在一个窗口上点击，能以点击点的坐标为中心画三角形、矩形、圆形

不是什么高级程序，就是基础知识的练习，所以不叙述了。

代码如下：

package mt;

import java.awt.Graphics;
/**
 * 是否在窗口画图
 * @author mt
 *
 */
public interface Drawable {
	public abstract void draw(Graphics g);
}

package mt;

import java.awt.Color;

/**
 * 自定义工具类
 * @author jackfrued
 *
 */
public final class MyUtil {

	private MyUtil() {
	}
	
	/**
	 * 产生指定范围的随机整数
	 * @param min 最小值(闭区间)
	 * @param max 最大值(闭区间)
	 * @return 指定范围的随机整数
	 */
	public static int random(int min, int max) {
		return (int) (Math.random() * (max - min + 1) + min);
	}
	
	/**
	 * 生成随机颜色
	 * @return Color对象
	 */
	public static Color randomColor() {
		int r = random(0, 255);
		int g = random(0, 255);
		int b = random(0, 255);
		return new Color(r, g, b);
	}
}

package mt;

import java.awt.Color;
/**
 * 图像（抽象的）
 * @author mt
 *
 */
public abstract class Shape implements Drawable{
	protected int x , y ; //中心的横纵坐标

	protected Color color; //颜色
	/**
	 * 计算面积
	 * @return  面积值
	 */
	public abstract double area();
	/**
	 * 算周长
	 * @return  周长值
	 */
	public abstract double perimiter();

	public void setColor(Color color) {
		this.color = color;
	}


	public void setX(int x) {
		this.x = x;
	}

	public void setY(int y) {
		this.y = y;
	}

	public int getX() {
		return x;
	}

	public int getY() {
		return y;
	} 
	

}

package mt;

import java.awt.Graphics;
/**
 * 圆形
 * @author mt
 *
 */
public class Circle extends Shape {
	private int radius; //半径
	
	/**
	 * 构造器
	 * @param radius  半径
	 */
	public Circle(int radius) {
		this.radius = radius;
	}

	@Override
	public void draw(Graphics g) {
		g.setColor(color);
		g.drawOval(x-radius, y-radius, radius << 1, radius << 2);

	}

	@Override
	public double area() {	
		return Math.PI * radius * radius;
	}

	@Override
	public double perimiter() {
		return Math.PI * 2 *radius;
	}

}

package mt;

import java.awt.Graphics;
/**
 * 矩形
 * @author mt
 *
 */
public class Rect extends Shape {
	private int widgt;  //宽带
	private int height;  //高度

/**
 * 构造器
 * @param widgt  宽度
 * @param height  高度
 */
	public Rect(int widgt, int height) {
		this.widgt = widgt;
		this.height = height;
	}

	@Override
	public void draw(Graphics g) {
		g.setColor(color);
		g.drawRect(x - widgt / 2, y - height / 2, widgt, height);
	}

	@Override
	public double area() {		
		return widgt * height;
	}

	@Override
	public double perimiter() {
		return (widgt + height) << 1 ;
	}

}

package mt;

import java.awt.Graphics;
/**
 * 三角形
 * @author mt
 *
 */
public class Trigon extends Shape {
	private int length;  //边长
	/* a = {x,(int) (y-length/2/0.866)};
	   b = {x-length/2,(int) (y+length/2*0.577)};
	   c = {x+length/2,(int) (y+length/2*0.577)};  三点坐标*/
	
	
	/**
	 * 构造器
	 * @param length  边长
	 */
	public Trigon(int length) {
		this.length = length;
	}

	@Override
	public void draw(Graphics g) {
		g.setColor(color);
		g.drawLine(x, (int) (y-length/2/0.866),x-length/2, (int) (y+length/2*0.577));
		g.drawLine(x, (int) (y-length/2/0.866), x+length/2,(int) (y+length/2*0.577));
		g.drawLine(x-length/2, (int) (y+length/2*0.577),x+length/2, (int) (y+length/2*0.577));

	}

	@Override
	public double area() {
	double p = 3 * length /2;
		return Math.sqrt(p*((p-length)*(p-length)*(p-length)));
	}

	@Override
	public double perimiter() {
	
		return 3 * length;
	}

}

package mt;

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.awt.image.BufferedImage;
import java.util.ArrayList;
import java.util.List;

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

@SuppressWarnings("serial")
public class MyFrame extends JFrame {
	private BufferedImage offImage = new BufferedImage(900, 600, 1);
	private String temp = "Circle";  //临时字符串（用于记录按按钮读取到的字符串）
	private Shape shape = null;    //创建一个形状对象
	private List<Shape> shapeList = new ArrayList<Shape>();  //装形状的数组
	private JButton circleBtton = new JButton();  //画圆形的按钮
	private JButton rectBtton = new JButton();    //画矩形的按钮
	private JButton trigonBtton = new JButton();  //画三角形的按钮
	
	public MyFrame() {
		this.setSize(900,600);
		this.setTitle("绘图");
		this.setResizable(false);
		this.setLocationRelativeTo(null);
		this.setDefaultCloseOperation(EXIT_ON_CLOSE);
		circleBtton = new JButton("Circle");
		rectBtton = new JButton("Rect");
		trigonBtton = new JButton("Trigon");
		this.setLayout(null);
		this.add(circleBtton);
		circleBtton.setBounds(200, 20, 100, 30);
		this.add(rectBtton);
		rectBtton.setBounds(400, 20, 100, 30);
		this.add(trigonBtton);
		trigonBtton.setBounds(600, 20, 100, 30);

		ActionListener cilck = new CilckButton();
		circleBtton.addActionListener(cilck);
		rectBtton.addActionListener(cilck);
		trigonBtton.addActionListener(cilck);
		this.addMouseListener(new MouseAdapter() {

			@Override
			public void mousePressed(MouseEvent e) {
				if (temp.equals("Circle")) {
					shape = new Circle(MyUtil.random(10, 250));
				}
				else if(temp.equals("Rect")) {
					shape = new Rect(MyUtil.random(10, 300),
							MyUtil.random(20, 400));
				}
				else  {
					shape = new Trigon(MyUtil.random(100, 300));
				}
				
				
				shape.setX(e.getX());
				shape.setY(e.getY());
				shape.setColor(MyUtil.randomColor());
				shapeList.add(shape);
				
				repaint();
			}
		});
	}

	private class CilckButton implements ActionListener {

		@Override
		public void actionPerformed(ActionEvent e) {
			temp = e.getActionCommand();
		}
	}


	@Override
	public void paint(Graphics g) {
		Graphics g2 = offImage.getGraphics();
		super.paint(g2);
		for(Shape temp : shapeList) {
			temp.draw(g2);
		}
		if (shape != null) {			
			shape.draw(g2);
			g2.drawString(String.format("周长: %.2f", shape.perimiter()), 
					shape.getX(), shape.getY());
			g2.drawString(String.format("面积: %.2f", shape.area()), 
					shape.getX(), shape.getY()+50);
		}
		g.drawImage(offImage, 0, 0, null);
	}
}

package mt;

public class Test {
	public static void main(String[] args) {
		new MyFrame().setVisible(true);
	}
}