Collections Framework A very brief look at Java’s Collection Framework David Davenport May 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 QueueAbstractQueue 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 s = new HashSet (); 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 m = new HashMap (); // 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 void swap( List 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 list = new ArrayList (); 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 list = Arrays.asList( "dog", "cat", "mouse", "elephant", "horse", "camel"); // List list = Arrays.asList( animals); // List list = new ArrayList ( Arrays.asList( animals) ); List list = new LinkedList ( 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 s = new HashSet (); // Set s = new LinkedHashSet (); Set s = new TreeSet (); 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") );