Module java.base
Package java.util

Interface Collection<E>

Type Parameters:
E - the type of elements in this collection
All Superinterfaces:
Iterable<E>
All Known Subinterfaces:
BeanContext, BeanContextServices, BlockingDeque<E>, BlockingQueue<E>, Deque<E>, EventSet, List<E>, NavigableSet<E>, Queue<E>, Set<E>, SortedSet<E>, TransferQueue<E>
All Known Implementing Classes:
AbstractCollection, AbstractList, AbstractQueue, AbstractSequentialList, AbstractSet, ArrayBlockingQueue, ArrayDeque, ArrayList, AttributeList, BeanContextServicesSupport, BeanContextSupport, ConcurrentHashMap.KeySetView, ConcurrentLinkedDeque, ConcurrentLinkedQueue, ConcurrentSkipListSet, CopyOnWriteArrayList, CopyOnWriteArraySet, DelayQueue, EnumSet, HashSet, JobStateReasons, LinkedBlockingDeque, LinkedBlockingQueue, LinkedHashSet, LinkedList, LinkedTransferQueue, PriorityBlockingQueue, PriorityQueue, RoleList, RoleUnresolvedList, Stack, SynchronousQueue, TreeSet, 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. 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.

Certain methods are specified to be optional. If a collection implementation doesn't implement a particular operation, it should define the corresponding method to throw UnsupportedOperationException. Such methods are marked "optional operation" in method specifications of the collections interfaces.

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.

Some collection operations which perform recursive traversal of the collection may fail with an exception for self-referential instances where the collection directly or indirectly contains itself. This includes the clone(), equals(), hashCode() and toString() methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so.

View Collections

Most collections manage storage for elements they contain. By contrast, view collections themselves do not store elements, but instead they rely on a backing collection to store the actual elements. Operations that are not handled by the view collection itself are delegated to the backing collection. Examples of view collections include the wrapper collections returned by methods such as Collections.checkedCollection, Collections.synchronizedCollection, and Collections.unmodifiableCollection. Other examples of view collections include collections that provide a different representation of the same elements, for example, as provided by List.subList, NavigableSet.subSet, or Map.entrySet. Any changes made to the backing collection are visible in the view collection. Correspondingly, any changes made to the view collection — if changes are permitted — are written through to the backing collection. Although they technically aren't collections, instances of Iterator and ListIterator can also allow modifications to be written through to the backing collection, and in some cases, modifications to the backing collection will be visible to the Iterator during iteration.

Unmodifiable Collections

Certain methods of this interface are considered "destructive" and are called "mutator" methods in that they modify the group of objects contained within the collection on which they operate. They can be specified to throw UnsupportedOperationException if this collection implementation does not support the operation. Such methods should (but are not required to) throw an UnsupportedOperationException if the invocation would have no effect on the collection. For example, consider a collection that does not support the add operation. What will happen if the addAll method is invoked on this collection, with an empty collection as the argument? The addition of zero elements has no effect, so it is permissible for this collection simply to do nothing and not to throw an exception. However, it is recommended that such cases throw an exception unconditionally, as throwing only in certain cases can lead to programming errors.

An unmodifiable collection is a collection, all of whose mutator methods (as defined above) are specified to throw UnsupportedOperationException. Such a collection thus cannot be modified by calling any methods on it. For a collection to be properly unmodifiable, any view collections derived from it must also be unmodifiable. For example, if a List is unmodifiable, the List returned by List.subList is also unmodifiable.

An unmodifiable collection is not necessarily immutable. If the contained elements are mutable, the entire collection is clearly mutable, even though it might be unmodifiable. For example, consider two unmodifiable lists containing mutable elements. The result of calling list1.equals(list2) might differ from one call to the next if the elements had been mutated, even though both lists are unmodifiable. However, if an unmodifiable collection contains all immutable elements, it can be considered effectively immutable.

Unmodifiable View Collections

An unmodifiable view collection is a collection that is unmodifiable and that is also a view onto a backing collection. Its mutator methods throw UnsupportedOperationException, as described above, while reading and querying methods are delegated to the backing collection. The effect is to provide read-only access to the backing collection. This is useful for a component to provide users with read access to an internal collection, while preventing them from modifying such collections unexpectedly. Examples of unmodifiable view collections are those returned by the Collections.unmodifiableCollection, Collections.unmodifiableList, and related methods.

Note that changes to the backing collection might still be possible, and if they occur, they are visible through the unmodifiable view. Thus, an unmodifiable view collection is not necessarily immutable. However, if the backing collection of an unmodifiable view is effectively immutable, or if the only reference to the backing collection is through an unmodifiable view, the view can be considered effectively immutable.

Serializability of Collections

Serializability of collections is optional. As such, none of the collections interfaces are declared to implement the Serializable interface. However, serializability is regarded as being generally useful, so most collection implementations are serializable.

The collection implementations that are public classes (such as ArrayList or HashMap) are declared to implement the Serializable interface if they are in fact serializable. Some collections implementations are not public classes, such as the unmodifiable collections. In such cases, the serializability of such collections is described in the specification of the method that creates them, or in some other suitable place. In cases where the serializability of a collection is not specified, there is no guarantee about the serializability of such collections. In particular, many view collections are not serializable.

A collection implementation that implements the Serializable interface cannot be guaranteed to be serializable. The reason is that in general, collections contain elements of other types, and it is not possible to determine statically whether instances of some element type are actually serializable. For example, consider a serializable Collection<E>, where E does not implement the Serializable interface. The collection may be serializable, if it contains only elements of some serializable subtype of E, or if it is empty. Collections are thus said to be conditionally serializable, as the serializability of the collection as a whole depends on whether the collection itself is serializable and on whether all contained elements are also serializable.

An additional case occurs with instances of SortedSet and SortedMap. These collections can be created with a Comparator that imposes an ordering on the set elements or map keys. Such a collection is serializable only if the provided Comparator is also serializable.

This interface is a member of the Java Collections Framework.

Implementation Requirements:
The default method implementations (inherited or otherwise) do not apply any synchronization protocol. If a Collection implementation has a specific synchronization protocol, then it must override default implementations to apply that protocol.
Since:
1.2
See Also:

  • Method Summary

    Modifier and Type
    Method
    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.
    default Stream<E>
    parallelStream()
    Returns a possibly parallel Stream with this collection as its source.
    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).
    default boolean
    removeIf(Predicate<? super E> filter)
    Removes all of the elements of this collection that satisfy the given predicate.
    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.
    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.
    Object[]
    toArray()
    Returns an array containing all of the elements in this collection.
    default <T> T[]
    toArray(IntFunction<T[]> generator)
    Returns an array containing all of the elements in this collection, using the provided generator function to allocate the returned array.
    <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.

    Methods declared in interface java.lang.Iterable

    forEach

  • Method Details

    • 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

    • isEmpty

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

    • 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 Objects.equals(o, 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)

    • 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

    • 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's runtime component type is Object.

      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.

      API Note:
      This method acts as a bridge between array-based and collection-based APIs. It returns an array whose runtime type is Object[]. Use toArray(T[]) to reuse an existing array, or use toArray(IntFunction) to control the runtime type of the array.
      Returns:
      an array, whose runtime component type is Object, 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.

      API Note:
      This method acts as a bridge between array-based and collection-based APIs. It allows an existing array to be reused under certain circumstances. Use toArray() to create an array whose runtime type is Object[], or use toArray(IntFunction) to control the runtime type of the array.

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

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

      The return value is reassigned to the variable y, because a new array will be allocated and returned if the collection x has too many elements to fit into the existing array y.

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

      Type Parameters:
      T - the component type of the array to contain the collection
      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 any element in this collection is not assignable to the runtime component type of the specified array
      NullPointerException - if the specified array is null

    • toArray

      default <T> T[] toArray(IntFunction<T[]> generator)
      Returns an array containing all of the elements in this collection, using the provided generator function to allocate the returned array.

      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.

      API Note:
      This method acts as a bridge between array-based and collection-based APIs. It allows creation of an array of a particular runtime type. Use toArray() to create an array whose runtime type is Object[], or use toArray(T[]) to reuse an existing array.

      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(String[]::new);
      Implementation Requirements:
      The default implementation calls the generator function with zero and then passes the resulting array to toArray(T[]).
      Type Parameters:
      T - the component type of the array to contain the collection
      Parameters:
      generator - a function which produces a new array of the desired type and the provided length
      Returns:
      an array containing all of the elements in this collection
      Throws:
      ArrayStoreException - if the runtime type of any element in this collection is not assignable to the runtime component type of the generated array
      NullPointerException - if the generator function is null
      Since:
      11

    • 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

    • 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 Objects.equals(o, 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

    • 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:

    • 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:

    • 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:

    • removeIf

      default 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 Iterator.remove(). If the collection's iterator does not support removal then an UnsupportedOperationException will be thrown on the first matching element.
      Parameters:
      filter - a predicate which returns true for elements to be removed
      Returns:
      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.
      Since:
      1.8

    • 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:

    • 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

    • 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:

    • 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:

    • spliterator

      default 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 Spliterator.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.

      Specified by:
      spliterator in interface Iterable<E>
      Implementation Requirements:
      The default implementation creates a late-binding spliterator from the collection's Iterator. The spliterator inherits the fail-fast properties of the collection's iterator.

      The created Spliterator reports Spliterator.SIZED.

      Implementation Note:
      The created Spliterator additionally reports Spliterator.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 Spliterators.emptySpliterator()) for empty collections, and enables clients to determine if such a spliterator covers no elements.

      Returns:
      a Spliterator over the elements in this collection
      Since:
      1.8

    • stream

      default 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:
      a sequential Stream over the elements in this collection
      Since:
      1.8

    • parallelStream

      default 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:
      a possibly parallel Stream over the elements in this collection
      Since:
      1.8