[hardwired]

Code:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class LineTest extends JPanel
{
    Point p1 = new Point();
    Point p2 = new Point();

    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setPaint(Color.red);
        g2.fillOval(p1.x-2, p1.y-2, 4, 4);
        g2.setPaint(Color.green.darker());
        g2.fillOval(p2.x-2, p2.y-2, 4, 4);
        g2.setPaint(Color.blue);
        drawLine(p1.x, p1.y, p2.x, p2.y, g2);
    }

    private void drawLine(int x1, int y1, int x2, int y2, Graphics2D g)
    {
	int dX = 0, dY = 0;   //device x and y
	int xMin = Math.min(x1, x2);
	int yMin;             // Everything is upside-down.
	//Calculate the slope
        // Origin of this is at upper left; origin of
        // cartesian coordinate system is at lower left.
        // Therefore, reverse the sign for the rise:
        double rise = -(double)(y2 - y1);
        double run  =  (double)(x2 - x1);
	double slope = rise / run;
        System.out.printf("slope = %.3f  rise = %.0f  run = %.0f%n",
                           slope, rise, run);
	if (slope > 0)        // posetive slope rises left to right
	{
            yMin = Math.max(y1, y2);
	    if (Math.abs(rise) >= Math.abs(run))  // slope > 1.0
	    {
		for (int y = 0; y <= Math.abs(rise); y++)
		{
		    dY = yMin - y;
		    dX = (int)(xMin + (y/slope));
		    g.drawOval(dX-2, dY-2, 4, 4);
		}
	    }
            else  // 0 < slope < 1.0
            {
                for(int x = 0; x <= Math.abs(run); x++)
                {
                    dY = (int)(yMin - (slope*x));
                    dX = xMin + x;
                    g.drawOval(dX-2, dY-2, 4, 4);
                }
            }
	}
    }

    public static void main(String[] args)
    {
        LineTest test = new LineTest();
        test.addMouseListener(test.ma);
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(test);
        f.setSize(400,400);
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    private MouseAdapter ma = new MouseAdapter() {
        int pointCount = 0;

        public void mousePressed(MouseEvent e) {
            Point p = e.getPoint();
            switch(pointCount) {
                case 0:
                    p1 = p;
                    break;
                case 1:
                    p2 = p;
            }
            System.out.printf("p1 = [%d, %d]  p2 = [%d, %d]%n",
                               p1.x, p1.y, p2.x, p2.y);
            repaint();
            pointCount++;
            if(pointCount > 1)
                pointCount = 0;
        }
    };
}