public interface Collection<E>

ExtendsIterable<E>

Known subtypesComponentSelector, AbstractCollection, AbstractList, AbstractQueue, AbstractSequentialList, AbstractSet, ArrayDeque, ArrayList, Deque, HashSet, LinkedHashSet, LinkedList, List, NavigableSet, PriorityQueue, Queue, Set, SortedSet, Stack, TreeSet, Vector

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.

Methods

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

Inherited methods

From Iterable

forEach

Method details

add

public abstract 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 E
the object to add.

Returns

true if this Collection is modified, false otherwise.

Throws

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

public abstract 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 Collection<? extends E>
the Collection of objects.

Returns

true if this Collection is modified, false otherwise.

Throws

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

public abstract void clear()
Removes all elements from this Collection, leaving it empty (optional).

Throws

UnsupportedOperationException
if removing from this Collection is not supported.

See also

contains

public abstract 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 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

public abstract 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 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.

equals

public abstract boolean equals(Object object)
Compares the argument to the receiver, and returns true if they represent the same object using a class specific comparison.

Parameters

object 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

public abstract int hashCode()
Returns an integer hash code for the receiver. Objects which are equal return the same value for this method.

Returns

the receiver’s hash.

See also

isEmpty

public abstract boolean isEmpty()
Returns if this Collection contains no elements.

Returns

true if this Collection has no elements, false otherwise.

See also

iterator

public abstract 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.

Returns

an iterator for accessing the Collection contents.

remove

public abstract 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 Object
the object to remove.

Returns

true if this Collection is modified, false otherwise.

Throws

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

public abstract 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 Collection<?>
the collection of objects to remove.

Returns

true if this Collection is modified, false otherwise.

Throws

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.

retainAll

public abstract 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 Collection<?>
the collection of objects to retain.

Returns

true if this Collection is modified, false otherwise.

Throws

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.

size

public abstract 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

public abstract 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

public abstract <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.

Parameters

array T[]
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.

removeIf

public default boolean removeIf(Predicate<? super E> filter)
Removes all of the elements of this collection that satisfy the given predicate. Stubbed in the CLDC11 subset; the actual implementation comes from the platform’s JDK at runtime.