CSC152 2005F, Class 37: Inheritance

Admin:
* Exam 2 ready.  Due next Friday.
* No additional homework for the next week.
* Swing tonight at 9:30 in Harris this weekend. Extra credit!
  * Lessons tonight in South lounge at 5:30 for beginners,
    6:30 for advanced.  Extra credit!
* Two more football games for extra credit, too.

Overview:
* Reuse
* Inheritance
* Inheritance in Java
* Special issue: Constructors
* Example
* Inheritance and interfaces

About the exam
* Making change
  * Suppose in Samsville, the demoniations are 25, 10, and 1
    *To find the fewest coins to make N cents
      (a) Find out how many coins to make N-25 cents, call that a
      (b) Find out how many coins to make N-10 cents, call that b
      (c) Find out how many coins to make N-1 cents, call that c
      * The fewest coins to make N cents is 1 + min(a,b,c)
   * In Foosburg, the denominations are 49, 18, 7, and 3
    *To find the fewest coins to make N cents
      (a) Find out how many coins to make N-49 cents, call that a
      (b) Find out how many coins to make N-18 cents, call that b
      (c) Find out how many coins to make N-7 cents, call that c
      (d) Find out how many coins to make N-3 cents, call that d
      * The fewest coins to make N cents is 1 + min(a,b,c,d)

Reuse
* When we need to do something similar to what we've already done,
  it doesn't require lots of extra work
  * One strategy: Copy, paste, and modify
    * Problem: If the original code was wrong or slow, you need
      to remember to propagate any changes you make
    * Problem: Your programs are big and repetitious
* We need more general ways to write things
  * Strategy one: Interfaces and polymophism
* This strategy does not cover all the cases in which we have code resuse
  * Does not cover "I want to design an object similar to one I've designed before"
* Inheritance permits you to define new classes in terms of old *without copying code*

If class A inherits from class B
* A automatically gets all the fields (not just static) of B
* A automatically gets all the methods of B
* A can add new methods
* A can add new fields
* A can *change* the methods of B
* Polymorphism: A's can be used wherever B's are expected

In Java, we write

public class A
  extends B
{
}