Jun 15, 2 022 Lists in Java Part of the Collections Framework
Dec 22, 2015
Apr 19, 2023
Lists in Java
Part of the Collections Framework
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 Exampleimport 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 3Shuffled: [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
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}
public interface Iterator<E> {
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 Defined in java.util.Collection interface, so it applies to all classes
that implement Collection Example:
Object[ ] a = c.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 hereString[ ] a = new String[20];list.toArray(a);String[] b = list.toArray(new String[0]);
Set operations
A B
Set union: A B
A B
Set intersection: A B
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
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( );
boolean containsAll(Collection<?> c);boolean addAll(Collection<? extends E> c);
boolean removeAll(Collection<?> c);
boolean retainAll(Collection<?> c);void clear( );
Object[ ] toArray( );<T> T[ ] toArray(T a[ ]);
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