Part of the Collections Framework Lists in Java Part of the Collections Framework 25-May-19

Kinds of Collections Collection—a group of objects, called elements Set—An unordered collection with no duplicates SortedSet—An ordered collection with no duplicates List—an ordered collection, duplicates are allowed Map—a collection that maps keys to values SortedMap—a collection ordered by the keys Note that there are two distinct hierarchies

Using Collections import java.util.* or import java.util.Collection; There is a sister class, java.util.Collections; that provides a number of algorithms for use with collections: sort, binarySearch, copy, shuffle, reverse, max, min, etc.

Collections Example import java.util.*; // importing Arrays, List, and Collections public class TestCollections { public static void main(String args[]) { String[] array = {"Phil", "Mary", "Betty", "bob"}; List<String> myList = Arrays.asList(array); Collections.sort(myList); System.out.println("Sorted: " + myList); int where = Collections.binarySearch(myList, "bob"); System.out.println("bob is at " + where); Collections.shuffle(myList); System.out.println("Shuffled: " + myList); } Sorted: [Betty, Mary, Phil, bob] bob is at 3 Shuffled: [Betty, bob, Phil, Mary]

Collections are interfaces Collection is actually an interface Each kind of Collection has one or more implementations You can create new kinds of Collections When you implement an interface, you promise to supply the required methods Some Collection methods are optional How can an interface declare an optional method?

Creating a Collection All Collection implementations should have two constructors: A no-argument constructor to create an empty collection A constructor with another Collection as argument All the Sun-supplied implementations obey this rule, but— —if you implement your own Collection type, this rule cannot be enforced, because an Interface cannot specify constructors

Collection<E>: Basic operations int size( ); boolean isEmpty( ); boolean contains(Object element); boolean add(E element); // Optional boolean remove(Object element); // Optional Iterator<E> iterator( );

Collection: Iterator public interface Iterator<E> { boolean hasNext( ); // true if there is another element E next( ); // returns the next element (advances the iterator) void remove( ); // Optional // removes the element returned by next }

Using an Iterator static void printAll (Collection coll) { Iterator iter = coll.iterator( ); while (iter.hasNext( )) { System.out.println(iter.next( ) ); } } hasNext() just checks if there are any more elements next() returns the next element and advances in the collection Note that this code is polymorphic—it will work for any collection

Object[] toArray() Defined in java.util.Collection interface, so it applies to all classes that implement Collection Object[ ] toArray( ) Creates a new array of Objects Example: Object[ ] newArray = myCollection.toArray( ); Problem: Returns an array of Objects, so we have to cast every time we want to use something from the array

<T> T[] toArray(T[] a) Also defined in java.util.Collection interface, so it applies to all classes that implement Collection More complex, but definitely worth knowing This version of toArray: Is a message to some collection of type T objects Takes as parameter an array of some type T Puts the objects from the collection into the array If the array is longer than needed, a null is put in after the data If the array is too short, a new array (of the correct type) is created and returned Examples: ArrayList<String> list = new ArrayList<String>(); // Put some Strings into list here String[ ] a = new String[20]; list.toArray(a); String[] b = list.toArray(new String[0]);

Set operations Set union: A  B Set intersection: A  B Set difference: A – B

Collection: Bulk operations boolean containsAll(Collection<?> c); boolean addAll(Collection<? extends E> c); boolean removeAll(Collection<?> c); boolean retainAll(Collection<?> c); void clear( ); The last four operations are optional, in order to allow for immutable collections That is, they might throw an UnsupportedOperationException addAll, removeAll, retainAll return true if the object receiving the message was modified

Set operations Set union: A  B setC = setA.addAll(setB); Set intersection: A  B setC = setA.retainAll(setB); A B Set difference: A – B setC = setA.removeAll(setB);

Mixing Collection types Note that most methods, such as boolean containsAll(Collection<?> c); are defined for any type of Collection, and take any type of Collection as an argument This makes it very easy to work with different types of Collections

singleton static <T> Set<T>Collections.singleton(T e) returns an immutable set containing only the element e This is handy when you have a single element but you would like to use a Set operation c.removeAll(Collections.singleton(e)); will remove all occurrences of e from the Collection c

The List interface The order of elements in a List is important, and there may be duplicate elements Operations are exactly those for Collection int size( ); boolean isEmpty( ); boolean contains(Object e); boolean add(E e); boolean remove(Object e); Iterator iterator( ); Object[ ] toArray( ); <T> T[ ] toArray(T a[ ]); boolean containsAll(Collection<?> c); boolean addAll(Collection<? extends E> c); boolean removeAll(Collection<?> c); boolean retainAll(Collection<?> c); void clear( );

List implementations List is an interface; you can’t say new List ( ) There are two implementations: LinkedList gives faster insertions and deletions ArrayList gives faster random access It’s poor style to expose the implementation unnecessarily, so: Good: List list = new LinkedList ( ); Not as good: LinkedList list = new LinkedList ( );

Inherited List methods list.remove(e) removes the first e add and addAll add to the end of the list To append one list to another: list1.addAll(list2); To append two lists into a new list: List list3 = new ArrayList(list1); list3.addAll(list2); Again, it's good style to hide the implementation

List: Positional access E get(int index); // Required -- the rest are optional E set(int index, E element); void add(int index, E element); E remove(int index); boolean addAll(int index, Collection<? extends E> c); These operations are more efficient with the ArrayList implementation

List: Searching int indexOf(Object o); int lastIndexOf(Object o); equals and hashCode work even if implementations are different

Interface ListIterator Iterators specific to Lists: ListIterator listIterator( ); ListIterator listIterator(int index); starts at the position indicated (0 is first element) Methods that ListIterator inherits from Iterator: boolean hasNext( ); E next( ); void remove( ); Additional methods: boolean hasPrevious() E previous()

List: Iterating backwards boolean hasPrevious( ); E previous( ); int nextIndex( ); int previousIndex( ); Think of the iterator as “between” elements Hence, next followed by previous gives you the same element each time

ListIterator: More operations void add(E o); Inserts an object at the cursor position void set(E o); // Optional Replaces the current element void remove(int index); // Optional Removes the last element that was returned by next() or previous()

List: Range-view List<E> subList(int from, int to) allows you to manipulate part of a list Notice the unusual capitalization A subList is a “view” into the actual list; manipulating it is manipulating the original list. A subList may be used just like any other list However, it is dangerous to modify the original list and expect the subList to remain consistent

The End References: http://java.sun.com/docs/books/tutorial/collections/interfaces/collection.html http://java.sun.com/docs/books/tutorial/collections/interfaces/list.html