This is my Solution on CS106A - Section 20 - Problem 4 - Drawing Lines from the Computer Science Course CS106A of Prof. Mehran Sahami at the STANFORD University.

DrawLines.java

/*
 * File: DrawLines.java
 * -------------------
 * Name: Axel Werner [mail@awerner.myhome-server.de]
 * Section Leader:
 * 
 * CS106A - Section 20 - Problem 4 - Draw Lines
 * 
 * Write a GraphicsProgram that allows the user to 
 * draw lines on the canvas. Pressing the mouse 
 * button sets the starting point for the line. 
 * Dragging the mouse moves the other endpoint 
 * around as the drag proceeds. Releasing the 
 * mouse fixes the line in its current position 
 * and gets ready to start a new line. 
 * 
 * For example, suppose that you press the mouse 
 * button somewhere on the screen and then drag it
 * rightward an inch, holding the button down. 
 * What you’d like to see is the following picture:
 * 
 * {picture1}
 * 
 * If you then move the mouse downward without 
 * releasing the button, the displayed line will 
 * track the mouse, so that you might see the 
 * following picture:
 * 
 * {picture2}
 * 
 * Because the original point and the mouse position 
 * appear to be joined by some elastic string, this 
 * technique is called rubber-banding. 
 * Although this program may seem quite powerful, it 
 * is also simple to implement. The entire program 
 * requires fewer than 20 lines of code.
 *
 * PERSONAL FEATURE BONUS:
 * 
 * The temporary Line while in "rubber band mode" is 
 * colored in RED to indicate the mode. When mouse
 * button is released the Line is drawn in standard
 * Color BLACK as supposed to. 
 * 
 */
 
import acm.graphics.*;
import acm.program.*;
import acm.util.*;
 
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
 
public class DrawLines extends GraphicsProgram {
 
 
	public void run() {
		addMouseListeners();
	}
 
	public void mousePressed (MouseEvent e) {
		tmpLine.setStartPoint(	e.getX(), e.getY()	);
		tmpLine.setEndPoint(	e.getX(), e.getY()	);
		tmpLine.setColor( Color.RED ); 
		add(tmpLine);
	}
 
	public void mouseDragged(MouseEvent e) {
		tmpLine.setEndPoint( e.getX(), e.getY()	);
	}
 
	public void mouseReleased(MouseEvent e) {
		GLine newLine = new GLine(	tmpLine.getX(), tmpLine.getY(),
									e.getX(), e.getY() );
		add(newLine);
		remove(tmpLine);
	}
 
 
	/*
	 * Define some Instance Variables
	 */
 
	/**
	 * Define a temporary Line to work with
	 */
	GLine tmpLine = new GLine(0,0,0,0);
 
}

— Axel Werner 2012-04-04 00:10