June 13th , 2011

Abstract Data Types

Example: Stack ADT
- elements are added and removed from the same end; LIFO (Last In First Out)

push() ==> Add (Push) an element
pop() ==> Remove (pop) an element
peek() ==> Look at the top element w/o removing it


is Empty()
size()

Assume some type T to be held by the stack
where T is just a placeholder

push(T elem)
T pop()
T peek ()
boolean is Empty()

Generic Types

- Some way to postpone specifying a type without some of the hazards of polymorphism.

Animal[] creatures = new Mammal[25];

.
..
...
creatures[1] = new Lizard () ;

This causes a runtime error because Lizard is not a subclass of Mammal.

Generic types allow us to help the compiler by providing additional hints about data types.

Recall the comparable interface

class Apple implements Comparable {
.
..
...
public int compareTo (Object otherObject)
.
..
...
}

Comparable is actually a generic interface :

public interface Comparable   { 
public int compareTo ( T thing) ;
}

To compare Apples to Apples (only!)

class Apple implements Comparable {
.
..
..
public int compareTo (Apple otherApple) {.....} ;

Text uses this language:

"Then, when a generic class is needed, it is instantiated with a specific class used in place of T."

Would be nice to allow:

class Apple implement Comparable , Comparable {.......

But doesnt work!

Subtly wrong - information about the actual type used for T is not part of the runtime hidden bookkeeping information. Generics are enforced by the compiler.