The Collections API

1. The Collections API CS 242 Fall 2008 Karl R. Wurst

2. The Collections API • Provides data structures that can be reused • All have a standard interface and provide a consistent set of operations (methods) • Underlying implementations differ and provide different benefits

3. The Iterator Pattern • An Iterator allows stepping though all elements in a collection • You have done this with arrays: for (int i = 0; i < v.length; i++) System.out.println( v[ i ] ); • But this only works for arrays - we want something more general

4. The Iterator Pattern • A more general Iterator could look something like this: IteratorType itr; for (itr = v.first(); itr.isValid; itr.advance()) System.out.println( itr.getData() ); • We have two objects here • v - our Collection which holds the data • itr - our Iterator which allows us to step through the

5. Basic Iterator Design • Our first version has two classes and three methods • MyContainer - holds the data • MyContainerIterator iterator() - returns a MyContainerIterator object • MyContainerIterator - has methods to step through the data • Boolean hasNext() - are there more items to iterate over • Object next() - returns the next item in the collection and advances the current position

6. Using the Container and Iterator

7. MyContainer Class • Uses an array internally

8. MyContainerIterator Class

9. Inheritance-Based Iterator

10. Inheritance-Based Iterator

11. Using the Inheritance-Based Iterator

12. The Collection Interface • Represents a group of objects (elements) • May be ordered or unordered, depending on implementation • May allow duplicates, depending on implementation • All provide a minimum set of operations

14. The Iterator Interface

15. Interface issues • To implement an interface, you are required to provide all methods specified in the interface • Some methods may not make sense for our implementation - say remove() • Implement the method to throw UnsupportedOperationException • And be sure to document it so anyone using your class is not surprised!

16. Using the Collection and Iterator • The second version works because Iterator implements Iterable

17. The List Interface • A list is a collection of items with positions

18. The List Interface

19. The ListIterator Interface • Works forwards and backwards • The method to get the Iterator takes a starting position • 0 - start at the beginning and go forward • size of the list - start at the end and go backward

22. LinkedList • Objects are stored in nodes containing the object and a reference to the next node

23. LinkedList • Additional operations: • void addLast(AnyType element) • void addFirst(AnyType element) • AnyType getFirst() • AnyType element() • AnyType getLast() • AnyType removeFirst() • AnyType remove() • AnyType removeLast()

24. Stacks and Queues • Do not allow access to all elements directly • Stack - Last In-First Out (LIFO) • Queue - First In-First Out (FIFO)

25. Stack • LIFO

26. Stack

27. Stack • Java provides Stack class, but it is based on Vector, so it can be slow • LinkedList can be used to implement Stack • add() for push() • element() for top() • remove() for pop()

28. Queue • FIFO

29. Queue

30. Queue • Java does not provide Queue class • LinkedList can be used to implement Queue • addLast() for enqueue() • element() for getFront() • remove() for dequeue()

31. Set • Sets contain no duplicates • SortedSet keeps items in sorted order • Elements are either Comparable (Natural Order) • Or, a Comparator is provided • Iteratorwill provide elements in sorted order • AnyType first() - smallest element • AnyType last() - largest element • TreeSet implements SortedSet • O(log N)

32. HashSet • Does not keep elements in sorted order • On average, performs in constant time • Will work with no additional information, but works better if you provide int hashCode() • int hashCode() should always provide the same value if equals() is true • int hashCode() should try to provide different values for items that are not equal

33. Maps • Maps store pairs of keys and values • Keys must be unique • Values do not have to be unique • HashMap does not maintain sorted keys • TreeMap maintains sorted keys • Provides no Iterator, instead • Returns a Set of keys - keySet() • Returns a Collection of values - values() • Returns a Set of Entry - entrySet() • Entry is a class containing Key/Value pairs

35. PriorityQueue • Elements are added to the queue, but minimum value is removed first • Used often in operating systems • Java provides a PriorityQueue class

36. Summary