java.lang.Objectjava.util.AbstractCollection<E>
public abstract class AbstractCollection<E>
This class provides a skeletal implementation of the Collection interface, to minimize the effort required to implement this interface.
To implement an unmodifiable collection, the programmer needs only to extend this class and provide implementations for the iterator and size methods. (The iterator returned by the iterator method must implement hasNext and next .)
To implement a modifiable collection, the programmer must additionally override this class's add method (which otherwise throws an UnsupportedOperationException ), and the iterator returned by the iterator method must additionally implement its remove method.
The programmer should generally provide a void (no argument) and Collection constructor, as per the recommendation in the Collection interface specification.
The documentation for each non-abstract
method
methods
in this class describes its implementation in detail. Each of these methods may be overridden if the collection being implemented admits a more efficient implementation.
This class is a member of the
Java Collections Framework
Java Collections Framework
.
Constructor Summary | |
---|---|
protected |
AbstractCollection
() Sole constructor. |
Method Summary | ||
---|---|---|
boolean |
add
(
E
e) Ensures that this collection contains the specified element (optional operation). |
|
boolean |
addAll
(
Collection
<? extends
E
> 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 |
isEmpty
() Returns true if this collection contains no elements. |
|
abstract Iterator < E > |
iterator
() Returns an iterator over the elements contained 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 of 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). |
|
abstract int |
size
() Returns the number of elements in this collection. |
|
Object [] |
toArray
() Returns an array containing all of the elements in this collection. |
|
|
toArray
(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. |
|
String |
toString
() Returns a string representation of this collection. |
Methods inherited from class java.lang. Object |
---|
clone , equals , finalize , getClass , hashCode , notify , notifyAll , wait , wait , wait |
Methods inherited from interface java.util. Collection |
---|
equals , hashCode |
Constructor Detail |
---|
protected AbstractCollection()
Method Detail |
---|
public abstract Iterator<E> iterator()
public abstract int size()
public boolean isEmpty()
This implementation returns size() == 0 .
public boolean contains(Object o)
This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
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.
This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index 0. The length of the returned array is equal to the number of elements returned by the iterator, even if the size of this collection changes during iteration, as might happen if the collection permits concurrent modification during iteration. The size method is called only as an optimization hint; the correct result is returned even if the iterator returns a different number of elements.
This implementation allocates the array to be returned, and iterates over the elements in the collection, storing each object reference in the next consecutive element of the array, starting with element 0.
List<E> list = new ArrayList<E>(size()); for (E e : this) list.add(e); return list.toArray();
public <T> T[] toArray(T[] 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 Collection.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 x is a collection known to contain only strings. The following code can be used to dump the collection into a newly allocated array of String :
String[] y = x.toArray(new String[0]);Note that toArray(new Object[0]) is identical in function to toArray() .
This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index 0. If the number of elements returned by the iterator is too large to fit into the specified array, then the elements are returned in a newly allocated array with length equal to the number of elements returned by the iterator, even if the size of this collection changes during iteration, as might happen if the collection permits concurrent modification during iteration. The size method is called only as an optimization hint; the correct result is returned even if the iterator returns a different number of elements.
This implementation checks if the array is large enough to contain the collection; if not, it allocates a new array of the correct size and type (using reflection). Then, it iterates over the collection, storing each object reference in the next consecutive element of the array, starting with element 0. If the array is larger than the collection, a
null
is stored in the first location after the end of the collection.
List<E> list = new ArrayList<E>(size()); for (E e : this) list.add(e); return list.toArray(a);
public boolean add(E e)
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.
This implementation always throws an UnsupportedOperationException .
public boolean remove(Object o)
This implementation iterates over the collection looking for the specified element. If it finds the element, it removes the element from the collection using the iterator's remove method.
Note that this implementation throws an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection contains the specified object.
public boolean containsAll(Collection<?> c)
This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection. If all elements are so contained true is returned, otherwise false .
public boolean addAll(Collection<? extends E> c)
This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
Note that this implementation will throw an UnsupportedOperationException unless add is overridden (assuming the specified collection is non-empty).
public boolean removeAll(Collection<?> c)
This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection. If it's so contained, it's removed from this collection with the iterator's remove method.
Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method and this collection contains one or more elements in common with the specified collection.
public boolean retainAll(Collection<?> c)
This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection. If it's not so contained, it's removed from this collection with the iterator's remove method.
Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method and this collection contains one or more elements not present in the specified collection.
public void clear()
This implementation iterates over this collection, removing each element using the Iterator.remove operation. Most implementations will probably choose to override this method for efficiency.
Note that this implementation will throw an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection is non-empty.
public String toString()