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java.util

Class AbstractCollection

java.lang.Object
|
+--java.util.AbstractCollection

All Implemented Interfaces:

Collection


public abstract class AbstractCollection

extends Object

implements Collection

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.

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Constructor Summary

AbstractCollection()

The main constructor, for use by subclasses.

Method Summary

booleanadd(java.lang.Object o)

Add an object to the collection (optional operation).
booleanaddAll(java.util.Collection c)

Add all the elements of a given collection to this collection (optional operation).
voidclear()

Remove all elements from the collection (optional operation).
booleancontains(java.lang.Object o)

Test whether this collection contains a given object.
booleancontainsAll(java.util.Collection c)

Tests whether this collection contains all the elements in a given collection.
booleanisEmpty()

Test whether this collection is empty.
java.util.Iteratoriterator()

Return an Iterator over this collection.
booleanremove(java.lang.Object o)

Remove a single instance of an object from this collection (optional operation).
booleanremoveAll(java.util.Collection c)

Remove from this collection all its elements that are contained in a given collection (optional operation).
booleanretainAll(java.util.Collection c)

Remove from this collection all its elements that are not contained in a given collection (optional operation).
intsize()

Return the number of elements in this collection.
java.lang.Object[]toArray()

Return an array containing the elements of this collection.
java.lang.Object[]toArray(java.lang.Object[] 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.
java.lang.StringtoString()

Creates a String representation of the Collection.

Constructor Details

AbstractCollection

protected AbstractCollection()

The main constructor, for use by subclasses.


Method Details

add

public boolean add(java.lang.Object 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.

Parameters:

Returns:

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addAll

public boolean addAll(java.util.Collection 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.

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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.

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contains

public boolean contains(java.lang.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.

Parameters:

Returns:


containsAll

public boolean containsAll(java.util.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.

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isEmpty

public boolean isEmpty()

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

Returns:

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iterator

public Iterator 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.

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remove

public boolean remove(java.lang.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.

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removeAll

public boolean removeAll(java.util.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).

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retainAll

public boolean retainAll(java.util.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).

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size

public int size()

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

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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.

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toArray

public Object[] toArray(java.lang.Object[] 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.

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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.

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