Collections Framework

A very brief look at Java’s Collection Framework

David DavenportMay 2010

Core Collection Interfaces

Set: A collection that cannot contain duplicate elements.

List: An ordered collection (sometimes called a sequence). Lists can contain duplicate elements. The user of a List generally has precise control over where in the List each element is inserted, and can access elements by their integer index (position).

Queue: A collection used to hold multiple elements prior to processing.

Map: An object that maps keys to values. Maps cannot contain duplicate keys: Each key can map to at most one value.

The Hierarchy

Collection

Set

List

SortedSet

AbstractCollection

AbstractList

AbstractSet

AbstractSequentialList

TreeSet

HashSet

Vector

ArrayList

LinkedList

Stack

LinkedHashSet

Map

SortedMap

AbstractMap

TreeMap

HashMap LinkedHashMap

Queue AbstractQueue PriorityQueue

Examples…

for ( Object o : collection) System.out.println( o);

Iterator i = collection.iterator(); while ( i.hasNext() )

System.out.println( i.next() );

import java.util.*;public class FindDups { public static void main( String args[]) { Set<String> s = new HashSet<String>(); for ( String a : args) if ( !s.add( a) ) System.out.println( "Duplicate: " + a); System.out.println( s.size() + " distinct words: “ + s ); }}

Note use of Set for variable s allows

change to TreeSet, for example

Examples…

Run with…java Freq if it is to be it is up to me to delegate

output: 8 distinct words:{to=3, delegate=1, be=1, it=2, up=1, if=1, me=1, is=2}

import java.util.*;public class Freq { public static void main( String args[] ) { Map<String, Integer> m = new HashMap<String, Integer>();

// Initialize frequency table from command line for ( String a : args) { Integer freq = m.get(a); m.put( a, (freq == null ? 1 : freq + 1) ); } System.out.println( m.size() + " distinct words:“ ); System.out.println(m); }}

Try changing the implementation to

TreeMap or LinkedHashMap

Collections class…public static <E> void swap( List<E> a, int i, int j) { E tmp = a.get(i); a.set( i, a.get(j) ); a.set( j, tmp);}

import java.util.*;public class Shuffle { public static void main( String args[]) { List<String> list = new ArrayList<String>(); for( String a : args) list.add( a); Collections.shuffle( list, new Random()); System.out.println( list); }}

public static void shuffle( List<?> list, Random rnd) { for( int i = list.size(); i > 1; i--) swap( list, i - 1, rnd.nextInt(i) );}

Can also…sort, reverse, fill, copy, swap, addAll, retainAll, …

and binarySearch on sorted list..int pos = Collections.binarySearch( list, key);if (pos < 0)

l.add(-pos-1);

From & To Arrays…// *********************************************************************// Create List from an Array// - asList uses original array, or use new to construct new copy.// *********************************************************************String[] animals = { "dog", "cat", "mouse", "mouse", "elephant", "horse", "camel"};

// List<String> list = Arrays.asList( "dog", "cat", "mouse", "elephant", "horse", "camel");// List<String> list = Arrays.asList( animals); // List<String> list = new ArrayList<String>( Arrays.asList( animals) ); List<String> list = new LinkedList<String>( Arrays.asList( animals) );

System.out.println( list);

// *********************************************************************// Convert List back to Array// *********************************************************************

// Object[] zoo = list.toArray();String[] zoo = list.toArray( new String[0] );

for ( String creature : zoo)System.out.println( creature);

Polymorphic Algorithms…// *********************************************************************// Use some of the polymorphic List algorithms // *********************************************************************

// Collections.sort( list, Collections.reverseOrder() );Collections.sort( list );

System.out.println( list);

String key = "giraffe";int pos = Collections.binarySearch( list, key);if ( pos < 0){ System.out.println( "\"" + key + "\" not found.. adding"); list.add(-pos-1, key);}else

System.out.println( "\"" + key + "\" found at " + pos);

Collections.reverse( list);Collections.shuffle( list);

Sets…

// *********************************************************************// Sets cannot have duplicate elements// - HashSet (unordered), LinkedHashSet (order added), TreeSet (ordered)// *********************************************************************

// Set<String> s = new HashSet<String>();// Set<String> s = new LinkedHashSet<String>();Set<String> s = new TreeSet<String>();

for (String a : animals)if ( !s.add( a) )

System.out.println( "Duplicate detected: " + a);

System.out.println( s.size() + " distinct words: " + s);

Arrays class…

// *********************************************************************// The Arrays class provides some useful methods too...// *********************************************************************

String[] animals = { "dog", "cat", "mouse", "mouse", "elephant", "horse", "camel"};

System.out.println( animals); // doesn't work!System.out.println( Arrays.toString(animals)); // does work!

Arrays.sort( animals);

System.out.println( Arrays.toString(animals)); // after sorting, can search...

System.out.println( "\"elephant\" is at " + Arrays.binarySearch( animals, "elephant") );