# Class AbstractCollection<E>

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

Type Parameters:
  `E` - the type of elements in this collection

All Implemented Interfaces:
  `Iterable<E>, Collection<E>`

Direct Known Subclasses:
  `AbstractList, AbstractQueue, AbstractSet, ArrayDeque, ConcurrentLinkedDeque`

public abstract class AbstractCollection<E> extends Object implements Collection<E>
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.

Since:
  1.2
See Also:
  - `Collection`

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `AbstractCollection()`
  Sole constructor.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `add(E e)`
  Ensures that this collection contains the specified element (optional operation).
  `boolean`
  `addAll(Collection<? extends E> c)`
  Adds all of the elements in the specified collection to this collection (optional operation).
  `void`
  `clear()`
  Removes all of the elements from this collection (optional operation).
  `boolean`
  `contains(Object o)`
  Returns `true` if this collection contains the specified element.
  `boolean`
  `containsAll(Collection<?> c)`
  Returns `true` if this collection contains all of the elements in the specified collection.
  `boolean`
  `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).
  `Object[]`
  `toArray()`
  Returns an array containing all of the 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.
  `String`
  `toString()`
  Returns a string representation of this collection.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

  ### Methods declared in interface Collection

  `equals, hashCode, parallelStream, removeIf, size, spliterator, stream, toArray`

  ### Methods declared in interface Iterable

  `forEach`

- ## Constructor Details

  - ### AbstractCollection

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

- ## Method Details

  - ### iterator

    public abstract Iterator<E> iterator()
    Returns an iterator over the elements contained in this collection.
    Specified by:
      `iterator` in interface `Collection<E>`
    Specified by:
      `iterator` in interface `Iterable<E>`
    Returns:
      an iterator over the elements contained in this collection

  - ### isEmpty

    public boolean isEmpty()
    Returns `true` if this collection contains no elements.
    Specified by:
      `isEmpty` in interface `Collection<E>`
    Implementation Requirements:
      This implementation returns `size() == 0`.
    Returns:
      `true` if this collection contains no elements

  - ### 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 `Objects.equals(o, e)`.
    Specified by:
      `contains` in interface `Collection<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` - element whose presence in this collection is to be tested
    Returns:
      `true` if this collection contains the specified element
    Throws:
      `ClassCastException` - if the type of the specified element is incompatible with this collection (optional)
      `NullPointerException` - if the specified element is null and this collection does not permit null elements (optional)

  - ### 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'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.

    Specified by:
      `toArray` in interface `Collection<E>`
    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:
      an array, whose runtime component type is `Object`, containing all of the elements in this collection

  - ### toArray

    public <T> T[] toArray(T[] a)
    Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.

    If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to `null`. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain any `null` elements.)

    If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

    Specified by:
      `toArray` in interface `Collection<E>`
    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);

    Type Parameters:
      `T` - the component type of the array to contain the collection
    Parameters:
      `a` - the array into which the elements of this collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
    Returns:
      an array containing all of the elements in this collection
    Throws:
      `ArrayStoreException` - if the runtime type of any element in this collection is not assignable to the runtime component type of the specified array
      `NullPointerException` - if the specified array is null

  - ### 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.

    Specified by:
      `add` in interface `Collection<E>`
    Implementation Requirements:
      This implementation always throws an `UnsupportedOperationException`.
    Parameters:
      `e` - element whose presence in this collection is to be ensured
    Returns:
      `true` if this collection changed as a result of the call
    Throws:
      `UnsupportedOperationException` - if the `add` operation is not supported by this collection
      `ClassCastException` - if the class of the specified element prevents it from being added to this collection
      `NullPointerException` - if the specified element is null and this collection does not permit null elements
      `IllegalArgumentException` - if some property of the element prevents it from being added to this collection
      `IllegalStateException` - if the element cannot be added at this time due to insertion restrictions

  - ### remove

    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).
    Specified by:
      `remove` in interface `Collection<E>`
    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` - element to be removed from this collection, if present
    Returns:
      `true` if an element was removed as a result of this call
    Throws:
      `UnsupportedOperationException` - if the `remove` operation is not supported by this collection
      `ClassCastException` - if the type of the specified element is incompatible with this collection (optional)
      `NullPointerException` - if the specified element is null and this collection does not permit null elements (optional)

  - ### containsAll

    public boolean containsAll(Collection<?> c)
    Returns `true` if this collection contains all of the elements in the specified collection.
    Specified by:
      `containsAll` in interface `Collection<E>`
    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 to be checked for containment in this collection
    Returns:
      `true` if this collection contains all of the elements in the specified collection
    Throws:
      `ClassCastException` - if the types of one or more elements in the specified collection are incompatible with this collection (optional)
      `NullPointerException` - if the specified collection contains one or more null elements and this collection does not permit null elements (optional) or if the specified collection is null.
    See Also:
      - `contains(Object)`

  - ### 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.) If the specified collection has a defined encounter order, processing of its elements generally occurs in that order.
    Specified by:
      `addAll` in interface `Collection<E>`
    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 containing elements to be added to this collection
    Returns:
      `true` if this collection changed as a result of the call
    Throws:
      `UnsupportedOperationException` - if the `addAll` operation is not supported by this collection
      `ClassCastException` - if the class of an element of the specified collection prevents it from being added to this collection
      `NullPointerException` - if the specified collection contains a null element and this collection does not permit null elements, or if the specified collection is null
      `IllegalArgumentException` - if some property of an element of the specified collection prevents it from being added to this collection
      `IllegalStateException` - if not all the elements can be added at this time due to insertion restrictions
    See Also:
      - `add(Object)`

  - ### 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.
    Specified by:
      `removeAll` in interface `Collection<E>`
    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 containing elements to be removed from this collection
    Returns:
      `true` if this collection changed as a result of the call
    Throws:
      `UnsupportedOperationException` - if the `removeAll` operation is not supported by this collection
      `ClassCastException` - if the types of one or more elements in this collection are incompatible with the specified collection (optional)
      `NullPointerException` - if this collection contains one or more null elements and the specified collection does not support null elements (optional) or if the specified collection is null
    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.
    Specified by:
      `retainAll` in interface `Collection<E>`
    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 containing elements to be retained in this collection
    Returns:
      `true` if this collection changed as a result of the call
    Throws:
      `UnsupportedOperationException` - if the `retainAll` operation is not supported by this collection
      `ClassCastException` - if the types of one or more elements in this collection are incompatible with the specified collection (optional)
      `NullPointerException` - if this collection contains one or more null elements and the specified collection does not permit null elements (optional) or if the specified collection is null
    See Also:
      - `remove(Object)`
      - `contains(Object)`

  - ### clear

    public void clear()
    Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns.
    Specified by:
      `clear` in interface `Collection<E>`
    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` - if the `clear` operation is not supported by 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)`.
    Overrides:
      `toString` in class `Object`
    Returns:
      a string representation of this collection