The Java Collections Package

C. DeJong

Fall 2001

Introduction

• A collection is an object that contains other objects

• Ex: a poker hand, a phone directory, a mail folder

Collection framework features

• Interfaces

• Implementations

• Algorithms– Polymorphic

• cf. the Standard Template Library (STL) in C++

Benefits

• Data structures and algorithms for free

• Tested

• A well-known standard

• Software reuse

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

Collection methods (basic)

• int size()

• boolean isEmpty()

• boolean contains(Object element)

• boolean add(Object element)

• boolean remove(Object element)

• Iterator iterator()

The Iterator interface

• Used to traverse elements of a Collection

• Can remove elements

• Methods– boolean hasNext()– Object next()– void remove()

// 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());

}

}

Using Iterator

Collection methods (bulk ops)

• boolean containsAll(Collection c)

• boolean addAll(Collection c)

• boolean removeAll(Collection c)

• boolean retainAll(Collection c)

• void clear()

Collection methods (arrays)

• Object[] toArray()

• Object[] toArray(Object a[])

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…)

The Set interface

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

Set interface methods

• Basic (size, isEmpty, contains, add, remove, iterator)

• Bulk (containsAll, addAll, removeAll, retainAll, clear)

• Arrays (toArray)

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: iDuplicate detected: i4 distinct words detected: [came, left, saw, i]

Using the Set interface

• 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

List interface methods

• all methods from Collection interface

• Positional access (get, set, add, remove, addAll)

• Search (indexOf, lastIndexOf)

• Iteration (listIterator)

• Range-view (subList)

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

Using the List interface

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)

Map interface methods

• no methods from Collection interface

• Basic (put, get, remove, containsKey, containsValue, size, isEmpty)

• Bulk (putAll, clear)

• Collection views (keySet, values, entrySet)

// sample Map to store state capitols by state abbreviationprivate 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 elementsSystem.out.println("Map has " + capitols.size() + " elements");

using the Map interface

The Collections class

• Not the same as the Collection interface

• Static utility methods to operate on Collection objects

Collections class methods

• Searching• Sorting• Copying• Min/Max• Reverse• Shuffle• Synchronization

• Unmodifiable• Singletons• Fill• Constants for empty…

– List

– Set

– Map

• Etc.

using the Collections classimport 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);

}}