::::::::::::::
Triangle.java
::::::::::::::
class Experiment {
  public static void main(String[] args) {
      Triangle a = new Triangle(new Point(0, 3), 
                                new Point(4, 0), 
                                Point.origin); 
      System.out.println(a.area()); // should be 6.0 
  }
}

class Triangle {
    Line a, b, c; 
    Triangle (Point a, Point b, Point c) {
        this.a = new Line(a, b); 
        this.b = new Line(a, c); 
        this.c = new Line(b, c); 
    }
    double area() {
        double s = (this.a.length() + this.b.length() + this.c.length()) / 2; 
        return Math.sqrt(s * (s - a.length()) * 
                             (s - b.length()) * 
                             (s - c.length())); 
    }
}

class Line {
    Point a, b;
    double length() {
        return a.distanceTo(b);
    }
    Line(Point u1, Point u2) {
        this.a = u1;
        this.b = u2;
    }
}

class Point {
    double x, y;
    Point(double x, double y) { this.x = x; this.y = y; }
    Point() { this(0, 0); }
    static Point origin = new Point(); 
    double distanceTo(Point other) {
        double dx = this.x - other.x;
        double dy = this.y - other.y;
        return Math.sqrt(dx * dx + dy * dy);
    }
}

::::::::::::::
Clock.java
::::::::::::::
class One {
    public static void main(String[] args) {
	Clock one = new Clock("2350"); 
	for (int i = 0; i < 20; i++) {
	    one.tick(); 
	    System.out.println(one.report()); 
        }
    }
}

class Clock {
    int hours, minutes; 
    Clock(String time) {
	hours = Integer.parseInt(time.substring(0, 2)); 
	minutes = Integer.parseInt(time.substring(2)); 
    }
    void tick() {
        minutes += 1; 
        if (minutes == 60) { 
	    minutes = 0; 
	    hours += 1; 
	}
        if (hours == 24) {
	    hours = 0; 
	}
    }
    String report() {
        String hours = "00" + this.hours; 
	String minutes = "00" + this.minutes;
	return 
	    hours.substring(hours.length() - 2) + ":" + 
	    minutes.substring(minutes.length() - 2); 
    }
}

::::::::::::::
Line.java
::::::::::::::
class One {
    public static void main(String[] args) {
	Line a = new Line(new Point(0, 0), new Point (1, 1)); 
	System.out.println(a.length()); 
    }
}

class Line {
    Point a, b; 
    Line(Point a, Point b) {
	this.a = a; 
	this.b = b; 
    }
    double length() {
	return (this.a).distanceTo(this.b); 
    }
}

class Point {
    double x, y;
    Point(double x, double y) {
	this.x = x; this.y = y; 
    }
    double distanceTo(Point other) {
	double 
	    dX = this.x - other.x, 
	    dY = this.y - other.y; 
	return Math.sqrt(dX * dX + dY * dY); 
    }
}

::::::::::::::
Paper.java
::::::::::::::
class One {
    public static void main(String[] args) {

	Player bonaparte, wellington;

	bonaparte = new Player(); 
	wellington = new Player(); 

	System.out.println("Let the game begin!"); 

	bonaparte.makeGuess(); 
	wellington.makeGuess(); 

        System.out.println("The guesses have been made: "); 

	System.out.println("  Bonaparte has chosen .... " + bonaparte.report());
	System.out.println("  Wellington has chosen ... " + wellington.report()); 

	if (bonaparte.strongerThan(wellington)) 
	    System.out.println("Bonaparte wins!"); 
        else if (wellington.strongerThan(bonaparte))
	    System.out.println("Wellington wins!"); 
	else System.out.println("It's a draw...");     }
}

class Player {
    String guess; 
    String makeGuess() {
	int value = (int) (Math.random() * 3); 
        if (value == 0) this.guess = "paper"; 
        if (value == 1) this.guess = "rock"; 
        if (value == 2) this.guess = "scissors"; 
	return this.guess; 
    }
    boolean strongerThan(Player other) {
	if (this.guess.equals("paper")    && other.guess.equals("rock"    ) || 
	    this.guess.equals("rock" )    && other.guess.equals("scissors") ||
	    this.guess.equals("scissors") && other.guess.equals("paper")) 
	    return true; 
	else return false; 
    }
    String report() {
	return guess; 
    }
}

::::::::::::::
Tigger.java
::::::::::::::
class Tigger {
    String x, y; 
    Tigger(int x, int y) {
	this.x = x + ""; 
	this.y = y + ""; 
    }
    void bounce() {
	int a = calculate(x), 
	    b = calculate(y); 
	this.x = a + "";
	this.y = b + ""; 

    }
    int calculate(String a) {
	int sum = 0; 
	for (int i = 0; i < a.length(); i++) {
	    sum += (a.charAt(i) - '0') * (a.charAt(i) - '0'); 
        } 
        return sum; 
    }
    String report() {
	String 
	    x = "   " + this.x, 
	    y = "   " + this.y; 
	return 
	    "Tigger just bounced to (" + 
	    x.substring(x.length() - 3) + ", " 
	    + y.substring(y.length() - 3) + ")"; 
    }
} 

::::::::::::::
Elevator.java
::::::::::::::
class One {
    public static void main(String[] args) {
	Elevator e = new Elevator(20); 
	e.up(26); 
	e.down(14); 
        e.up(10); 
	e.down(30); 
        e.up(e.currentFloor() + 3); 
    }
}

class Elevator {
    int floor; 
    Elevator(int floor) {
	this.floor = floor; 
    }  
    void up(int to) {
	if (this.floor >= to) {
	    System.out.println("Sorry, from floor " + this.floor + 
			       " we can't go up to floor " + to); 
	} else {
	    System.out.println("Elevator going up (" + 
			       this.floor +
			       " --> " + to + ")"); 
	    for (int i = this.floor; i <= to; i++) {
		this.floor = i; 
		this.report(); 
	    }
	    System.out.println("Elevator now on floor: " + this.floor); 
	}
    }
    void down(int to) {
	if (this.floor <= to) {
	    System.out.println("Sorry, from floor " + this.floor + 
			       " we can't go down to floor " + to); 
	} else {
	    System.out.println("Elevator going down: (" + 
			       this.floor +
			       " --> " + to + ")"); 
	    for (int i = this.floor; i >= to; i--) {
		this.floor = i; 
		this.report(); 
	    }
	    System.out.println("Elevator now on floor: " + this.floor); 
	}
    }
    void report() {
	System.out.println("   The elevator is now on floor " + this.floor); 
    }
    int currentFloor() { return this.floor; } 
}

::::::::::::::
Robot.java
::::::::::::::
class Robot {
    String direction;
    int x, y;
    String name;
    Robot(String n, int x, int y, String dir) {
        name = n; this.x = x; this.y = y; direction = dir; 
    } 
    void moveForward() {
        System.out.println("Robot " + name + " now moves forward."); 
        if (direction.equals("North")) {
            y -= 1; 
        } else if (direction.equals("South")) {
            y += 1; 
        } else if (direction.equals("East")) {
            x += 1; 
        } else if (direction.equals("West")) {
            x -= 1; 
        } else {
            // nothing 
        }
    } 
    void turnLeft() {
        System.out.println("Robot " + name + " now turns left."); 
        if (direction.equals("North")) {
            direction = "West"; 
        } else if (direction.equals("South")) {
            direction = "East";          
        } else if (direction.equals("East")) {
            direction = "North";         
        } else if (direction.equals("West")) {
            direction = "South";         
        } else {
            // nothing 
        }
    }
    int getX() { return x; }
    int getY() { return y; }
    String direction() { return direction; } 
    void report() {
        System.out.println( "Robot " + name + " located at (" + x + 
                            ", " + y + ") facing " + direction ); 
    } 
}

So here's an example of using this class: 

class Walk {
    public static void main(String[] args) {
        Robot a = new Robot("Alice", 2, 3, "North"); 
        a.report(); 
        Robot q = new Robot("Queen", -4, -1, "West"); 
        q.report(); 
        a.turnLeft(); 
        a.report(); 
        a.moveForward(); 
        a.report(); 
        a.turnLeft(); 
        a.report(); 
        a.moveForward();
        a.report(); 
        a.moveForward();
        a.report();  
        a.moveForward();
        a.report(); 
        q.moveForward();
        q.report(); 
        q.turnLeft(); 
        q.report(); 
    } 
}