Collection and List

• Recall we said a List is a type of Collection.
• Informally we understand this to mean
  • A List does everything a Collection does... and then some
  • For example
    • List has an isEmpty method
    • List has an add() method
    • But List also has a get method
    • and List also has an add(int index) method
  • Such a relationship is called inheritance
    • List inherits from Collection
    • Collection is called a superinterface of List
    • List is called a subinterface of Collection
  • Consequences of the inheritance relationship
    • Anything that is a List is also a Collection
    • Anything that is a Collection is not necessarily a List (e.g. Set)
  • Java allows us to formally state this relationship

    interface List extends Collection {
    	void add(int index, Object o);
    	int indexOf(Object o);
    	Object get(int index);
    	Object remove(int index);
    	Object set(int index);
    }
    		

  • Again, this in in addition to the methods inherited from the Collection interface.

  Other Examples That We've Seen

  Subinterface/Superinterface
  • SortedSet and Set
  • SortedMap and Map
  • Set and Collection
    • This one is interesting
    • No new methods, simply additional constraints upon inherited methods
      • No duplicate elements

  Coding Java Interfaces

  • Just like classes
  • List interface goes in List.java
  • Compiles and produces List.class file

  ArrayList - An Implementation of a Subinterface

  class ArrayList implements List {
  	...
  }
  

  • Must implement ALL of List's methods
  • Includes ALL of Collection's methods
  • Once implemented:
    • An ArrayList can be used wherever a List or a Collection is called for

    List [] listArr = new List[10];
    ...
    listArr[i] = new ArrayList();
    		

    void doSomething(Collection c) {
    	Iterator iter = c.iterator();
    	while (iter.hasNext()) {
    		... = iter.next();
    		...
    	}
    }
    ...
    // elsewhere
    ArrayList alist = new ArrayList();
    ...
    doSomething(aList);