Overview

• Admin.
• Interfaces.
• Polymorphism.
• An example: Text blocks.

Admin

• Reading for Friday: Inheritance
• Sorry for the mixup/delay on today's readings. I'll do my best to guide you through the same material.
  • (No lab.)
• EC Opportunities
  • Convocation noon, Wednesday.
  • Learning from Alumni Thursday @ 2:15 Sam Tape and company. (3821)
  • CS Extras Thursday @ 4:30: Kim Spasaro on Linguistics Programming (3821)
  • CS Table Friday (pair programming)
  • Other?
• Are there questions on HW4?
  • The registers should hold fractions

Interfaces

• Goal in program design: Separate WHAT your code does from HOW your code achieves that goal.
  • You can change your implementation without affecting your client code
  • Your clients can't "mess up" your code.
  • You think differently about programming if you separate the two
• Example: Points in the plane
  • Get the coordinates of the point:
    • distance from the x axis and y axis (x, y)
      • getX()
      • getY()
    • distance from the origin and angleq
      • getAngle()
      • getDistance()
• Java encourages this approach

• Interfaces provide the WHAT, not the how

  public interface Point2D {
      double getX();
      double getY();
      double getAngle();
      double getDistance();
      Point2D add(Point2D other);
      // DOT PRODUCT!
      Point2D multiply(Point2D other);
  } // interface Point2D
  
  
  public class XYPoint implements Point2D {
     ...
  }
  
  
  public class Vector2D implements Point2D {
     ...
  }
  

• Two magic things happen with the "implements" keywords

  • Java forces you to implement all of the methods in Point2D.
  • If someone writes code that expects a Point2D, it will work with an XYPoint

Polymorphism

• Polymorphism is an approach to save code / avoid repetitous code.

• Consider squaring numbers

  • Scheme

    (define square (lambda (num) (* num num)))a

  • C

    int squareInt(int x) { return xx; } double squareDouble(double x) { return xx; }

  • Java permits overloading

    int square(int x) { return xx; } double square(double x) { return xx; }

  • BUt the code is repetitious!

  • Why not code with copy-paste-change?
    • Inelegant - You can do it better by writing a single procedure (we hope)
    • What if we have to change something?
    • Code bloat

• Scheme's approach is great, but not

An example: Text blocks

    public interface TextBlock {
       int getWidth();
       int getHeight();
       String getRow(int i) throws Exception;
    } // interface TextBlock

    public class TextLine implements TextBlock {
       ...
    }

    public class VerticallyComposeTextBlock implements TextBlock {
      ...
    }

    TextBlock fiona = new TextLine("Hello");
    TextBlock john = new TextLine("Goodbye");
    TextBlock adam = new VCTB(fiona,john);
    TextBlock mark = new TextLine("Mark");
    TextBlock sunshine = new VCTB(adam, mark);