The Java Collections Package

1. The Java Collections Package C. DeJong Fall 2001

2. Introduction • A collection is an object that contains other objects • Ex: a poker hand, a phone directory, a mail folder

3. Collection framework features • Interfaces • Implementations • Algorithms • Polymorphic • cf. the Standard Template Library (STL) in C++

4. Benefits • Data structures and algorithms for free • Tested • A well-known standard • Software reuse

5. The Collection interface • Root of the collections hierarchy • Other interfaces extend Collection • Java provides implementations of subinterfaces (Set, List, etc) • For passing and manipulating collections generically • Implementations have a constructor that accepts a Collection parameter

6. Collection methods (basic) • int size() • boolean isEmpty() • boolean contains(Object element) • boolean add(Object element) • boolean remove(Object element) • Iterator iterator()

7. The Iterator interface • Used to traverse elements of a Collection • Can remove elements • Methods • boolean hasNext() • Object next() • void remove()

8. Using Iterator // simple method to print out each element in a collection // using an Iterator public void printContents(Collection stuff) { for(Iterator i=stuff.iterator();i.hasNext();) { System.out.println(i.next()); } }

9. Collection methods (bulk ops) • boolean containsAll(Collection c) • boolean addAll(Collection c) • boolean removeAll(Collection c) • boolean retainAll(Collection c) • void clear()

10. Collection methods (arrays) • Object[] toArray() • Object[] toArray(Object a[])

11. A note on Collection methods • All Collection interfaces (Set, List, Map) have these methods (why?) • Not all are supported • Optional operations not supported throw UnsupportedOperationException • Java uses Exceptions to represent errors at runtime (more on this later…)

12. extends Collection interface Cannot contain duplicate elements Models the mathematical Set abstraction Implemented by HashSet and TreeSet classes Use HashSet in most cases The Set interface

13. Set interface methods • Basic (size, isEmpty, contains, add, remove, iterator) • Bulk (containsAll, addAll, removeAll, retainAll, clear) • Arrays (toArray)

14. Using the Set interface import java.util.*; public class FindDups { public static void main(String[] args) { Set s = new HashSet(); for (int i=0;i<args.length;i++) if (!s.add(args[i])) System.out.println("Duplicate detected " + args[i]); System.out.println(s.size() + " distinct words detected: " + s); } } java FindDups i came i saw i left Duplicate detected: i Duplicate detected: i 4 distinct words detected: [came, left, saw, i]

15. extends Collection interface Ordered Duplicates OK Elements have a position and index number Similar to arrays Implemented by ArrayList, LinkedList and Vector (use ArrayList) beware of java.awt.List The List interface

16. List interface methods • all methods from Collection interface • Positional access (get, set, add, remove, addAll) • Search (indexOf, lastIndexOf) • Iteration (listIterator) • Range-view (subList)

17. Using the List interface List list1 = new ArrayList(); list1.add("John"); list1.add("George"); List list2 = new Vector(); list2.add("Paul"); list2.add("Ringo"); list1.addAll(list2); for (int i=0;i<list1.size();i++) { String name = (String)list1.get(i); System.out.println(name); // John, then George, etc. } System.out.println((String)list1.get(1)); // George

18. The Map Interface • does not extend Collection interface • Maps keys to values ("associative array") • Like List and arrays, but key instead of index number • implemented by HashMap, TreeMap and HashTable (use HashMap)

19. Map interface methods • no methods from Collection interface • Basic (put, get, remove, containsKey, containsValue, size, isEmpty) • Bulk (putAll, clear) • Collection views (keySet, values, entrySet)

20. using the Map interface // sample Map to store state capitols by state abbreviation private Map capitols = new HashMap(); capitols.put("MI","Lansing"); capitols.put("IL","Springfield"); capitols.put("TX","Austin"); capitols.put("CA","Sacramento"); String michiganCap = (String)capitols.get("MI"); System.out.println("The capitol of Michigan is " + michiganCap); System.out.println("Map has " + capitols.size() + " elements"); capitols.clear(); // deletes all elements System.out.println("Map has " + capitols.size() + " elements");

21. The Collections class • Not the same as the Collection interface • Static utility methods to operate on Collection objects

22. Searching Sorting Copying Min/Max Reverse Shuffle Synchronization Unmodifiable Singletons Fill Constants for empty… List Set Map Etc. Collections class methods

23. using the Collections class import java.util.*; public class CollectionsUse { public static void main(String args[]) { List states = new ArrayList(); states.add("AL"); states.add("IL"); states.add("MI"); states.add("CA"); System.out.println("first state is " + Collections.min(states)); System.out.println("last state is " + Collections.max(states)); System.out.println("states in original order " + states); Collections.sort(states); System.out.println("states in sorted order " + states); Collections.reverse(states); System.out.println("states in reverse sorted order " + states); Collections.shuffle(states); System.out.println("states in random order " + states); } }