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Java Platform 1.2 |
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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 JDK 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 JDK comply.
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 |
public int size()
public boolean isEmpty()
public boolean contains(Object o)
o - element whose presence in this collection is to be tested.public Iterator iterator()
public Object[] toArray()
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.
public Object[] toArray(Object[] a)
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().
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.public boolean add(Object o)
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.
o - element whose presence in this collection is to be ensured.public boolean remove(Object o)
o - element to be removed from this collection, if present.public boolean containsAll(Collection c)
c - collection to be checked for containment in this collection.contains(Object)public boolean addAll(Collection c)
c - elements to be inserted into this collection.add(Object)public boolean removeAll(Collection c)
c - elements to be removed from this collection.remove(Object),
contains(Object)public boolean retainAll(Collection c)
c - elements to be retained in this collection.remove(Object),
contains(Object)public void clear()
public boolean equals(Object o)
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.)
o - Object to be compared for equality with this collection.Object.equals(Object),
Set.equals(Object),
List.equals(Object)public int hashCode()
Object.hashCode(),
Object.equals(Object)
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Java Platform 1.2 |
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