The Collections Framework A Brief Introduction

Collections A collection is a structured group of objects –An array is a kind of collection –A Vector is a kind of collection –A linked list is a kind of collection Java 1.2 introduced the Collections Framework and provided many great implementations –Vector s have been redefined to implement Collection –Trees, linked lists, stacks, hash tables, and other classes are implementations of Collection –Arrays do not implement the Collection interfaces

Types of Collection Java supplies several types of Collection : –Set : cannot contain duplicate elements, order is not important –SortedSet : like a Set, but order is important –List : may contain duplicate elements, order is important Java also supplies some “collection-like” things: –Map : a “dictionary” that associates keys with values, order is not important –SortedMap : like a Map, but order is important

The Collections framework There are two groups: Collection s of single elements, and Map s containing key/value pairs These are all interfaces, but there are classes implementing each interface Collection SortedSet ListSet SortedMap Map

Some implementations Each class (in green) implements one or more interfaces (in white), hence implements all the methods declared in those interfaces We will look at the interface methods Collection Set SortedSetHashSet TreeSet List ArrayListLinkedList Vector Stack Map HashtableHashMap SortedMap TreeMap

Collections are ADTs The interfaces of the Collections framework describe the functionality of a data structure but not its implementation Collections are one of the best-designed parts of Java, because –They are elegant: they combine maximum power with maximum simplicity –They are uniform: when you know how to use one, you almost know how to use them all –You can easily convert from one to another

The Collection interface Some simple Collection methods: –int size( ) –boolean isEmpty( ) –boolean contains(Object e) –boolean add(Object e)* –boolean remove(Object e)* –Iterator iterator( ) Some “bulk” Collection methods: –boolean containsAll(Collection c) –boolean addAll(Collection c)* –boolean removeAll(Collection c)* –boolean retainAll(Collection c)* –void clear( ) * Returns true if the Collection was changed, false otherwise

The Set interface A Set is a Collection that does not contain duplicate (“equal”) elements –“Equal” is defined by whatever equals(Object) method applies to the elements of the collection –You need to be sure you have a working equals method –For a HashSet, you also need a working hashCode method The methods of Set are the same as those of Collection, but their effects may vary –Example: If you add an object to a set, and the set already contains that object (as defined by equals ), it will not be added

The SortedSet interface A SortedSet keeps its elements in increasing order Therefore, it must be possible to sort the elements! This means: –The elements must be objects of a type that implement the Comparable interface (alternatively, you need to supply a Comparator when you create the SortedSet ) –Elements must be mutually comparable (e.g. you can’t compare a String to a Button ) –The ordering must be consistent with equals Additional methods include first() and last()

The List interface List implements all the methods defined by Collection, and adds a few of its own: –Object get(int index) –Object set(int index, Object element) –void add(int index, Object element) –Object remove(int index) –boolean addAll(int index, Collection c) –int indexOf(Object o) –int lastIndexOf(Object o) In addition, List supplies a ListIterator that can iterate through the elements forwards or backwards

The Map interface A Map is an object that maps keys to values A map cannot contain duplicate keys –Depending on the implementation, null may or may not be an allowable key Each key can map to at most one value Examples: dictionary, phone book, etc.

Map: Basic operations Object put(Object key, Object value) –Returns the old value for this key, or null if no old value Object get(Object key) Object remove(Object key) boolean containsKey(Object key) boolean containsValue(Object value) int size( ) boolean isEmpty( )

Additional Map methods void putAll(Map t) –Copies one Map into another (adding to whatever is already there) –Example: newMap.putAll(oldMap) void clear() –Example: oldMap.clear() public Set keySet( ) public Collection values( )

A simple map: Hashtable To create a Hashtable, use: import java.util.*; Hashtable table = new Hashtable(); To put things into a Hashtable, use: table.put (key, value ); To retrieve a value from a Hashtable, use: value = ( type )table.get( key );

The SortedMap interface A hash table keeps elements in an (apparently) random order Sometimes you want the keys of a map to be in sorted order (e.g. phone book, dictionary) A map can be implemented with a hash table, but it doesn’t have to be The SortedMap interface implements the Map interface and provides additional methods For efficiency, you want an implementation that keeps its elements in some kind of order

Requirements for SortedMap A SortedMap keeps its elements in the order of increasing key values Therefore, it must be possible to sort the keys! This means: –The keys must be objects of a type that implement the Comparable interface (or the SortedMap must be created with a Comparator ) –Keys must be mutually comparable (e.g. you can’t compare a String to a Button ) –The ordering must be consistent with equals

Some SortedMap Methods Comparator comparator() –Returns the comparator associated with this sorted map, or null if it uses its keys' natural ordering. Object firstKey() –Returns the first (lowest) key currently in this sorted map. Object lastKey() –Returns the last (highest) key currently in this sorted map. There are some other methods, but they are not particularly useful What is more important than these methods is that the Iterator for a SortedMap returns elements in sorted order

Vectors The class Vector has been retrofitted to implement the Collection interface Vector supplies add(Object) and iterator() methods (among others) Let’s look at creating a Vector and iterating through the elements

The Iterator interface interface iterator { // java.lang.util boolean hasNext(); // Returns true if the iteration has more // elements. Object next(); // Returns the next element in the // interation. void remove(); // Removes from the underlying collection // the last element returned by the // iterator (optional operation).

Using a Vector Collection numerals = new Vector(); numerals.add("one"); numerals.add("two"); numerals.add("three"); Iterator iter = numerals.iterator(); while (iter.hasNext()) { System.out.println(iter.next()); } Results: one two three

Using a TreeSet Collection numerals = new TreeSet(); numerals.add("one"); numerals.add("two"); numerals.add("three"); Iterator iter = numerals.iterator(); while (iter.hasNext()) { System.out.println(iter.next()); } Results: one three two

Constructors Although constructors cannot be defined in an interface, every Java-supplied class that implements Collection has a constructor that takes any other Collection as an argument –Example: Vector myVector = new Vector(myTreeSet); Similarly, most classes that implement the Map interface have a constructor that takes any other Map as an argument –Example: TreeMap myMap = new TreeMap(myHashtable); Thus, it is very easy to convert from one kind of Collection or Map to another

Back to arrays Arrays are not part of the new Collections Framework, but they haven’t been ignored Java 1.2 introduced the new Arrays class, with some useful operations, for example: –static void sort(Object[] a) –static int binarySearch(Object[] a, Object key) –static boolean equals(Object[] a, Object[] a2) –static void fill(Object[] a, Object val) –static void fill(Object[] a, Object val, int from, int to) –static List asList(Object[] a) Although I’ve shown these methods for arrays of Objects, there are corresponding methods for arrays of each of the primitive types

The End