AbstractCollection

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## java.util
Class AbstractCollection

    java.lang.Object
      java.util.AbstractCollection

All Implemented Interfaces:
  Collection

Direct Known Subclasses:
  AbstractList, AbstractSet

    public abstract class AbstractCollection

extends Object
implements Collection

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 methods 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
Version:
  1.16, 02/02/00
Author:
  Josh Bloch
See Also:
  `Collection`

Method Summary
`boolean` | `add(Object o)` Ensures that this collection contains the specified element (optional operation).
`boolean` | `addAll(Collection 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` | `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 from this collection all of its elements that are 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.
`Object[]` | `toArray()` Returns an array containing all of the elements in this collection.
`Object[]` | `toArray(Object[] 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 inherited from class java.lang.Object
`equals, getClass, hashCode, notify, notifyAll, wait, wait, wait`

Methods inherited from interface java.util.Collection
`equals, hashCode`

Method Detail

### iterator

    public abstract Iterator iterator()

  Returns an iterator over the elements contained in this collection.

  Specified by:
    `iterator` in interface `Collection`
  Returns:
    an iterator over the elements contained in this collection.

### size

    public abstract int size()

  Returns the number of elements in this collection. If the collection contains more than `Integer.MAX_VALUE` elements, returns `Integer.MAX_VALUE`.

  Specified by:
    `size` in interface `Collection`
  Returns:
    the number of elements in this collection.

### isEmpty

    public boolean isEmpty()

  Returns `true` if this collection contains no elements.

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

  Specified by:
    `isEmpty` in interface `Collection`
  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 `(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.

  Specified by:
    `contains` in interface `Collection`
  Parameters:
    `o` - object to be checked for containment in this collection.
  Returns:
    `true` if this collection contains the specified element.

### toArray

    public Object[] toArray()

  Returns an array containing all of the elements in this collection. If the 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 the collection. (In other words, this method must allocate a new array even if the collection is backed by an Array). The caller is thus free to modify the returned array.

  This implementation allocates the array to be returned, and iterates over the elements in the collection, storing each object reference in the next consecutive element of the array, starting with element 0.

  Specified by:
    `toArray` in interface `Collection`
  Returns:
    an array containing all of the elements in this collection.

### toArray

    public Object[] toArray(Object[] 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 the collection fits in the specified array with room to spare (i.e., the array has more elements than the 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 the collection only if the caller knows that the 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.

  This implementation checks if the array is large enough to contain the collection; if not, it allocates a new array of the correct size and type (using reflection). Then, it iterates over the collection, storing each object reference in the next consecutive element of the array, starting with element 0. If the array is larger than the collection, a `null` is stored in the first location after the end of the collection.

  Specified by:
    `toArray` in interface `Collection`
  Parameters:
    `a` - the array into which the elements of the 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 the elements of the collection.
  Throws:
    `NullPointerException` - if the specified array is `null`.
    `ArrayStoreException` - if the runtime type of the specified array is not a supertype of the runtime type of every element in this collection.

### add

    public boolean add(Object o)

  Ensures that this collection contains the specified element (optional operation). Returns `true` if the 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 the 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.

  This implementation always throws an `UnsupportedOperationException`.

  Specified by:
    `add` in interface `Collection`
  Parameters:
    `o` - element whose presence in this collection is to be ensured.
  Returns:
    `true` if the collection changed as a result of the call.
  Throws:
    `UnsupportedOperationException` - if the `add` method is not supported by this collection.
    `NullPointerException` - if this collection does not permit `null` elements, and the specified element is `null`.
    `ClassCastException` - if the class of the specified element prevents it from being added to this collection.
    `IllegalArgumentException` - if some aspect of this element prevents it from being added to 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 the collection contains one or more such elements. Returns `true` if the collection contained the specified element (or equivalently, if the 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.

  Specified by:
    `remove` in interface `Collection`
  Parameters:
    `o` - element to be removed from this collection, if present.
  Returns:
    `true` if the collection contained the specified element.
  Throws:
    `UnsupportedOperationException` - if the `remove` method is not supported by this collection.

### 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`.

  Specified by:
    `containsAll` in interface `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:
    `NullPointerException` - if the specified collection is null.
  See Also:
    `contains(Object)`

### addAll

    public boolean addAll(Collection 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).

  Specified by:
    `addAll` in interface `Collection`
  Parameters:
    `c` - collection whose elements are to be added to this collection.
  Returns:
    `true` if this collection changed as a result of the call.
  Throws:
    `UnsupportedOperationException` - if this collection does not support the `addAll` method.
    `NullPointerException` - if the specified collection is null.
  See Also:
    `add(Object)`

### removeAll

    public boolean removeAll(Collection c)

  Removes from this collection all of its elements 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. 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.

  Specified by:
    `removeAll` in interface `Collection`
  Parameters:
    `c` - elements to be removed from this collection.
  Returns:
    `true` if this collection changed as a result of the call.
  Throws:
    `UnsupportedOperationException` - if the `removeAll` method is not supported by this collection.
    `NullPointerException` - 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.

  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.

  Specified by:
    `retainAll` in interface `Collection`
  Parameters:
    `c` - elements to be retained in this collection.
  Returns:
    `true` if this collection changed as a result of the call.
  Throws:
    `UnsupportedOperationException` - if the `retainAll` method is not supported by this Collection.
    `NullPointerException` - 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 call returns (unless it throws an exception).

  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.

  Specified by:
    `clear` in interface `Collection`
  Throws:
    `UnsupportedOperationException` - if the `clear` method 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)`.

  This implementation creates an empty string buffer, appends a left square bracket, and iterates over the collection appending the string representation of each element in turn. After appending each element except the last, the string `", "` is appended. Finally a right bracket is appended. A string is obtained from the string buffer, and returned.

  Overrides:
    `toString` in class `Object`
  Returns:
    a string representation of this collection.

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