JavaTM 2 Platform
Standard Ed. 5.0

java.util
Class AbstractCollection<E>

java.lang.Object
  extended by java.util.AbstractCollection<E>
All Implemented Interfaces:
Iterable<E>, Collection<E>
Direct Known Subclasses:
AbstractList, AbstractQueue, AbstractSet

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

This class provides a skeletal implementation of the Collection interface, to minimize the effort required to implement this interface.

To implement an unmodifiable collection, the programmer needs only to extend this class and provide implementations for the iterator and size methods. (The iterator returned by the iterator method must implement hasNext and next.)

To implement a modifiable collection, the programmer must additionally override this class's add method (which otherwise throws an UnsupportedOperationException), and the iterator returned by the iterator method must additionally implement its remove method.

The programmer should generally provide a void (no argument) and Collection constructor, as per the recommendation in the Collection interface specification.

The documentation for each non-abstract methods in this class describes its implementation in detail. Each of these methods may be overridden if the collection being implemented admits a more efficient implementation.

This class is a member of the Java Collections Framework.

Since:
1.2
See Also:
Collection

Constructor Summary
protected AbstractCollection()
          Sole constructor.
 
Method Summary
 boolean add(E o)
          Ensures that this collection contains the specified element (optional operation).
 boolean addAll(Collection<? extends E> c)
          Adds all of the elements in the specified collection to this collection (optional operation).
 void clear()
          Removes all of the elements from this collection (optional operation).
 boolean contains(Object o)
          Returns true if this collection contains the specified element.
 boolean containsAll(Collection<?> c)
          Returns true if this collection contains all of the elements in the specified collection.
 boolean isEmpty()
          Returns true if this collection contains no elements.
abstract  Iterator<E> iterator()
          Returns an iterator over the elements contained in this collection.
 boolean remove(Object o)
          Removes a single instance of the specified element from this collection, if it is present (optional operation).
 boolean removeAll(Collection<?> c)
          Removes from this collection all of its elements that are contained in the specified collection (optional operation).
 boolean retainAll(Collection<?> c)
          Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract  int size()
          Returns the number of elements in this collection.
 Object[] toArray()
          Returns an array containing all of the elements in this collection.
<T> T[]
toArray(T[] a)
          Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
 String toString()
          Returns a string representation of this collection.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 
Methods inherited from interface java.util.Collection
equals, hashCode
 

Constructor Detail

AbstractCollection

protected AbstractCollection()
Sole constructor. (For invocation by subclass constructors, typically implicit.)

Method Detail

iterator

public abstract Iterator<E> iterator()
Returns an iterator over the elements contained in this collection.

Specified by:
iterator in interface Iterable<E>
Specified by:
iterator in interface Collection<E>
Returns:
an iterator over the elements contained in this collection.

size

public abstract int size()
Returns the number of elements in this collection. If the collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Specified by:
size in interface Collection<E>
Returns:
the number of elements in this collection.

isEmpty

public boolean isEmpty()
Returns true if this collection contains no elements.

This implementation returns size() == 0.

Specified by:
isEmpty in interface Collection<E>
Returns:
true if this collection contains no elements.

contains

public boolean contains(Object o)
Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e)).

This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.

Specified by:
contains in interface Collection<E>
Parameters:
o - object to be checked for containment in this collection.
Returns:
true if this collection contains the specified element.

toArray

public Object[] toArray()
Returns an array containing all of the elements in this collection. If the collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. The returned array will be "safe" in that no references to it are maintained by the collection. (In other words, this method must allocate a new array even if the collection is backed by an Array). The caller is thus free to modify the returned array.

This implementation allocates the array to be returned, and iterates over the elements in the collection, storing each object reference in the next consecutive element of the array, starting with element 0.

Specified by:
toArray in interface Collection<E>
Returns:
an array containing all of the elements in this collection.

toArray

public <T> T[] toArray(T[] a)
Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.

If the collection fits in the specified array with room to spare (i.e., the array has more elements than the collection), the element in the array immediately following the end of the collection is set to null. This is useful in determining the length of the collection only if the caller knows that the collection does not contain any null elements.)

If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

This implementation checks if the array is large enough to contain the collection; if not, it allocates a new array of the correct size and type (using reflection). Then, it iterates over the collection, storing each object reference in the next consecutive element of the array, starting with element 0. If the array is larger than the collection, a null is stored in the first location after the end of the collection.

Specified by:
toArray in interface Collection<E>
Parameters:
a - the array into which the elements of the collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
Returns:
an array containing the elements of the collection.
Throws:
NullPointerException - if the specified array is null.
ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this collection.

add

public boolean add(E o)
Ensures that this collection contains the specified element (optional operation). Returns true if the collection changed as a result of the call. (Returns false if this collection does not permit duplicates and already contains the specified element.) Collections that support this operation may place limitations on what elements may be added to the collection. In particular, some collections will refuse to add null elements, and others will impose restrictions on the type of elements that may be added. Collection classes should clearly specify in their documentation any restrictions on what elements may be added.

This implementation always throws an UnsupportedOperationException.

Specified by:
add in interface Collection<E>
Parameters:
o - element whose presence in this collection is to be ensured.
Returns:
true if the collection changed as a result of the call.
Throws:
UnsupportedOperationException - if the add method is not supported by this collection.
NullPointerException - if this collection does not permit null elements, and the specified element is null.
ClassCastException - if the class of the specified element prevents it from being added to this collection.
IllegalArgumentException - if some aspect of this element prevents it from being added to this collection.

remove

public boolean remove(Object o)
Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if the collection contains one or more such elements. Returns true if the collection contained the specified element (or equivalently, if the collection changed as a result of the call).

This implementation iterates over the collection looking for the specified element. If it finds the element, it removes the element from the collection using the iterator's remove method.

Note that this implementation throws an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection contains the specified object.

Specified by:
remove in interface Collection<E>
Parameters:
o - element to be removed from this collection, if present.
Returns:
true if the collection contained the specified element.
Throws:
UnsupportedOperationException - if the remove method is not supported by this collection.

containsAll

public boolean containsAll(Collection<?> c)
Returns true if this collection contains all of the elements in the specified collection.

This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection. If all elements are so contained true is returned, otherwise false.

Specified by:
containsAll in interface Collection<E>
Parameters:
c - collection to be checked for containment in this collection.
Returns:
true if this collection contains all of the elements in the specified collection.
Throws:
NullPointerException - if the specified collection is null.
See Also:
contains(Object)

addAll

public boolean addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this collection (optional operation). The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (This implies that the behavior of this call is undefined if the specified collection is this collection, and this collection is nonempty.)

This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.

Note that this implementation will throw an UnsupportedOperationException unless add is overridden (assuming the specified collection is non-empty).

Specified by:
addAll in interface Collection<E>
Parameters:
c - collection whose elements are to be added to this collection.
Returns:
true if this collection changed as a result of the call.
Throws:
UnsupportedOperationException - if this collection does not support the addAll method.
NullPointerException - if the specified collection is null.
See Also:
add(Object)

removeAll

public boolean removeAll(Collection<?> c)
Removes from this collection all of its elements that are contained in the specified collection (optional operation).

This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection. If it's so contained, it's removed from this collection with the iterator's remove method.

Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method and this collection contains one or more elements in common with the specified collection.

Specified by:
removeAll in interface Collection<E>
Parameters:
c - elements to be removed from this collection.
Returns:
true if this collection changed as a result of the call.
Throws:
UnsupportedOperationException - if the removeAll method is not supported by this collection.
NullPointerException - if the specified collection is null.
See Also:
remove(Object), contains(Object)

retainAll

public boolean retainAll(Collection<?> c)
Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.

This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection. If it's not so contained, it's removed from this collection with the iterator's remove method.

Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method and this collection contains one or more elements not present in the specified collection.

Specified by:
retainAll in interface Collection<E>
Parameters:
c - elements to be retained in this collection.
Returns:
true if this collection changed as a result of the call.
Throws:
UnsupportedOperationException - if the retainAll method is not supported by this Collection.
NullPointerException - if the specified collection is null.
See Also:
remove(Object), contains(Object)

clear

public void clear()
Removes all of the elements from this collection (optional operation). The collection will be empty after this call returns (unless it throws an exception).

This implementation iterates over this collection, removing each element using the Iterator.remove operation. Most implementations will probably choose to override this method for efficiency.

Note that this implementation will throw an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection is non-empty.

Specified by:
clear in interface Collection<E>
Throws:
UnsupportedOperationException - if the clear method is not supported by this collection.

toString

public String toString()
Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String.valueOf(Object).

This implementation creates an empty string buffer, appends a left square bracket, and iterates over the collection appending the string representation of each element in turn. After appending each element except the last, the string ", " is appended. Finally a right bracket is appended. A string is obtained from the string buffer, and returned.

Overrides:
toString in class Object
Returns:
a string representation of this collection.

JavaTM 2 Platform
Standard Ed. 5.0

Submit a bug or feature
For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

Copyright 2004 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.


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