Added in API level 1

Summary: Protected Ctors | Methods | Inherited Methods

AbstractCollection

public abstract class AbstractCollection
extends Object implements Collection<E>

java.lang.Object
↳	java.util.AbstractCollection<E>

Known direct subclasses AbstractList<E>, AbstractQueue<E>, AbstractSet<E>, ArrayDeque<E>, ConcurrentLinkedDeque<E> AbstractList<E> This class provides a skeletal implementation of the List interface to minimize the effort required to implement this interface backed by a "random access" data store (such as an array).  AbstractQueue<E> This class provides skeletal implementations of some Queue operations.  AbstractSet<E> This class provides a skeletal implementation of the Set interface to minimize the effort required to implement this interface.  ArrayDeque<E> Resizable-array implementation of the Deque interface.  ConcurrentLinkedDeque<E> An unbounded concurrent deque based on linked nodes.

Known indirect subclasses AbstractSequentialList<E>, ArrayBlockingQueue<E>, ArrayList<E>, ConcurrentLinkedQueue<E>, ConcurrentSkipListSet<E>, CopyOnWriteArraySet<E>, DelayQueue<E extends Delayed>, EnumSet<E extends Enum<E>>, HashSet<E>, LinkedBlockingDeque<E>, LinkedBlockingQueue<E>, LinkedHashSet<E>, LinkedList<E>, LinkedTransferQueue<E>, PriorityBlockingQueue<E>, PriorityQueue<E>, Stack<E>, SynchronousQueue<E>, TreeSet<E>, Vector<E> AbstractSequentialList<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).  ArrayBlockingQueue<E> A bounded blocking queue backed by an array.  ArrayList<E> Resizable-array implementation of the List interface.  ConcurrentLinkedQueue<E> An unbounded thread-safe queue based on linked nodes.  ConcurrentSkipListSet<E> A scalable concurrent NavigableSet implementation based on a ConcurrentSkipListMap.  CopyOnWriteArraySet<E> A Set that uses an internal CopyOnWriteArrayList for all of its operations.  DelayQueue<E extends Delayed> An unbounded blocking queue of Delayed elements, in which an element generally becomes eligible for removal when its delay has expired.  EnumSet<E extends Enum<E>> A specialized Set implementation for use with enum types.  HashSet<E> This class implements the Set interface, backed by a hash table (actually a HashMap instance).  LinkedBlockingDeque<E> An optionally-bounded blocking deque based on linked nodes.  LinkedBlockingQueue<E> An optionally-bounded blocking queue based on linked nodes.  LinkedHashSet<E> Hash table and linked list implementation of the Set interface, with well-defined encounter order.  LinkedList<E> Doubly-linked list implementation of the List and Deque interfaces.  LinkedTransferQueue<E> An unbounded TransferQueue based on linked nodes.  PriorityBlockingQueue<E> An unbounded blocking queue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations.  PriorityQueue<E> An unbounded priority queue based on a priority heap.  Stack<E> The Stack class represents a last-in-first-out (LIFO) stack of objects.  SynchronousQueue<E> A blocking queue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa.  TreeSet<E> A NavigableSet implementation based on a TreeMap.  Vector<E> The Vector class implements a growable array of objects.

---

This class provides a skeletal implementation of the Collection interface, to minimize the effort required to implement this interface.

To implement an unmodifiable collection, the programmer needs only to extend this class and provide implementations for the iterator and size methods. (The iterator returned by the iterator method must implement hasNext and next.)

To implement a modifiable collection, the programmer must additionally override this class's add method (which otherwise throws an UnsupportedOperationException), and the iterator returned by the iterator method must additionally implement its remove method.

The programmer should generally provide a void (no argument) and Collection constructor, as per the recommendation in the Collection interface specification.

The documentation for each non-abstract method in this class describes its implementation in detail. Each of these methods may be overridden if the collection being implemented admits a more efficient implementation.

This class is a member of the Java Collections Framework.

See also:

• Collection

Summary

Protected constructors
AbstractCollection() Sole constructor.

Public methods
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	isEmpty() Returns true if this collection contains no elements.
abstract Iterator<E>	iterator() Returns an iterator over the elements contained in this collection.
boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation).
boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).
boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this collection.
<T> T[]	toArray(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
Object[]	toArray() Returns an array containing all of the elements in this collection.
String	toString() Returns a string representation of this collection.

Inherited methods
From class java.lang.Object Object clone() Creates and returns a copy of this object. boolean equals(Object obj) Indicates whether some other object is "equal to" this one. void finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. final Class<?> getClass() Returns the runtime class of this Object. int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long timeoutMillis, int nanos) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait(long timeoutMillis) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait() Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
From interface java.util.Collection 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. 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.
From interface java.lang.Iterable default void forEach(Consumer<? 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<E> iterator() Returns an iterator over elements of type T. default Spliterator<E> spliterator() Creates a Spliterator over the elements described by this Iterable.

Protected constructors

AbstractCollection

Added in API level 1

protected AbstractCollection ()

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

Public methods

add

Added in API level 1

public 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.

Implementation Requirements:

• This implementation always throws an UnsupportedOperationException.

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
ClassCastException
NullPointerException
IllegalArgumentException
IllegalStateException

addAll

Added in API level 1

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

Adds all of the elements in the specified collection to this collection (optional operation). The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (This implies that the behavior of this call is undefined if the specified collection is this collection, and this collection is nonempty.) If the specified collection has a defined encounter order, processing of its elements generally occurs in that order.

Implementation Requirements:

• This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.

  Note that this implementation will throw an UnsupportedOperationException unless add is overridden (assuming the specified collection is non-empty).

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
ClassCastException
NullPointerException
IllegalArgumentException
IllegalStateException

See also:

• add(Object)

clear

Added in API level 1

public void clear ()

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

Implementation Requirements:

• This implementation iterates over this collection, removing each element using the Iterator.remove operation. Most implementations will probably choose to override this method for efficiency.

  Note that this implementation will throw an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection is non-empty.

Throws
UnsupportedOperationException

contains

Added in API level 1

public boolean contains (Object o)

Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that Objects.equals(o, e).

Implementation Requirements:

• This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.

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
NullPointerException

containsAll

Added in API level 1

public boolean containsAll (Collection<?> c)

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

Implementation Requirements:

• This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection. If all elements are so contained true is returned, otherwise false.

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
NullPointerException

See also:

• contains(Object)

isEmpty

Added in API level 1

public boolean isEmpty ()

Returns true if this collection contains no elements.

Implementation Requirements:

• This implementation returns size() == 0.

Returns
boolean	true if this collection contains no elements

iterator

Added in API level 1

public abstract Iterator<E> iterator ()

Returns an iterator over the elements contained in this collection.

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

remove

Added in API level 1

public boolean remove (Object o)

Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element e such that 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).

Implementation Requirements:

• This implementation iterates over the collection looking for the specified element. If it finds the element, it removes the element from the collection using the iterator's remove method.

  Note that this implementation throws an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection contains the specified object.

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
UnsupportedOperationException
ClassCastException
NullPointerException

removeAll

Added in API level 1

public 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.

Implementation Requirements:

• This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection. If it's so contained, it's removed from this collection with the iterator's remove method.

  Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method and this collection contains one or more 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
ClassCastException
NullPointerException

See also:

• remove(Object)
• contains(Object)

retainAll

Added in API level 1

public boolean retainAll (Collection<?> c)

Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.

Implementation Requirements:

• This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection. If it's not so contained, it's removed from this collection with the iterator's remove method.

  Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method and this collection contains one or more elements not present 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
ClassCastException
NullPointerException

See also:

• remove(Object)
• contains(Object)

size

Added in API level 1

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
int	the number of elements in this collection

toArray

Added in API level 1

public T[] toArray (T[] a)

Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.

If 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.

Implementation Requirements:

• This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index 0. If the number of elements returned by the iterator is too large to fit into the specified array, then the elements are returned in a newly allocated array with length equal to the number of elements returned by the iterator, even if the size of this collection changes during iteration, as might happen if the collection permits concurrent modification during iteration. The size method is called only as an optimization hint; the correct result is returned even if the iterator returns a different number of elements.

  This method is equivalent to:

   List<E> list = new ArrayList<E>(size());
   for (E e : this)
       list.add(e);
   return list.toArray(a);
   

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
NullPointerException

toArray

Added in API level 1

public 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.

Implementation Requirements:

• This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index 0. The length of the returned array is equal to the number of elements returned by the iterator, even if the size of this collection changes during iteration, as might happen if the collection permits concurrent modification during iteration. The size method is called only as an optimization hint; the correct result is returned even if the iterator returns a different number of elements.

  This method is equivalent to:

   List<E> list = new ArrayList<E>(size());
   for (E e : this)
       list.add(e);
   return list.toArray();
   

Returns
Object[]	an array, whose type is Object, containing all of the elements in this collection

toString

Added in API level 1

public String toString ()

Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String.valueOf(Object).

Returns
String	a string representation of this collection