©2006 Felleisen, Proulx, et. al.

4  Abstracting over Data Definitions

A bank customer can have three different accounts: a checking account, a savings account, and a line of credit account.

4.1  Review of Designing Methods for Unions of Classes.

The code in lab4-banking.bjava defines the classes that represent this information.

  1. Make examples of data for these classes, then make sure you understand the rules for withdrawals.

    Now design the methods that will manage the banking records:

  2. Design the method canWithdraw that determines whether the customer can withdraw some desired amount.

  3. Design the method makeDeposit that allows the customer to deposit a given amount of money into the account.

  4. Design the method maxWithdrawal that computes the maximum that the customer can withdraw from an account.

  5. Design the method moreAvailable that produces the account that has more money available for withdrawal.

4.2  Abstracting over Data Definitions: Lifting Fields

Save your work and open it again with the file type 'ijava'. Change the language level to Intermediate ProfessorJ.

Look at the code and identify all places where the code repeats -- the opportunity for abstraction.

Lift the common fields to an abstract class ABanking. Make sure you include a constructor in the abstract class, and change the constructors in the derived classes accordingly. Run the program and make sure all test cases work as before.

4.3  Abstracting over Data Definitions: Lifting Methods

For each method that is defined in all three classes decide to which category it belongs:

  1. The method bodies in the different classes are all different, and so the method has to be declared as abstract in the abstract class.

  2. The method bodies are the same in all classes and it can be implemented completely in the abstract class.

  3. The methods look very similar, but each produces a different variant of the union -- therefore it cannot be lifted to the super class.

  4. The method bodies are the same for two of the classes, but are different in one class -- therefore we can define the common body in the abstract class and override it in only one derived class.

Now, lift the methods that can be lifted and run all tests again.

Use the book and lecture notes for Lecture 10 as a guide.

4.4  Quiz

You have 10 minutes.

4.5  Part 2: Designing the Pong Game

A Game of Pong

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A ball starts at a random height on the left and falls from the left side diagonally down. At the bottom is a paddle that can move left and right controlled by the arrow keys.When the ball hits the bottom, but misses the paddle, it disappears from the game. When the ball hits the paddle, it bounces back and continues diagonally up to the right. When the ball exits the playing field, a new ball comes into play.

Classes needed:

Ball - a Posn and the direction in which the ball moves (up or down)

Paddle - a Posn

PongWorld - contains one Ball and one Paddle, has a fixed width and height

The code in the file pong-game-skeleton.ijava defines the classes that represent the ball, the paddle, and the world. (For now, we ignore the world).

The class PongWorld extends the class World in the teachpack. Therefore, we need to use ProfessorJ Intermediate Language.

4.6  Designing methods in the class Ball

  1. Design the method draw that displays the ball on a canvas. The following code (that can be written within the Examples class shows how you can draw one circle:

    import draw.*;
    import colors.*;
    import geometry.*;
    class Examples{
      Examples() {}
      Canvas c = new Canvas(200, 200);
      boolean makeDrawing = 
        this.c.show() && 
        this.c.drawDisk(new Posn(100, 150), 50, new Red());

    The three import statements on the top indicate that we are using the code programmed by someone else and available in the libraries named draw, colors, and geometry. Open the Help Desk and look under the Teachpacks for the teachpacks for How to Design Classes to find out more about the drawing and the Canvas.

  2. Design the method moveBall that moves the ball five pixels in its direction.

  3. Design the method bounce that produces a ball after it bounced up to the right, and with its direction set to move up to the right.

  4. Design the method hitBottom that determines whether the ball hit the bottom of the canvas of the given height.

  5. Design the method outOfBounds that determines whether the ball is out of bounds of the canvas of the given width and height.

4.7  Designing methods in the class Paddle

  1. Design the method draw that displays the paddle on a given canvas of the given height.

  2. Design the method movePaddle that consumes a String and moves the paddle either left or right depending on the String it receives as argument. For now, ignore the requirement that the paddle stays within the bounds of the canvas. (You may add it later, once the program is working.)

  3. Design the method hitBall that determines whether the paddle hit the given ball. For simplicity, just make sure that the distance between the center of the ball and the center of the top of the paddle is less than or equal to the radius of the ball. You may need to delegate the work to the class Ball.

4.8  Designing methods in the class PongWorld

  1. Design the methods draw and erase that show the background, the ball, and the paddle. Make the background black.

  2. Finally, add some interactions to your program, by letting the paddle move in response to the key events. Design the method onKeyEvent that consumes a String and moves the paddle left, or right by 5 pixels every time the user hits one of the corresponding arrow keys.

    Use the code in the program WorldDemo.ijava to figure out how to respond to the key events and to see how to run the program.

  3. Design the method onTick as follows:

    • If the ball is out of bounds, replace the ball with a new ball. Initially, start the new ball in the top left corner. When everything else is working, make the ball start on the left edge at a random height between the top and the middle.

    • If the ball hit the paddle, replace the ball with a new one going in the opposite direction - and moved ahead.

      Hint: Recall the method bounce in the class Ball.

    • Otherwise, just move the ball in its direction.

    • Currently the world never ends. When all is working, think of what may be the appropriate end of the game (count the number of balls that were put into play, the number of balls that hit the paddle, or the number of elapsed ticks. Then modify the appropriate methods so that the world eventually ends.

Save all your work -- the next lab may build on the work you have done here!

Last modified: Thursday, September 28th, 2006 11:25:37pm