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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 can only be taken 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 predictable iteration 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). This implementation always throws an `UnsupportedOperationException`.
`boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
`void` | `clear()` Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the `Iterator.remove` operation.
`boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
`boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection. 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.
`boolean` | `isEmpty()` Returns `true` if this collection contains no elements. This implementation returns `size() == 0`.
`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). This implementation iterates over the collection looking for the specified element.
`boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation). 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.
`boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation). 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.
`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. 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`.
`Object[]` | `toArray()` Returns an array containing all of the elements in this collection. 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`.
`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 timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
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.
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

    protected AbstractCollection ()

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

## Public methods

### add

    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.

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

    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.)

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

    public void clear ()

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

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

    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 `(o==null ? e==null : o.equals(e))`.

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

    public boolean containsAll (Collection<?> c)

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

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

    public boolean isEmpty ()

Returns `true` if this collection contains no elements.

This implementation returns `size() == 0`.

Returns
`boolean` | `true` if this collection contains no elements

### iterator

    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

    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 `(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).

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

    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.

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

    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.

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

    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

    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.

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

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

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

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

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

    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 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.

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 containing all of the elements in this collection

### toString

    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

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