All Superinterfaces:

Iterable<E>

All Known Subinterfaces:

List<E>, Queue<E>

All Known Implementing Classes:

ArrayList
```
public interface Collection<E>
extends Iterable<E>
```
Collection is the root of the collection hierarchy. It defines operations on data collections and the behavior that they will have in all implementations of Collections. All direct or indirect implementations of Collection should implement at least two constructors. One with no parameters which creates an empty collection and one with a parameter of type Collection. This second constructor can be used to create a collection of different type as the initial collection but with the same elements. Implementations of Collection cannot be forced to implement these two constructors but at least all implementations under java.util do. Methods that change the content of a collection throw an UnsupportedOperationException if the underlying collection does not support that operation, though it's not mandatory to throw such an Exception in cases where the requested operation would not change the collection. In these cases it's up to the implementation whether it throws an UnsupportedOperationException or not. Methods marked with (optional) can throw an UnsupportedOperationException if the underlying collection doesn't support that method.

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`boolean`	`add(E object)`	Attempts to add `object` to the contents of this `Collection` (optional).
`boolean`	`addAll(Collection<? extends E> collection)`	Attempts to add all of the objects contained in `Collection` to the contents of this `Collection` (optional).
`void`	`clear()`	Removes all elements from this `Collection`, leaving it empty (optional).
`boolean`	`contains(Object object)`	Tests whether this `Collection` contains the specified object.
`boolean`	`containsAll(Collection<?> collection)`	Tests whether this `Collection` contains all objects contained in the specified `Collection`.
`boolean`	`equals(Object object)`	Compares the argument to the receiver, and returns true if they represent the same object using a class specific comparison.
`int`	`hashCode()`	Returns an integer hash code for the receiver.
`boolean`	`isEmpty()`	Returns if this `Collection` contains no elements.
`Iterator<E>`	`iterator()`	Returns an instance of `Iterator` that may be used to access the objects contained by this `Collection`.
`boolean`	`remove(Object object)`	Removes one instance of the specified object from this `Collection` if one is contained (optional).
`boolean`	`removeAll(Collection<?> collection)`	Removes all occurrences in this `Collection` of each object in the specified `Collection` (optional).
`boolean`	`retainAll(Collection<?> collection)`	Removes all objects from this `Collection` that are not also found in the `Collection` passed (optional).
`int`	`size()`	Returns a count of how many objects this `Collection` contains.
`Object[]`	`toArray()`	Returns a new array containing all elements contained in this `Collection`.
`<T> T[]`	`toArray(T[] array)`	Returns an array containing all elements contained in this `Collection`.

- Method Detail
  - add
```
boolean add(E object)
```
    Attempts to add object to the contents of this Collection (optional). After this method finishes successfully it is guaranteed that the object is contained in the collection. If the collection was modified it returns true, false if no changes were made. An implementation of Collection may narrow the set of accepted objects, but it has to specify this in the documentation. If the object to be added does not meet this restriction, then an IllegalArgumentException is thrown. If a collection does not yet contain an object that is to be added and adding the object fails, this method must throw an appropriate unchecked Exception. Returning false is not permitted in this case because it would violate the postcondition that the element will be part of the collection after this method finishes.
    
    Parameters:
    
    object - the object to add.
    
    Returns:
    
    true if this Collection is modified, false otherwise.
    
    Throws:
    
    java.lang.UnsupportedOperationException - if adding to this Collection is not supported.
    
    ClassCastException - if the class of the object is inappropriate for this collection.
    
    IllegalArgumentException - if the object cannot be added to this Collection.
    
    NullPointerException - if null elements cannot be added to the Collection.
  - addAll
```
boolean addAll(Collection<? extends E> collection)
```
    Attempts to add all of the objects contained in Collection to the contents of this Collection (optional). If the passed Collection is changed during the process of adding elements to this Collection, the behavior is not defined.
    
    Parameters:
    
    collection - the Collection of objects.
    
    Returns:
    
    true if this Collection is modified, false otherwise.
    
    Throws:
    
    java.lang.UnsupportedOperationException - if adding to this Collection is not supported.
    
    ClassCastException - if the class of an object is inappropriate for this Collection.
    
    IllegalArgumentException - if an object cannot be added to this Collection.
    
    NullPointerException - if collection is null, or if it contains null elements and this Collection does not support such elements.
  - clear
```
void clear()
```
    Removes all elements from this Collection, leaving it empty (optional).
    
    Throws:
    
    java.lang.UnsupportedOperationException - if removing from this Collection is not supported.
    
    See Also:
    
    isEmpty(), size()
  - contains
```
boolean contains(Object object)
```
    Tests whether this Collection contains the specified object. Returns true if and only if at least one element elem in this Collection meets following requirement: (object==null ? elem==null : object.equals(elem)).
    
    Parameters:
    
    object - the object to search for.
    
    Returns:
    
    true if object is an element of this Collection, false otherwise.
    
    Throws:
    
    ClassCastException - if the object to look for isn't of the correct type.
    
    NullPointerException - if the object to look for is null and this Collection doesn't support null elements.
  - containsAll
```
boolean containsAll(Collection<?> collection)
```
    Tests whether this Collection contains all objects contained in the specified Collection. If an element elem is contained several times in the specified Collection, the method returns true even if elem is contained only once in this Collection.
    
    Parameters:
    
    collection - the collection of objects.
    
    Returns:
    
    true if all objects in the specified Collection are elements of this Collection, false otherwise.
    
    Throws:
    
    ClassCastException - if one or more elements of collection isn't of the correct type.
    
    NullPointerException - if collection contains at least one null element and this Collection doesn't support null elements.
    
    NullPointerException - if collection is null.
  - equals
```
boolean equals(Object object)
```
    Compares the argument to the receiver, and returns true if they represent the same object using a class specific comparison.
    
    Overrides:
    
    equals in class Object
    
    Parameters:
    
    object - the object to compare with this object.
    
    Returns:
    
    true if the object is the same as this object and false if it is different from this object.
    
    See Also:
    
    hashCode()
  - hashCode
```
int hashCode()
```
    Returns an integer hash code for the receiver. Objects which are equal return the same value for this method.
    
    Overrides:
    
    hashCode in class Object
    
    Returns:
    
    the receiver's hash.
    
    See Also:
    
    equals(java.lang.Object)
  - isEmpty
```
boolean isEmpty()
```
    Returns if this Collection contains no elements.
    
    Returns:
    
    true if this Collection has no elements, false otherwise.
    
    See Also:
    
    size()
  - iterator
```
Iterator<E> iterator()
```
    Returns an instance of Iterator that may be used to access the objects contained by this Collection. The order in which the elements are returned by the iterator is not defined. Only if the instance of the Collection has a defined order the elements are returned in that order.
    
    Specified by:
    
    iterator in interface Iterable<E>
    
    Returns:
    
    an iterator for accessing the Collection contents.
  - remove
```
boolean remove(Object object)
```
    Removes one instance of the specified object from this Collection if one is contained (optional). The element elem that is removed complies with (object==null ? elem==null : object.equals(elem).
    
    Parameters:
    
    object - the object to remove.
    
    Returns:
    
    true if this Collection is modified, false otherwise.
    
    Throws:
    
    java.lang.UnsupportedOperationException - if removing from this Collection is not supported.
    
    ClassCastException - if the object passed is not of the correct type.
    
    NullPointerException - if object is null and this Collection doesn't support null elements.
  - removeAll
```
boolean removeAll(Collection<?> collection)
```
    Removes all occurrences in this Collection of each object in the specified Collection (optional). After this method returns none of the elements in the passed Collection can be found in this Collection anymore.
    
    Parameters:
    
    collection - the collection of objects to remove.
    
    Returns:
    
    true if this Collection is modified, false otherwise.
    
    Throws:
    
    java.lang.UnsupportedOperationException - if removing from this Collection is not supported.
    
    ClassCastException - if one or more elements of collection isn't of the correct type.
    
    NullPointerException - if collection contains at least one null element and this Collection doesn't support null elements.
    
    NullPointerException - if collection is null.
  - retainAll
```
boolean retainAll(Collection<?> collection)
```
    Removes all objects from this Collection that are not also found in the Collection passed (optional). After this method returns this Collection will only contain elements that also can be found in the Collection passed to this method.
    
    Parameters:
    
    collection - the collection of objects to retain.
    
    Returns:
    
    true if this Collection is modified, false otherwise.
    
    Throws:
    
    java.lang.UnsupportedOperationException - if removing from this Collection is not supported.
    
    ClassCastException - if one or more elements of collection isn't of the correct type.
    
    NullPointerException - if collection contains at least one null element and this Collection doesn't support null elements.
    
    NullPointerException - if collection is null.
  - size
```
int size()
```
    Returns a count of how many objects this Collection contains.
    
    Returns:
    
    how many objects this Collection contains, or Integer.MAX_VALUE if there are more than Integer.MAX_VALUE elements in this Collection.
  - toArray
```
Object[] toArray()
```
    Returns a new array containing all elements contained in this Collection. If the implementation has ordered elements it will return the element array in the same order as an iterator would return them. The array returned does not reflect any changes of the Collection. A new array is created even if the underlying data structure is already an array.
    
    Returns:
    
    an array of the elements from this Collection.
  - toArray
```
<T> T[] toArray(T[] array)
```
    Returns an array containing all elements contained in this Collection. If the specified array is large enough to hold the elements, the specified array is used, otherwise an array of the same type is created. If the specified array is used and is larger than this Collection, the array element following the Collection elements is set to null. If the implementation has ordered elements it will return the element array in the same order as an iterator would return them. toArray(new Object[0]) behaves exactly the same way as toArray() does.
    
    Type Parameters:
    
    T - the type.
    
    Parameters:
    
    array - the array.
    
    Returns:
    
    an array of the elements from this Collection.
    
    Throws:
    
    ArrayStoreException - if the type of an element in this Collection cannot be stored in the type of the specified array.

Interface Collection<E>

Method Summary

Method Detail

add

addAll

clear

contains

containsAll

equals

hashCode

isEmpty

iterator

remove

removeAll

retainAll

size

toArray

toArray