java.util

Interface Collection<E>

Type Parameters:

E - the type of elements in this collection

All Superinterfaces:

Iterable<E>

All Known Subinterfaces:

List<E>, Set<E>

All Known Implementing Classes:

AbstractCollection, AbstractList, AbstractSet, ArrayList, Stack, Vector

public interface Collection<E>
extends Iterable<E>

The root interface in the collection hierarchy. A collection represents a group of objects, known as its elements. Some collections allow duplicate elements and others do not. Some are ordered and others unordered.

Bags or multisets (unordered collections that may contain duplicate elements) should implement this interface directly.

All general-purpose Collection implementation classes (which typically implement Collection indirectly through one of its subinterfaces) should provide two "standard" constructors: a void (no arguments) constructor, which creates an empty collection, and a constructor with a single argument of type Collection, which creates a new collection with the same elements as its argument. In effect, the latter constructor allows the user to copy any collection, producing an equivalent collection of the desired implementation type. There is no way to enforce this convention (as interfaces cannot contain constructors) but all of the general-purpose Collection implementations in the Java platform libraries comply.

The "destructive" methods contained in this interface, that is, the methods that modify the collection on which they operate, are specified to throw UnsupportedOperationException if this collection does not support the operation. If this is the case, these methods may, but are not required to, throw an UnsupportedOperationException if the invocation would have no effect on the collection. For example, invoking the addAll(Collection) method on an unmodifiable collection may, but is not required to, throw the exception if the collection to be added is empty.

Some collection implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException or ClassCastException. Attempting to query the presence of an ineligible element may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible element whose completion would not result in the insertion of an ineligible element into the collection may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.

It is up to each collection to determine its own synchronization policy. In the absence of a stronger guarantee by the implementation, undefined behavior may result from the invocation of any method on a collection that is being mutated by another thread; this includes direct invocations, passing the collection to a method that might perform invocations, and using an existing iterator to examine the collection.

Many methods in Collections Framework interfaces are defined in terms of the equals method. For example, the specification for the contains(Object o) method says: "returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e))." This specification should not be construed to imply that invoking Collection.contains with a non-null argument o will cause o.equals(e) to be invoked for any element e. Implementations are free to implement optimizations whereby the equals invocation is avoided, for example, by first comparing the hash codes of the two elements. (The Object.hashCode() specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlying Object methods wherever the implementor deems it appropriate.

This interface is a member of the Java Collections Framework

Method Summary

Methods

Modifier and Type	Method and Description
boolean	add(E e) 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	equals(Object o) Compares the specified object with this collection for equality.
int	hashCode() Returns the hash code value for this collection.
boolean	isEmpty() Returns true if this collection contains no elements.
Iterator<E>	iterator() Returns an iterator over the elements 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 all of this collection's elements that are also 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).
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.

Method Detail

add

boolean add(E e)

Ensures that this collection contains the specified element (optional operation). Returns true if this 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 this 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.

If a collection refuses to add a particular element for any reason other than that it already contains the element, it must throw an exception (rather than returning false ). This preserves the invariant that a collection always contains the specified element after this call returns.

Parameters:

e - element whose presence in this collection is to be ensured

Returns:

true if this collection changed as a result of the call

Throws:

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

ClassCastException - if the class of the specified element prevents it from being added to this collection

NullPointerException - if the specified element is null and this collection does not permit null elements

IllegalArgumentException - if some property of the element prevents it from being added to this collection

IllegalStateException - if the element cannot be added at this time due to insertion restrictions

addAll

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.)

Parameters:

c - collection containing elements to be added to this collection

Returns:

true if this collection changed as a result of the call

Throws:

UnsupportedOperationException - if the addAll operation is not supported by this collection

ClassCastException - if the class of an element of the specified collection prevents it from being added to this collection

NullPointerException - if the specified collection contains a null element and this collection does not permit null elements, or if the specified collection is null

IllegalArgumentException - if some property of an element of the specified collection prevents it from being added to this collection

IllegalStateException - if not all the elements can be added at this time due to insertion restrictions

See Also:

add(Object)

clear

void clear()

Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns.

Throws:

UnsupportedOperationException - if the clear operation is not supported by this collection

contains

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)).

Parameters:

o - element whose presence in this collection is to be tested

Returns:

true if this collection contains the specified element

Throws:

ClassCastException - if the type of the specified element is incompatible with this collection (optional)

NullPointerException - if the specified element is null and this collection does not permit null elements (optional)

containsAll

boolean containsAll(Collection<?> c)

Returns true if this collection contains all of the elements in the specified collection.

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:

ClassCastException - if the types of one or more elements in the specified collection are incompatible with this collection (optional)

NullPointerException - if the specified collection contains one or more null elements and this collection does not permit null elements (optional), or if the specified collection is null.

See Also:

contains(Object)

equals

boolean equals(Object o)

Compares the specified object with this collection for equality.

While the Collection interface adds no stipulations to the general contract for the Object.equals, programmers who implement the Collection interface "directly" (in other words, create a class that is a Collection but is not a Set or a List) must exercise care if they choose to override the Object.equals. It is not necessary to do so, and the simplest course of action is to rely on Object's implementation, but the implementor may wish to implement a "value comparison" in place of the default "reference comparison." (The List and Set interfaces mandate such value comparisons.)

The general contract for the Object.equals method states that equals must be symmetric (in other words, a.equals(b) if and only if b.equals(a)). The contracts for List.equals and Set.equals state that lists are only equal to other lists, and sets to other sets. Thus, a custom equals method for a collection class that implements neither the List nor Set interface must return false when this collection is compared to any list or set. (By the same logic, it is not possible to write a class that correctly implements both the Set and List interfaces.)

Overrides:

equals in class Object

Parameters:

o - object to be compared for equality with this collection

Returns:

true if the specified object is equal to this collection

See Also:

Object.equals(Object), Set.equals(Object), List.equals(Object)

hashCode

int hashCode()

Returns the hash code value for this collection. While the Collection interface adds no stipulations to the general contract for the Object.hashCode method, programmers should take note that any class that overrides the Object.equals method must also override the Object.hashCode method in order to satisfy the general contract for the Object.hashCode method. In particular, c1.equals(c2) implies that c1.hashCode()==c2.hashCode().

Overrides:

hashCode in class Object

Returns:

the hash code value for this collection

See Also:

Object.hashCode(), Object.equals(Object)

isEmpty

boolean isEmpty()

Returns true if this collection contains no elements.

Returns:

true if this collection contains no elements

iterator

Iterator<E> iterator()

Returns an iterator over the elements in this collection. There are no guarantees concerning the order in which the elements are returned (unless this collection is an instance of some class that provides a guarantee).

Specified by:

iterator in interface Iterable<E>

Returns:

an Iterator over the elements in this collection

remove

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 this collection contains one or more such elements. Returns true if this collection contained the specified element (or equivalently, if this collection changed as a result of the call).

Parameters:

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

Returns:

true if an element was removed as a result of this call

Throws:

ClassCastException - if the type of the specified element is incompatible with this collection (optional)

NullPointerException - if the specified element is null and this collection does not permit null elements (optional)

UnsupportedOperationException - if the remove operation is not supported by this collection

removeAll

boolean removeAll(Collection<?> c)

Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.

Parameters:

c - collection containing 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

ClassCastException - if the types of one or more elements in this collection are incompatible with the specified collection (optional)

NullPointerException - if this collection contains one or more null elements and the specified collection does not support null elements (optional), or if the specified collection is null

See Also:

remove(Object), contains(Object)

retainAll

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.

Parameters:

c - collection containing elements to be retained in this collection

Returns:

true if this collection changed as a result of the call

Throws:

UnsupportedOperationException - if the retainAll operation is not supported by this collection

ClassCastException - if the types of one or more elements in this collection are incompatible with the specified collection (optional)

NullPointerException - if this collection contains one or more null elements and the specified collection does not permit null elements (optional), or if the specified collection is null

See Also:

remove(Object), contains(Object)

size

int size()

Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Returns:

the number of elements in this collection

toArray

Object[] toArray()

Returns an array containing all of the elements in this collection. 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.

The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection is backed by an array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based APIs.

Returns:

an array containing all of the elements in this collection

toArray

<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 this collection fits in the specified array with room to spare (i.e., the array has more elements than this 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 this collection only if the caller knows that this 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.

Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

Suppose x is a collection known to contain only strings. The following code can be used to dump the collection into a newly allocated array of String:

 String[] y = x.toArray(new String[0]);
 

Note that toArray(new Object[0]) is identical in function to toArray().

Parameters:

a - the array into which the elements of this 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 all of the elements in this collection

Throws:

ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this collection

NullPointerException - if the specified array is null