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## java.util
Interface Collection

All Known Subinterfaces:
  List, Set, SortedSet

All Known Implementing Classes:
  AbstractCollection

public interface Collection

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 SDK 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 SDK comply.

Since:
  1.2
See Also:
  `Set`, `List`, `Map`, `SortedSet`, `SortedMap`, `HashSet`, `TreeSet`, `ArrayList`, `LinkedList`, `Vector`, `Collections`, `Arrays`, `AbstractCollection`

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` | `equals(Object o)` Compares the specified object with this collection for equality.
`int` | `hashCode()` Returns the hash code value for this collection.
`boolean` | `isEmpty()` Returns `true` if this collection contains no elements.
`Iterator` | `iterator()` Returns an iterator over the elements 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 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).
`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 whose runtime type is that of the specified array.

Method Detail

### size

    public 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

### isEmpty

    public boolean isEmpty()

  Returns `true` if this collection contains no elements.
  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))`.
  Parameters:
    `o` - element whose presence in this collection is to be tested.
  Returns:
    `true` if this collection contains the specified element

### iterator

    public Iterator 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

### 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 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

### toArray

    public Object[] toArray(Object[] a)

  Returns an array containing all of the elements in this collection whose runtime type 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 `l` is a `List` known to contain only strings. The following code can be used to dump the list into a newly allocated array of `String`:

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

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

  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 the elements of this collection
  Throws:
    `ArrayStoreException` - 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 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:
    `o` - element whose presence in this collection is to be ensured.
  Returns:
    `true` if this collection changed as a result of the call
  Throws:
    `UnsupportedOperationException` - add is not supported by this collection.
    `ClassCastException` - class of the specified element prevents it from being added to this collection.
    `IllegalArgumentException` - 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 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 this collection changed as a result of the call
  Throws:
    `UnsupportedOperationException` - remove 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.
  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
  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.)
  Parameters:
    `c` - elements to be inserted into 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.
    `ClassCastException` - if the class of an element of the specified collection prevents it from being added to this collection.
    `IllegalArgumentException` - some aspect of an element of the specified collection prevents it from being added to this collection.
  See Also:
    `add(Object)`

### removeAll

    public boolean removeAll(Collection c)

  Removes all 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` - 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.
  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.
  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.
  See Also:
    `remove(Object)`, `contains(Object)`

### clear

    public void clear()

  Removes all of the elements from this collection (optional operation). This collection will be empty after this method returns unless it throws an exception.
  Throws:
    `UnsupportedOperationException` - if the `clear` method is not supported by this collection.

### equals

    public boolean equals(Object 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 implementer 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.)

  Overrides:
    `equals` in class `Object`
  Parameters:
    `o` - Object to be compared for equality with this collection.
  Returns:
    `true` if the specified object is equal to this collection
  See Also:
    `Object.equals(Object)`, `Set.equals(Object)`, `List.equals(Object)`

### hashCode

    public 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()`.
  Overrides:
    `hashCode` in class `Object`
  Returns:
    the hash code value for this collection
  See Also:
    `Object.hashCode()`, `Object.equals(Object)`

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