一、示例代码：

import java.awt.Graphics;
import javax.swing.JComponent;
import javax.swing.JFrame;

public class Main {
    public static void main(String[] a) {
        JFrame window = new JFrame();
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setBounds(200, 200, 200, 200);// 窗体大小
        window.setTitle("标题"); // 标题
        window.getContentPane().add(new MyCanvas());
        window.setVisible(true);
    }
}

class MyCanvas extends JComponent {
    public void paint(Graphics g) {
        // 在此处调用方法画图
        g.drawLine(0,0,100,100);
        g.drawLine(0,100,100,0);
        g.drawRoundRect(100,100,50,20,5,5);
    }
}

二、常用函数

1、直线
public abstract void drawLine(int x1, int y1, int x2, int y2)
2、矩形
public abstract void fillRect(int x, int y, int width, int height)
3、圆角矩形
public abstract void drawRoundRect(int x, int y, int width, int height, int arcWidth, int arcHeight);
4、椭圆
public abstract void drawOval(int x, int y, int width, int height)