# Collection

public interface **Collection** implements [Iterable](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html)\<E\>

|---|---|---|
| Known Indirect Subclasses [AbstractCollection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html)\<E\>, [AbstractList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractList.html)\<E\>, [AbstractQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractQueue.html)\<E\>, [AbstractSequentialList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractSequentialList.html)\<E\>, [AbstractSet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractSet.html)\<E\>, [ArrayBlockingQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ArrayBlockingQueue.html)\<E\>, [ArrayDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/ArrayDeque.html)\<E\>, [ArrayList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/ArrayList.html)\<E\>, [BlockingDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/BlockingDeque.html)\<E\>, [BlockingQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/BlockingQueue.html)\<E\>, [ConcurrentHashMap.KeySetView](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentHashMap.KeySetView.html)\<K, V\>, [ConcurrentLinkedDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedDeque.html)\<E\>, [ConcurrentLinkedQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html)\<E\>, [ConcurrentSkipListSet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentSkipListSet.html)\<E\>, [CopyOnWriteArrayList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CopyOnWriteArrayList.html)\<E\>, and [22 others.](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#) |---|---| | [AbstractCollection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html)\<E\> | This class provides a skeletal implementation of the Collection interface, to minimize the effort required to implement this interface. | | [AbstractList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractList.html)\<E\> | This class provides a skeletal implementation of the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/List.html` interface to minimize the effort required to implement this interface backed by a "random access" data store (such as an array). | | [AbstractQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractQueue.html)\<E\> | This class provides skeletal implementations of some `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html` operations. | | [AbstractSequentialList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractSequentialList.html)\<E\> | This class provides a skeletal implementation of the List interface to minimize the effort required to implement this interface backed by a "sequential access" data store (such as a linked list). | | [AbstractSet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractSet.html)\<E\> | This class provides a skeletal implementation of the Set interface to minimize the effort required to implement this interface. | | [ArrayBlockingQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ArrayBlockingQueue.html)\<E\> | A bounded {@linkplain BlockingQueue blocking queue} backed by an array. | | [ArrayDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/ArrayDeque.html)\<E\> | Resizable-array implementation of the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html` interface. | | [ArrayList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/ArrayList.html)\<E\> | Resizable-array implementation of the List interface. | | [BlockingDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/BlockingDeque.html)\<E\> | A `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html` that additionally supports blocking operations that wait for the deque to become non-empty when retrieving an element, and wait for space to become available in the deque when storing an element. | | [BlockingQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/BlockingQueue.html)\<E\> | A `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html` that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element. | | [ConcurrentHashMap.KeySetView](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentHashMap.KeySetView.html)\<K, V\> | A view of a ConcurrentHashMap as a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Set.html` of keys, in which additions may optionally be enabled by mapping to a common value. | | [ConcurrentLinkedDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedDeque.html)\<E\> | An unbounded concurrent {@linkplain Deque deque} based on linked nodes. | | [ConcurrentLinkedQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html)\<E\> | An unbounded thread-safe {@linkplain Queue queue} based on linked nodes. | | [ConcurrentSkipListSet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentSkipListSet.html)\<E\> | A scalable concurrent `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/NavigableSet.html` implementation based on a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentSkipListMap.html`. | | [CopyOnWriteArrayList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CopyOnWriteArrayList.html)\<E\> | A thread-safe variant of `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/ArrayList.html` in which all mutative operations (`add`, `set`, and so on) are implemented by making a fresh copy of the underlying array. | | [CopyOnWriteArraySet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CopyOnWriteArraySet.html)\<E\> | A `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Set.html` that uses an internal `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CopyOnWriteArrayList.html` for all of its operations. | | [DelayQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/DelayQueue.html)\<E extends [Delayed](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Delayed.html)\> | An unbounded {@linkplain BlockingQueue blocking queue} of `Delayed` elements, in which an element can only be taken when its delay has expired. | | [Deque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html)\<E\> | A linear collection that supports element insertion and removal at both ends. | | [EnumSet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/EnumSet.html)\<E extends [Enum](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Enum.html)\<E\>\> | A specialized `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Set.html` implementation for use with enum types. | | [HashSet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/HashSet.html)\<E\> | This class implements the Set interface, backed by a hash table (actually a HashMap instance). | | [LinkedBlockingDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/LinkedBlockingDeque.html)\<E\> | An optionally-bounded {@linkplain BlockingDeque blocking deque} based on linked nodes. | | [LinkedBlockingQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/LinkedBlockingQueue.html)\<E\> | An optionally-bounded {@linkplain BlockingQueue blocking queue} based on linked nodes. | | [LinkedHashSet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/LinkedHashSet.html)\<E\> | Hash table and linked list implementation of the Set interface, with predictable iteration order. | | [LinkedList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/LinkedList.html)\<E\> | Doubly-linked list implementation of the `List` and `Deque` interfaces. | | [LinkedTransferQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/LinkedTransferQueue.html)\<E\> | An unbounded `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TransferQueue.html` based on linked nodes. | | [List](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/List.html)\<E\> | An ordered collection (also known as a *sequence*). | | [NavigableSet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/NavigableSet.html)\<E\> | A `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/SortedSet.html` extended with navigation methods reporting closest matches for given search targets. | | [PriorityBlockingQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/PriorityBlockingQueue.html)\<E\> | An unbounded {@linkplain BlockingQueue blocking queue} that uses the same ordering rules as class `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/PriorityQueue.html` and supplies blocking retrieval operations. | | [PriorityQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/PriorityQueue.html)\<E\> | An unbounded priority {@linkplain Queue queue} based on a priority heap. | | [Queue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html)\<E\> | A collection designed for holding elements prior to processing. | | [Set](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Set.html)\<E\> | A collection that contains no duplicate elements. | | [SortedSet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/SortedSet.html)\<E\> | A `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Set.html` that further provides a *total ordering* on its elements. | | [Stack](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Stack.html)\<E\> | The `Stack` class represents a last-in-first-out (LIFO) stack of objects. | | [SynchronousQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/SynchronousQueue.html)\<E\> | A {@linkplain BlockingQueue blocking queue} in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa. | | [TransferQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TransferQueue.html)\<E\> | A `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/BlockingQueue.html` in which producers may wait for consumers to receive elements. | | [TreeSet](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/TreeSet.html)\<E\> | A `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/NavigableSet.html` implementation based on a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/TreeMap.html`. | | [Vector](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Vector.html)\<E\> | The `Vector` class implements a growable array of objects. | |||

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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#addAll(java.util.Collection<?%20extends%20E>)` 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#equals(java.lang.Object)` method. For example,
the specification for the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#contains(java.lang.Object)`
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
`https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html` 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.

This interface is a member of the
[Java Collections Framework](https://developers.google.com/j2objc/javadoc/jre/technotes/guides/collections/index.html).

##### See Also

- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Set.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/List.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Map.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/SortedSet.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/SortedMap.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/HashSet.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/TreeSet.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/ArrayList.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/LinkedList.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Vector.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collections.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Arrays.html`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html`

### Public Method Summary

|---|---|
| abstract boolean | [add](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#add(E))(E e) Ensures that this collection contains the specified element (optional operation). |
| abstract boolean | [addAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#addAll(java.util.Collection<?%20extends%20E>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<? extends E\> c) Adds all of the elements in the specified collection to this collection (optional operation). |
| abstract void | [clear](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#clear())() Removes all of the elements from this collection (optional operation). |
| abstract boolean | [contains](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#contains(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Returns true if this collection contains the specified element. |
| abstract boolean | [containsAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#containsAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> c) Returns true if this collection contains all of the elements in the specified collection. |
| abstract boolean | [equals](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#equals(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Compares the specified object with this collection for equality. |
| abstract int | [hashCode](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#hashCode())() Returns the hash code value for this collection. |
| abstract boolean | [isEmpty](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#isEmpty())() Returns true if this collection contains no elements. |
| abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\> | [iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#iterator())() Returns an iterator over the elements in this collection. |
| [Stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/stream/Stream.html)\<E\> | [parallelStream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#parallelStream())() Returns a possibly parallel `Stream` with this collection as its source. |
| abstract boolean | [remove](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#remove(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Removes a single instance of the specified element from this collection, if it is present (optional operation). |
| abstract boolean | [removeAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#removeAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation). |
| boolean | [removeIf](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#removeIf(java.util.function.Predicate<?%20super%20E>))([Predicate](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/function/Predicate.html)\<? super E\> filter) Removes all of the elements of this collection that satisfy the given predicate. |
| abstract boolean | [retainAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#retainAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> c) Retains only the elements in this collection that are contained in the specified collection (optional operation). |
| abstract int | [size](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#size())() Returns the number of elements in this collection. |
| [Spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html)\<E\> | [spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#spliterator())() Creates a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html` over the elements in this collection. |
| [Stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/stream/Stream.html)\<E\> | [stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#stream())() Returns a sequential `Stream` with this collection as its source. |
| abstract \<T\> T\[\] | [toArray](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#toArray(T[]))(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\[\]](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) | [toArray](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#toArray())() Returns an array containing all of the elements in this collection. |

### Inherited Method Summary

From interface [java.lang.Iterable](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html)

|---|---|
| void | [forEach](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html#forEach(java.util.function.Consumer<?%20super%20T>))([Consumer](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/function/Consumer.html)\<? super T\> action) Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception. |
| abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\> | [iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html#iterator())() Returns an iterator over elements of type `T`. |
| [Spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html)\<E\> | [spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html#spliterator())() Creates a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html` over the elements described by this `Iterable`. |

## Public Methods

#### public abstract 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/UnsupportedOperationException.html) | if the add operation is not supported by this collection |
| [ClassCastException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element prevents it from being added to this collection |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this collection does not permit null elements |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if some property of the element prevents it from being added to this collection |
| [IllegalStateException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalStateException.html) | if the element cannot be added at this time due to insertion restrictions |
|---|---|

#### public abstract boolean
**addAll**
([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<? 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/UnsupportedOperationException.html) | if the addAll operation is not supported by this collection |
| [ClassCastException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of an element of the specified collection prevents it from being added to this collection |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified collection contains a null element and this collection does not permit null elements, or if the specified collection is null |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if some property of an element of the specified collection prevents it from being added to this collection |
| [IllegalStateException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalStateException.html) | if not all the elements can be added at this time due to insertion restrictions |
|---|---|

##### See Also

- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#add(E)`

#### public abstract void
**clear**
()

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

##### Throws

| [UnsupportedOperationException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/UnsupportedOperationException.html) | if the clear operation is not supported by this collection |
|---|---|

#### public abstract boolean
**contains**
([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the type of the specified element is incompatible with this collection ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this collection does not permit null elements ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
|---|---|

#### public abstract boolean
**containsAll**
([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the types of one or more elements in the specified collection are incompatible with this collection ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified collection contains one or more null elements and this collection does not permit null elements ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)), or if the specified collection is null. |
|---|---|

##### See Also

- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#contains(java.lang.Object)`

#### public abstract boolean
**equals**
([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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 to be compared for equality with this collection |
|---|---|

##### Returns

- true if the specified object is equal to this collection

##### See Also

- `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#equals(java.lang.Object)`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Set.html#equals(java.lang.Object)`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/List.html#equals(java.lang.Object)`

#### public abstract 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

- the hash code value for this collection

##### See Also

- `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#hashCode()`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#equals(java.lang.Object)`

#### public abstract boolean
**isEmpty**
()

Returns true if this collection contains no elements.

##### Returns

- true if this collection contains no elements

#### public abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<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

- an Iterator over the elements in this collection

#### public [Stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/stream/Stream.html)\<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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#spliterator()`
method cannot return a spliterator that is `IMMUTABLE`,
`CONCURRENT`, or *late-binding* . (See `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#spliterator()`
for details.)

##### Returns

- a possibly parallel `Stream` over the elements in this collection

#### public abstract boolean
**remove**
([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the type of the specified element is incompatible with this collection ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this collection does not permit null elements ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
| [UnsupportedOperationException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/UnsupportedOperationException.html) | if the remove operation is not supported by this collection |
|---|---|

#### public abstract boolean
**removeAll**
([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/UnsupportedOperationException.html) | if the removeAll method is not supported by this collection |
| [ClassCastException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the types of one or more elements in this collection are incompatible with the specified collection ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if this collection contains one or more null elements and the specified collection does not support null elements ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)), or if the specified collection is null |
|---|---|

##### See Also

- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#remove(java.lang.Object)`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#contains(java.lang.Object)`

#### public boolean
**removeIf**
([Predicate](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/function/Predicate.html)\<? 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.

##### Parameters

| filter | a predicate which returns `true` for elements to be removed |
|---|---|

##### Returns

- `true` if any elements were removed

##### Throws

| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified filter is null |
| [UnsupportedOperationException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/UnsupportedOperationException.html) | 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. |
|---|---|

#### public abstract boolean
**retainAll**
([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/UnsupportedOperationException.html) | if the retainAll operation is not supported by this collection |
| [ClassCastException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the types of one or more elements in this collection are incompatible with the specified collection ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if this collection contains one or more null elements and the specified collection does not permit null elements ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)), or if the specified collection is null |
|---|---|

##### See Also

- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#remove(java.lang.Object)`
- `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#contains(java.lang.Object)`

#### public abstract 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

#### public [Spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html)\<E\>
**spliterator**
()

Creates a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html` 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html#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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#stream()` and
`https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#parallelStream()`} methods, spliterators should either have the
characteristic of `IMMUTABLE` or `CONCURRENT`, or be
*[late-binding](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator#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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#stream()` and `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#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
`https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#stream()` and `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#parallelStream()` methods will reflect the
contents of the collection as of initiation of the terminal stream
operation.

##### Returns

- a `Spliterator` over the elements in this collection

#### public [Stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/stream/Stream.html)\<E\>
**stream**
()

Returns a sequential `Stream` with this collection as its source.

This method should be overridden when the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#spliterator()`
method cannot return a spliterator that is `IMMUTABLE`,
`CONCURRENT`, or *late-binding* . (See `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#spliterator()`
for details.)

##### Returns

- a sequential `Stream` over the elements in this collection

#### public abstract 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#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().

<br />

##### 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ArrayStoreException.html) | if the runtime type of the specified array is not a supertype of the runtime type of every element in this collection |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified array is null |
|---|---|

#### public abstract [Object\[\]](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html)
**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