AbstractCollection (Java 2 Platform SE 5.0) (original) (raw)

java.util

Class AbstractCollection

java.lang.Object java.util.AbstractCollection

All Implemented Interfaces:

Iterable, Collection

Direct Known Subclasses:

AbstractList, AbstractQueue, AbstractSet

public abstract class AbstractCollection

extends Object

implements Collection

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 andsize 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 anUnsupportedOperationException), and the iterator returned by theiterator method must additionally implement its remove method.

The programmer should generally provide a void (no argument) andCollection constructor, as per the recommendation in theCollection 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

AbstractCollection

protected AbstractCollection()

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

iterator

public abstract Iterator<E> iterator()

Returns an iterator over the elements contained in this collection.

Specified by:

[iterator](../../java/lang/Iterable.html#iterator%28%29) in interface [Iterable](../../java/lang/Iterable.html "interface in java.lang")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

Specified by:

[iterator](../../java/util/Collection.html#iterator%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

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, returnsInteger.MAX_VALUE.

Specified by:

[size](../../java/util/Collection.html#size%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

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](../../java/util/Collection.html#isEmpty%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

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](../../java/util/Collection.html#contains%28java.lang.Object%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

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](../../java/util/Collection.html#toArray%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

Returns:

an array containing all of the elements in this collection.

toArray

public 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 tonull. 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](../../java/util/Collection.html#toArray%28T[]%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

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](../../java/lang/NullPointerException.html "class in java.lang") - if the specified array is null.

[ArrayStoreException](../../java/lang/ArrayStoreException.html "class in java.lang") - 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 anUnsupportedOperationException.

Specified by:

[add](../../java/util/Collection.html#add%28E%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

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](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the add method is not supported by this collection.

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if this collection does not permitnull elements, and the specified element isnull.

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the class of the specified element prevents it from being added to this collection.

[IllegalArgumentException](../../java/lang/IllegalArgumentException.html "class in java.lang") - 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 anUnsupportedOperationException 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](../../java/util/Collection.html#remove%28java.lang.Object%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

Parameters:

o - element to be removed from this collection, if present.

Returns:

true if the collection contained the specified element.

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - 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 containedtrue is returned, otherwise false.

Specified by:

[containsAll](../../java/util/Collection.html#containsAll%28java.util.Collection%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

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](../../java/lang/NullPointerException.html "class in java.lang") - 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 anUnsupportedOperationException unless add is overridden (assuming the specified collection is non-empty).

Specified by:

[addAll](../../java/util/Collection.html#addAll%28java.util.Collection%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

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](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if this collection does not support the addAll method.

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - 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 anUnsupportedOperationException if the iterator returned by theiterator method does not implement the remove method and this collection contains one or more elements in common with the specified collection.

Specified by:

[removeAll](../../java/util/Collection.html#removeAll%28java.util.Collection%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

Parameters:

c - elements to be removed from this collection.

Returns:

true if this collection changed as a result of the call.

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the removeAll method is not supported by this collection.

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - 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 anUnsupportedOperationException if the iterator returned by theiterator method does not implement the remove method and this collection contains one or more elements not present in the specified collection.

Specified by:

[retainAll](../../java/util/Collection.html#retainAll%28java.util.Collection%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

Parameters:

c - elements to be retained in this collection.

Returns:

true if this collection changed as a result of the call.

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the retainAll method is not supported by this Collection.

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - 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 anUnsupportedOperationException if the iterator returned by this collection's iterator method does not implement theremove method and this collection is non-empty.

Specified by:

[clear](../../java/util/Collection.html#clear%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/AbstractCollection.html "type parameter in AbstractCollection")>

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - 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](../../java/lang/Object.html#toString%28%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Returns:

a string representation of this collection.

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