java.util

Class AbstractCollection<E>

java.lang.Object

java.util.AbstractCollection<E>

All Implemented Interfaces:

Iterable<E>, Collection<E>

Direct Known Subclasses:

AbstractList, AbstractQueue, AbstractSet, ArrayDeque

public abstract class AbstractCollection<E>
extends Object
implements Collection<E>

A basic implementation of most of the methods in the Collection interface to make it easier to create a collection. To create an unmodifiable Collection, just subclass AbstractCollection and provide implementations of the iterator() and size() methods. The Iterator returned by iterator() need only provide implementations of hasNext() and next() (that is, it may throw an UnsupportedOperationException if remove() is called). To create a modifiable Collection, you must in addition provide an implementation of the add(Object) method and the Iterator returned by iterator() must provide an implementation of remove(). Other methods should be overridden if the backing data structure allows for a more efficient implementation. The precise implementation used by AbstractCollection is documented, so that subclasses can tell which methods could be implemented more efficiently.

The programmer should provide a no-argument constructor, and one that accepts another Collection, as recommended by the Collection interface. Unfortunately, there is no way to enforce this in Java.

Since:

1.2

Author:

Original author unknown, Bryce McKinlay, Eric Blake (ebb9@email.byu.edu), Tom Tromey (tromey@redhat.com), Andrew John Hughes (gnu_andrew@member.fsf.org)

See Also:

Collection, AbstractSet, AbstractList

Method Summary

Modifier and Type	Method and Description
boolean	add(E o) Add an object to the collection (optional operation).
boolean	addAll(Collection<? extends E> c) Add all the elements of a given collection to this collection (optional operation).
void	clear() Remove all elements from the collection (optional operation).
boolean	contains(Object o) Test whether this collection contains a given object.
boolean	containsAll(Collection<?> c) Tests whether this collection contains all the elements in a given collection.
boolean	isEmpty() Test whether this collection is empty.
abstract Iterator<E>	iterator() Return an Iterator over this collection.
boolean	remove(Object o) Remove a single instance of an object from this collection (optional operation).
boolean	removeAll(Collection<?> c) Remove from this collection all its elements that are contained in a given collection (optional operation).
boolean	retainAll(Collection<?> c) Remove from this collection all its elements that are not contained in a given collection (optional operation).
abstract int	size() Return the number of elements in this collection.
Object[]	toArray() Return an array containing the elements of this collection.
<T> T[]	toArray(T[] a) Copy the collection into a given array if it will fit, or into a dynamically created array of the same run-time type as the given array if not.
String	toString() Creates a String representation of the Collection.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Collection

equals, hashCode

Method Detail

add

public boolean add(E o)

Add an object to the collection (optional operation). This implementation always throws an UnsupportedOperationException - it should be overridden if the collection is to be modifiable. If the collection does not accept duplicates, simply return false. Collections may specify limitations on what may be added.

Specified by:

add in interface Collection<E>

Parameters:

o - the object to add

Returns:

true if the add operation caused the Collection to change

Throws:

UnsupportedOperationException - if the add operation is not supported on this collection

NullPointerException - if the collection does not support null

ClassCastException - if the object is of the wrong type

IllegalArgumentException - if some aspect of the object prevents it from being added

addAll

public boolean addAll(Collection<? extends E> c)

Add all the elements of a given collection to this collection (optional operation). This implementation obtains an Iterator over the given collection and iterates over it, adding each element with the add(Object) method (thus this method will fail with an UnsupportedOperationException if the add method does). The behavior is unspecified if the specified collection is modified during the iteration, including the special case of trying addAll(this) on a non-empty collection.

Specified by:

addAll in interface Collection<E>

Parameters:

c - the collection to add the elements of to this collection

Returns:

true if the add operation caused the Collection to change

Throws:

UnsupportedOperationException - if the add operation is not supported on this collection

NullPointerException - if the specified collection is null

ClassCastException - if the type of any element in c is not a valid type for addition.

IllegalArgumentException - if some aspect of any element in c prevents it being added.

NullPointerException - if any element in c is null and this collection doesn't allow null values.

See Also:

add(Object)

clear

public void clear()

Remove all elements from the collection (optional operation). This implementation obtains an iterator over the collection and calls next and remove on it repeatedly (thus this method will fail with an UnsupportedOperationException if the Iterator's remove method does) until there are no more elements to remove. Many implementations will have a faster way of doing this.

Specified by:

clear in interface Collection<E>

Throws:

UnsupportedOperationException - if the Iterator returned by iterator does not provide an implementation of remove

See Also:

Iterator.remove()

contains

public boolean contains(Object o)

Test whether this collection contains a given object. That is, if the collection has an element e such that (o == null ? e == null : o.equals(e)). This implementation obtains an iterator over the collection and iterates over it, testing each element for equality with the given object. If it is equal, true is returned. Otherwise false is returned when the end of the collection is reached.

Specified by:

contains in interface Collection<E>

Parameters:

o - the object to remove from this collection

Returns:

true if this collection contains an object equal to o

containsAll

public boolean containsAll(Collection<?> c)

Tests whether this collection contains all the elements in a given collection. This implementation iterates over the given collection, testing whether each element is contained in this collection. If any one is not, false is returned. Otherwise true is returned.

Specified by:

containsAll in interface Collection<E>

Parameters:

c - the collection to test against

Returns:

true if this collection contains all the elements in the given collection

Throws:

NullPointerException - if the given collection is null

See Also:

contains(Object)

isEmpty

public boolean isEmpty()

Test whether this collection is empty. This implementation returns size() == 0.

Specified by:

isEmpty in interface Collection<E>

Returns:

true if this collection is empty.

See Also:

size()

iterator

public abstract Iterator<E> iterator()

Return an Iterator over this collection. The iterator must provide the hasNext and next methods and should in addition provide remove if the collection is modifiable.

Specified by:

iterator in interface Iterable<E>

Specified by:

iterator in interface Collection<E>

Returns:

an iterator

remove

public boolean remove(Object o)

Remove a single instance of an object from this collection (optional operation). That is, remove one element e such that (o == null ? e == null : o.equals(e)), if such an element exists. This implementation obtains an iterator over the collection and iterates over it, testing each element for equality with the given object. If it is equal, it is removed by the iterator's remove method (thus this method will fail with an UnsupportedOperationException if the Iterator's remove method does). After the first element has been removed, true is returned; if the end of the collection is reached, false is returned.

Specified by:

remove in interface Collection<E>

Parameters:

o - the object to remove from this collection

Returns:

true if the remove operation caused the Collection to change, or equivalently if the collection did contain o.

Throws:

UnsupportedOperationException - if this collection's Iterator does not support the remove method

See Also:

Iterator.remove()

removeAll

public boolean removeAll(Collection<?> c)

Remove from this collection all its elements that are contained in a given collection (optional operation). This implementation iterates over this collection, and for each element tests if it is contained in the given collection. If so, it is removed by the Iterator's remove method (thus this method will fail with an UnsupportedOperationException if the Iterator's remove method does).

Specified by:

removeAll in interface Collection<E>

Parameters:

c - the collection to remove the elements of

Returns:

true if the remove operation caused the Collection to change

Throws:

UnsupportedOperationException - if this collection's Iterator does not support the remove method

NullPointerException - if the collection, c, is null.

See Also:

Iterator.remove()

retainAll

public boolean retainAll(Collection<?> c)

Remove from this collection all its elements that are not contained in a given collection (optional operation). This implementation iterates over this collection, and for each element tests if it is contained in the given collection. If not, it is removed by the Iterator's remove method (thus this method will fail with an UnsupportedOperationException if the Iterator's remove method does).

Specified by:

retainAll in interface Collection<E>

Parameters:

c - the collection to retain the elements of

Returns:

true if the remove operation caused the Collection to change

Throws:

UnsupportedOperationException - if this collection's Iterator does not support the remove method

NullPointerException - if the collection, c, is null.

See Also:

Iterator.remove()

size

public abstract int size()

Return the number of elements in this collection. If there are more than Integer.MAX_VALUE elements, return Integer.MAX_VALUE.

Specified by:

size in interface Collection<E>

Returns:

the size

toArray

public Object[] toArray()

Return an array containing the elements of this collection. This implementation creates an Object array of size size() and then iterates over the collection, setting each element of the array from the value returned by the iterator. The returned array is safe, and is not backed by the collection.

Specified by:

toArray in interface Collection<E>

Returns:

an array containing the elements of this collection

toArray

public <T> T[] toArray(T[] a)

Copy the collection into a given array if it will fit, or into a dynamically created array of the same run-time type as the given array if not. If there is space remaining in the array, the first element after the end of the collection is set to null (this is only useful if the collection is known to contain no null elements, however). This implementation first tests whether the given array is large enough to hold all the elements of the collection. If not, the reflection API is used to allocate a new array of the same run-time type. Next an iterator is obtained over the collection and the elements are placed in the array as they are returned by the iterator. Finally the first spare element, if any, of the array is set to null, and the created array is returned. The returned array is safe; it is not backed by the collection. Note that null may not mark the last element, if the collection allows null elements.

Specified by:

toArray in interface Collection<E>

Parameters:

a - the array to copy into, or of the correct run-time type

Returns:

the array that was produced

Throws:

NullPointerException - if the given array is null

ArrayStoreException - if the type of the array precludes holding one of the elements of the Collection

toString

public String toString()

Creates a String representation of the Collection. The string returned is of the form "[a, b, ...]" where a and b etc are the results of calling toString on the elements of the collection. This implementation obtains an Iterator over the Collection and adds each element to a StringBuffer as it is returned by the iterator. "" is inserted when the collection contains itself (only works for direct containment, not for collections inside collections).

Overrides:

toString in class Object

Returns:

a String representation of the Collection