x4,int y4,int x5,int y5,int n) {if(n<0) {return;}n--;MySerbinRect mys = new MySerbinRect();mys.drawTriangle(g, x3, y3, x4, y4, x5, y5);Polygon filledPolygon1=new Polygon();filledPolygon1.addPoint((x3+x4)/2,(y3+y4)/2);filledPolygon1.addPoint((x4+x5)/2,(y4+y5)/2);filledPolygon1.addPoint((x5+x3)/2,(y5+y3)/2);g.fillPolygon(filledPolygon1);drawMytri(g, x3, y3, (x4+x3)/2, (y3+y4)/2, (x3+x5)/2, (y5+y3)/2,n);drawMytri(g,(x4+x3)/2, (y3+y4)/2, x4, y4, (x4+x5)/2, (y5+y4)/2,n);drawMytri(g,(x5+x3)/2, (y3+y5)/2, (x4+x5)/2, (y5+y4)/2, x5,y5,n);}
}