The following code snippet show you how to draw a simple line using Graphics2D.draw() method. This method take a parameter that implements the java.awt.Shape interface.

To draw a line we can use the Line2D.Double static-inner class. This class constructor takes four integers values that represent the start (x1, y1) and end (x2, y2) coordinate of the line.

package org.kodejava.awt.geom;

import javax.swing.*;
import java.awt.*;
import java.awt.geom.Line2D;

public class DrawLine extends JComponent {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Draw Line");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(new DrawLine());
        frame.pack();
        frame.setSize(new Dimension(420, 440));
        frame.setVisible(true);
    }

    @Override
    public void paint(Graphics g) {
        // Draw a simple line using the Graphics2D draw() method.
        Graphics2D g2 = (Graphics2D) g;
        g2.setStroke(new BasicStroke(2f));
        g2.setColor(Color.RED);
        g2.draw(new Line2D.Double(50, 150, 250, 350));
        g2.setColor(Color.GREEN);
        g2.draw(new Line2D.Double(250, 350, 350, 250));
        g2.setColor(Color.BLUE);
        g2.draw(new Line2D.Double(350, 250, 150, 50));
        g2.setColor(Color.YELLOW);
        g2.draw(new Line2D.Double(150, 50, 50, 150));
        g2.setColor(Color.BLACK);
        g2.draw(new Line2D.Double(0, 0, 400, 400));
    }
}

When you run the snippet it will show you something like:

Draw 2D Line