public abstract class AbstractCollection<E> extends Object implements Collection<E>
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 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.
Collection
Modifier | Constructor and Description |
---|---|
protected |
AbstractCollection()
Sole constructor.
|
Modifier and Type | Method and Description |
---|---|
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.
|
<T> T[] |
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.
|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
equals, hashCode, parallelStream, removeIf, spliterator, stream
protected AbstractCollection()
public abstract Iterator<E> iterator()
public abstract int size()
Collection
size
in interface Collection<E>
public boolean isEmpty()
This implementation returns size() == 0.
isEmpty
in interface Collection<E>
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.
contains
in interface Collection<E>
o
- element whose presence in this collection is to be testedClassCastException
- if the type of the specified element
is incompatible with this collection
(optional)NullPointerException
- if the specified element is null and this
collection does not permit null elements
(optional)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 method is equivalent to:
List<E> list = new ArrayList<E>(size());
for (E e : this)
list.add(e);
return list.toArray();
toArray
in interface Collection<E>
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 method is equivalent to:
List<E> list = new ArrayList<E>(size());
for (E e : this)
list.add(e);
return list.toArray(a);
toArray
in interface Collection<E>
T
- the runtime type of the array to contain the collectiona
- 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.ArrayStoreException
- if the runtime type of the specified array
is not a supertype of the runtime type of every element in
this collectionNullPointerException
- if the specified array is nullpublic 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.
add
in interface Collection<E>
e
- element whose presence in this collection is to be ensuredUnsupportedOperationException
- if the add operation
is not supported by this collectionClassCastException
- if the class of the specified element
prevents it from being added to this collectionNullPointerException
- if the specified element is null and this
collection does not permit null elementsIllegalArgumentException
- if some property of the element
prevents it from being added to this collectionIllegalStateException
- if the element cannot be added at this
time due to insertion restrictionspublic 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.
remove
in interface Collection<E>
o
- element to be removed from this collection, if presentUnsupportedOperationException
- if the remove operation
is not supported by this collectionClassCastException
- if the type of the specified element
is incompatible with this collection
(optional)NullPointerException
- if the specified element is null and this
collection does not permit null elements
(optional)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.
containsAll
in interface Collection<E>
c
- collection to be checked for containment in this collectionClassCastException
- if the types of one or more elements
in the specified collection are incompatible with this
collection
(optional)NullPointerException
- if the specified collection contains one
or more null elements and this collection does not permit null
elements
(optional),
or if the specified collection is null.contains(Object)
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).
addAll
in interface Collection<E>
c
- collection containing elements to be added to this collectionUnsupportedOperationException
- if the addAll operation
is not supported by this collectionClassCastException
- if the class of an element of the specified
collection prevents it from being added to this collectionNullPointerException
- if the specified collection contains a
null element and this collection does not permit null elements,
or if the specified collection is nullIllegalArgumentException
- if some property of an element of the
specified collection prevents it from being added to this
collectionIllegalStateException
- if not all the elements can be added at
this time due to insertion restrictionsadd(Object)
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.
removeAll
in interface Collection<E>
c
- collection containing elements to be removed from this collectionUnsupportedOperationException
- if the removeAll method
is not supported by this collectionClassCastException
- if the types of one or more elements
in this collection are incompatible with the specified
collection
(optional)NullPointerException
- if this collection contains one or more
null elements and the specified collection does not support
null elements
(optional),
or if the specified collection is nullremove(Object)
,
contains(Object)
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.
retainAll
in interface Collection<E>
c
- collection containing elements to be retained in this collectionUnsupportedOperationException
- if the retainAll operation
is not supported by this collectionClassCastException
- if the types of one or more elements
in this collection are incompatible with the specified
collection
(optional)NullPointerException
- if this collection contains one or more
null elements and the specified collection does not permit null
elements
(optional),
or if the specified collection is nullremove(Object)
,
contains(Object)
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.
clear
in interface Collection<E>
UnsupportedOperationException
- if the clear operation
is not supported by this collectionpublic String toString()
String.valueOf(Object)
. Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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