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java.util
Interface Collection
- All Superinterfaces:
- java.lang.Iterable
- All Known Implementing Classes:
- AbstractCollection, AbstractList, AbstractSet, ArrayList, Collections.SynchronizedCollection, Collections.SynchronizedList, Collections.SynchronizedSet, Collections.SynchronizedSortedSet, Collections.UnmodifiableCollection, Collections.UnmodifiableList, Collections.UnmodifiableSet, Collections.UnmodifiableSortedSet, HashSet, LinkedHashSet, LinkedList, TreeSet, Vector
- public interface Collection
- extends java.lang.Iterable
Interface that represents a collection of objects. This interface is the root of the collection hierarchy, and does not provide any guarantees about the order of its elements or whether or not duplicate elements are permitted.
All methods of this interface that are defined to modify the collection are defined as optional. An optional operation may throw an UnsupportedOperationException if the data backing this collection does not support such a modification. This may mean that the data structure is immutable, or that it is read-only but may change ("unmodifiable"), or that it is modifiable but of fixed size (such as an array), or any number of other combinations.
A class that wishes to implement this interface should consider subclassing AbstractCollection, which provides basic implementations of most of the methods of this interface. Classes that are prepared to make guarantees about ordering or about absence of duplicate elements should consider implementing List or Set respectively, both of which are subinterfaces of Collection.
A general-purpose implementation of the Collection interface should in most cases provide at least two constructors: One which takes no arguments and creates an empty collection, and one which takes a Collection as an argument and returns a collection containing the same elements (that is, creates a copy of the argument using its own implementation).
- Since:
- 1.2
| Method Summary | |
boolean |
add(java.lang.Object o)
Add an element to this collection. |
boolean |
addAll(Collection c)
Add the contents of a given collection to this collection. |
void |
clear()
Clear the collection, such that a subsequent call to isEmpty() would return true. |
boolean |
contains(java.lang.Object o)
Test whether this collection contains a given object as one of its elements. |
boolean |
containsAll(Collection c)
Test whether this collection contains every element in a given collection. |
boolean |
equals(java.lang.Object o)
Test whether this collection is equal to some object. |
int |
hashCode()
Obtain a hash code for this collection. |
boolean |
isEmpty()
Test whether this collection is empty, that is, if size() == 0. |
Iterator |
iterator()
Obtain an Iterator over this collection. |
boolean |
remove(java.lang.Object o)
Remove a single occurrence of an object from this collection. |
boolean |
removeAll(Collection c)
Remove all elements of a given collection from this collection. |
boolean |
retainAll(Collection c)
Remove all elements of this collection that are not contained in a given collection. |
int |
size()
Get the number of elements in this collection. |
java.lang.Object[] |
toArray()
Copy the current contents of this collection into an array. |
java.lang.Object[] |
toArray(java.lang.Object[] a)
Copy the current contents of this collection into an array. |
| Method Detail |
add
public boolean add(java.lang.Object o)
- Add an element to this collection.
addAll
public boolean addAll(Collection c)
- Add the contents of a given collection to this collection.
clear
public void clear()
- Clear the collection, such that a subsequent call to isEmpty() would
return true.
contains
public boolean contains(java.lang.Object o)
- Test whether this collection contains a given object as one of its
elements.
containsAll
public boolean containsAll(Collection c)
- Test whether this collection contains every element in a given collection.
equals
public boolean equals(java.lang.Object o)
- Test whether this collection is equal to some object. The Collection
interface does not explicitly require any behaviour from this method, and
it may be left to the default implementation provided by Object. The Set
and List interfaces do, however, require specific behaviour from this
method.
If an implementation of Collection, which is not also an implementation of Set or List, should choose to implement this method, it should take care to obey the contract of the equals method of Object. In particular, care should be taken to return false when o is a Set or a List, in order to preserve the symmetry of the relation.
hashCode
public int hashCode()
- Obtain a hash code for this collection. The Collection interface does not
explicitly require any behaviour from this method, and it may be left to
the default implementation provided by Object. The Set and List interfaces
do, however, require specific behaviour from this method.
If an implementation of Collection, which is not also an implementation of Set or List, should choose to implement this method, it should take care to obey the contract of the hashCode method of Object. Note that this method renders it impossible to correctly implement both Set and List, as the required implementations are mutually exclusive.
isEmpty
public boolean isEmpty()
- Test whether this collection is empty, that is, if size() == 0.
iterator
public Iterator iterator()
- Obtain an Iterator over this collection.
- Specified by:
iteratorin interfacejava.lang.Iterable
remove
public boolean remove(java.lang.Object o)
- Remove a single occurrence of an object from this collection. That is,
remove an element e, if one exists, such that
o == null ? e == null : o.equals(e).
removeAll
public boolean removeAll(Collection c)
- Remove all elements of a given collection from this collection. That is,
remove every element e such that c.contains(e).
retainAll
public boolean retainAll(Collection c)
- Remove all elements of this collection that are not contained in a given
collection. That is, remove every element e such that !c.contains(e).
size
public int size()
- Get the number of elements in this collection.
toArray
public java.lang.Object[] toArray()
- Copy the current contents of this collection into an array.
toArray
public java.lang.Object[] toArray(java.lang.Object[] a)
- Copy the current contents of this collection into an array. If the array
passed as an argument has length less than the size of this collection, an
array of the same run-time type as a, and length equal to the size of this
collection, is allocated using Reflection. Otherwise, a itself is used.
The elements of this collection are copied into it, and if there is space
in the array, the following element is set to null. The resultant array is
returned.
Note: The fact that the following element is set to null is only useful
if it is known that this collection does not contain any null elements.
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