java.util.Collection Maven / Gradle / Ivy
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This work corresponds to the API signatures of JSR 219: Foundation
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*/
package java.util;
/**
* 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 Java platform libraries comply.
*
*
The "destructive" methods contained in this interface, that is, the
* methods that modify the collection on which they operate, are specified to
* throw UnsupportedOperationException if this collection does not
* support the operation. If this is the case, these methods may, but are not
* required to, throw an UnsupportedOperationException if the
* invocation would have no effect on the collection. For example, invoking
* the {@link #addAll(Collection)} method on an unmodifiable collection may,
* but is not required to, throw the exception if the collection to be added
* is empty.
*
*
Some collection implementations have restrictions on the elements that
* they may contain. For example, some implementations prohibit null elements,
* and some have restrictions on the types of their elements. Attempting to
* add an ineligible element throws an unchecked exception, typically
* NullPointerException or ClassCastException. Attempting
* to query the presence of an ineligible element may throw an exception,
* or it may simply return false; some implementations will exhibit the former
* behavior and some will exhibit the latter. More generally, attempting an
* operation on an ineligible element whose completion would not result in
* the insertion of an ineligible element into the collection may throw an
* exception or it may succeed, at the option of the implementation.
* Such exceptions are marked as "optional" in the specification for this
* interface.
*
*
This interface is a member of the
*
* Java Collections Framework.
*
* @author Josh Bloch
* @version 1.31, 02/02/00
* @see Set
* @see List
* @see Map
* @see SortedSet
* @see SortedMap
* @see HashSet
* @see TreeSet
* @see ArrayList
* @see LinkedList
* @see Vector
* @see Collections
* @see Arrays
* @see AbstractCollection
* @since 1.2
*/
public interface Collection
{
/**
* Returns the number of elements in this collection. If this collection
* contains more than Integer.MAX_VALUE elements, returns
* Integer.MAX_VALUE.
*
* @return the number of elements in this collection
*/
public int size();
/**
* Returns true if this collection contains no elements.
*
* @return true if this collection contains no elements
*/
public boolean isEmpty();
/**
* 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)).
*
* @param o element whose presence in this collection is to be tested.
* @return true if this collection contains the specified
* element
* @throws ClassCastException if the type of the specified element
* is incompatible with this collection (optional).
* @throws NullPointerException if the specified element is null and this
* collection does not support null elements (optional).
*/
public boolean contains(Object o);
/**
* 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).
*
* @return an Iterator over the elements in this collection
*/
public Iterator iterator();
/**
* 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.
*
* @return an array containing all of the elements in this collection
*/
public Object[] toArray();
/**
* 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.
*
* 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().
*
* @param 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.
* @return 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.
* @throws NullPointerException if the specified array is null.
*/
public Object[] toArray(Object[] a);
/**
* 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.
*
* @param o element whose presence in this collection is to be ensured.
* @return true if this collection changed as a result of the
* call
*
* @throws UnsupportedOperationException add is not supported by
* this collection.
* @throws ClassCastException class of the specified element prevents it
* from being added to this collection.
* @throws NullPointerException if the specified element is null and this
* collection does not support null elements.
* @throws IllegalArgumentException some aspect of this element prevents
* it from being added to this collection.
*/
public boolean add(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).
*
* @param o element to be removed from this collection, if present.
* @return true if this collection changed as a result of the
* call
*
* @throws ClassCastException if the type of the specified element
* is incompatible with this collection (optional).
* @throws NullPointerException if the specified element is null and this
* collection does not support null elements (optional).
* @throws UnsupportedOperationException remove is not supported by this
* collection.
*/
public boolean remove(Object o);
/**
* Returns true if this collection contains all of the elements
* in the specified collection.
*
* @param c collection to be checked for containment in this collection.
* @return 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).
* @throws NullPointerException if the specified collection contains one
* or more null elements and this collection does not support null
* elements (optional).
* @throws NullPointerException if the specified collection is
* null.
* @see #contains(Object)
*/
public boolean containsAll(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.)
*
* @param c elements to be inserted into this collection.
* @return true if this collection changed as a result of the
* call
*
* @throws UnsupportedOperationException if this collection does not
* support the addAll method.
* @throws ClassCastException if the class of an element of the specified
* collection prevents it from being added to this collection.
* @throws NullPointerException if the specified collection contains one
* or more null elements and this collection does not support null
* elements, or if the specified collection is null.
* @throws IllegalArgumentException some aspect of an element of the
* specified collection prevents it from being added to this
* collection.
* @see #add(Object)
*/
public boolean addAll(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.
*
* @param c elements to be removed from this collection.
* @return true if this collection changed as a result of the
* call
*
* @throws UnsupportedOperationException if the removeAll method
* is not supported by this collection.
* @throws ClassCastException if the types of one or more elements
* in this collection are incompatible with the specified
* collection (optional).
* @throws NullPointerException if this collection contains one or more
* null elements and the specified collection does not support
* null elements (optional).
* @throws NullPointerException if the specified collection is
* null.
* @see #remove(Object)
* @see #contains(Object)
*/
public boolean removeAll(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.
*
* @param c elements to be retained in this collection.
* @return true if this collection changed as a result of the
* call
*
* @throws UnsupportedOperationException if the retainAll method
* is not supported by this Collection.
* @throws ClassCastException if the types of one or more elements
* in this collection are incompatible with the specified
* collection (optional).
* @throws NullPointerException if this collection contains one or more
* null elements and the specified collection does not support null
* elements (optional).
* @throws NullPointerException if the specified collection is
* null.
* @see #remove(Object)
* @see #contains(Object)
*/
public boolean retainAll(Collection c);
/**
* 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.
*/
public void clear();
/**
* 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.)
*
* @param o Object to be compared for equality with this collection.
* @return true if the specified object is equal to this
* collection
*
* @see Object#equals(Object)
* @see Set#equals(Object)
* @see List#equals(Object)
*/
public boolean equals(Object o);
/**
* 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.hashCodemethod.
* In particular, c1.equals(c2) implies that
* c1.hashCode()==c2.hashCode().
*
* @return the hash code value for this collection
*
* @see Object#hashCode()
* @see Object#equals(Object)
*/
public int hashCode();
}