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Collection

public interface Collection
implements Iterable<E>

java.util.Collection<E>
Known Indirect Subclasses


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. The JDK does not provide any direct implementations of this interface: it provides implementations of more specific subinterfaces like Set and List. This interface is typically used to pass collections around and manipulate them where maximum generality is desired.

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 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.

See also:

Summary

Public methods

abstract boolean add(E e)

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

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

Adds all of the elements in the specified collection to this collection (optional operation).

abstract void clear()

Removes all of the elements from this collection (optional operation).

abstract boolean contains(Object o)

Returns true if this collection contains the specified element.

abstract boolean containsAll(Collection<?> c)

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

abstract boolean equals(Object o)

Compares the specified object with this collection for equality.

abstract int hashCode()

Returns the hash code value for this collection.

abstract boolean isEmpty()

Returns true if this collection contains no elements.

abstract Iterator<E> iterator()

Returns an iterator over the elements in this collection.

default Stream<E> parallelStream()

Returns a possibly parallel Stream with this collection as its source.

abstract boolean remove(Object o)

Removes a single instance of the specified element from this collection, if it is present (optional operation).

abstract boolean removeAll(Collection<?> c)

Removes all of this collection's elements that are also contained in the specified collection (optional operation).

default boolean removeIf(Predicate<? super E> filter)

Removes all of the elements of this collection that satisfy the given predicate.

abstract 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.

default Spliterator<E> spliterator()

Creates a Spliterator over the elements in this collection.

default Stream<E> stream()

Returns a sequential Stream with this collection as its source.

abstract <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.

abstract Object[] toArray()

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

Inherited methods

From interface java.lang.Iterable

Public methods

add

Added in API level 1
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 E: element whose presence in this collection is to be ensured
Returns
boolean 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

Added in API level 1
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: collection containing elements to be added to this collection
Returns
boolean 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:

clear

Added in API level 1
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

Added in API level 1
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 Object: element whose presence in this collection is to be tested
Returns
boolean 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

Added in API level 1
boolean containsAll (Collection<?> c)

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

Parameters
c Collection: collection to be checked for containment in this collection
Returns
boolean 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:

equals

Added in API level 1
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.)

Parameters
o Object: object to be compared for equality with this collection
Returns
boolean true if the specified object is equal to this collection

See also:

hashCode

Added in API level 1
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().

Returns
int the hash code value for this collection

See also:

isEmpty

Added in API level 1
boolean isEmpty ()

Returns true if this collection contains no elements.

Returns
boolean true if this collection contains no elements

iterator

Added in API level 1
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).

Returns
Iterator<E> an Iterator over the elements in this collection

parallelStream

Added in API level 24
Stream<E> parallelStream ()

Returns a possibly parallel Stream with this collection as its source. It is allowable for this method to return a sequential stream.

This method should be overridden when the spliterator() method cannot return a spliterator that is IMMUTABLE, CONCURRENT, or late-binding. (See spliterator() for details.)

Implementation Requirements:
  • The default implementation creates a parallel Stream from the collection's Spliterator.
Returns
Stream<E> a possibly parallel Stream over the elements in this collection

remove

Added in API level 1
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 Object: element to be removed from this collection, if present
Returns
boolean 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

Added in API level 1
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: collection containing elements to be removed from this collection
Returns
boolean 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:

removeIf

Added in API level 24
boolean removeIf (Predicate<? super E> filter)

Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.

Implementation Requirements:
  • The default implementation traverses all elements of the collection using its iterator(). Each matching element is removed using remove(). If the collection's iterator does not support removal then an UnsupportedOperationException will be thrown on the first matching element.
Parameters
filter Predicate: a predicate which returns true for elements to be removed
Returns
boolean true if any elements were removed
Throws
NullPointerException if the specified filter is null
UnsupportedOperationException if elements cannot be removed from this collection. Implementations may throw this exception if a matching element cannot be removed or if, in general, removal is not supported.

retainAll

Added in API level 1
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: collection containing elements to be retained in this collection
Returns
boolean 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:

size

Added in API level 1
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
int the number of elements in this collection

spliterator

Spliterator<E> spliterator ()

Creates a Spliterator over the elements in this collection. Implementations should document characteristic values reported by the spliterator. Such characteristic values are not required to be reported if the spliterator reports SIZED and this collection contains no elements.

The default implementation should be overridden by subclasses that can return a more efficient spliterator. In order to preserve expected laziness behavior for the stream() and parallelStream()} methods, spliterators should either have the characteristic of IMMUTABLE or CONCURRENT, or be late-binding. If none of these is practical, the overriding class should describe the spliterator's documented policy of binding and structural interference, and should override the stream() and parallelStream() methods to create streams using a Supplier of the spliterator, as in:

Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
 

These requirements ensure that streams produced by the stream() and parallelStream() methods will reflect the contents of the collection as of initiation of the terminal stream operation.

Implementation Requirements:
  • The default implementation creates a late-binding spliterator from the collections's Iterator. The spliterator inherits the fail-fast properties of the collection's iterator.

    The created Spliterator reports SIZED.

Implementation Note:
  • The created Spliterator additionally reports SUBSIZED.

    If a spliterator covers no elements then the reporting of additional characteristic values, beyond that of SIZED and SUBSIZED, does not aid clients to control, specialize or simplify computation. However, this does enable shared use of an immutable and empty spliterator instance (see emptySpliterator()) for empty collections, and enables clients to determine if such a spliterator covers no elements.

Returns
Spliterator<E> a Spliterator over the elements in this collection

stream

Added in API level 24
Stream<E> stream ()

Returns a sequential Stream with this collection as its source.

This method should be overridden when the spliterator() method cannot return a spliterator that is IMMUTABLE, CONCURRENT, or late-binding. (See spliterator() for details.)

Implementation Requirements:
  • The default implementation creates a sequential Stream from the collection's Spliterator.
Returns
Stream<E> a sequential Stream over the elements in this collection

toArray

Added in API level 1
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 T: 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
T[] 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

toArray

Added in API level 1
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
Object[] an array containing all of the elements in this collection
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