Collections Framework

1. Collections Framework A very brief look at Java’s Collection Framework David Davenport May 2010

2. 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.

3. The Hierarchy SortedSet TreeSet Set AbstractSet HashSet LinkedHashSet Collection AbstractCollection Vector Stack List AbstractList ArrayList AbstractSequentialList LinkedList Queue AbstractQueue PriorityQueue TreeMap SortedMap Map AbstractMap HashMap LinkedHashMap

4. 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

5. Examples… 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 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}

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

7. 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);

8. 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);

9. 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);

10. 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") );