使用Path绘制如下图所示三角形

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Path2D;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Triangle extends JPanel {

    private static final long serialVersionUID = 1L;

    public void paint(Graphics g) {
        Graphics2D g2d = (Graphics2D) g;

        Path2D path = new Path2D.Float();
        path.moveTo(50, 50);
        path.lineTo(150, 50);
        path.lineTo(100, 150);
        path.closePath();

        g2d.draw(path);
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("Triangle");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(new Triangle());
        frame.setSize(300, 300);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}

运行结果如下图所示：