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java.util
Class AbstractCollection<E>
java.lang.Object
java.util.AbstractCollection<E>
-
All Implemented Interfaces:
-
Iterable
<E>,
Collection
<E>
-
Direct Known Subclasses:
-
AbstractList
,
AbstractQueue
,
AbstractSet
,
ArrayDeque
-
public abstract class AbstractCollection<E>
- extends Object
- implements Collection<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 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
.
-
Since:
-
1.2
-
See Also:
-
Collection
Method Summary
|
boolean |
add
(
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
Removes
from this collection
all of
this collection's
its
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. |
AbstractCollection
protected AbstractCollection()
-
Sole constructor. (For invocation by subclass constructors, typically implicit.)
iterator
public abstract Iterator<E> iterator()
-
Returns an iterator over the elements contained in this collection.
-
-
Specified by:
-
iterator
in interface
Iterable
<
E
>
-
Specified by:
-
iterator
in interface
Collection
<
E
>
-
-
Returns:
-
an iterator over the elements contained in this
collection
collection.
size
public abstract int size()
-
Description copied from interface:
Collection
-
Returns the number of elements in this collection. If
this
the
collection contains more than
Integer.MAX_VALUE
elements, returns
Integer.MAX_VALUE
.
-
-
Specified by:
-
size
in interface
Collection
<
E
>
-
-
Returns:
-
the number of elements in this
collection
collection.
isEmpty
public boolean isEmpty()
-
Returns
true
if this collection contains no elements.
This implementation returns
size() == 0
.
-
-
Specified by:
-
isEmpty
in interface
Collection
<
E
>
-
-
Returns:
-
true
if this collection contains no
elements
elements.
contains
public boolean contains(Object o)
-
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))
(o==null ? e==null : o.equals(e))
.
This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
-
-
Specified by:
-
contains
in interface
Collection
<
E
>
-
-
Parameters:
-
o - element whose presence in this collection is to be tested
o - object to be checked for containment in this collection.
-
Returns:
-
true
if this collection contains the specified element
-
Throws:
-
ClassCastException
- 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)
if this collection contains the specified element.
toArray
public Object[] toArray()
-
Returns an array containing all of the elements in this collection. If
this
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
the
collection. (In other words, this method must allocate a new array even if
this
the
collection is backed by an
array).
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 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.
-
-
Specified by:
-
toArray
in interface
Collection
<
E
>
-
-
Returns:
-
an array containing all of the elements in this
collection
collection.
toArray
public <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. 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
the
collection fits in the specified array with room to spare (i.e., the array has more elements than
this
the
collection), the element in the array immediately following the end of the collection is set to
null
.
(This
This
is useful in determining the length of
this
the
collection
only
if the caller knows that
this
the
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 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.
-
-
Specified by:
-
toArray
in interface
Collection
<
E
>
-
-
Parameters:
-
a - the array into which the elements of
this
the
collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose. -
Returns:
-
an array containing
all
the elements
of the
elements in this collection
collection.
-
Throws:
-
ArrayStoreException
NullPointerException
- if the runtime type of the specified array is not a supertype of the runtime type of every element in this collection
- if the specified array is
null
.
-
NullPointerException
ArrayStoreException
- if the specified array is null
- if the runtime type of the specified array is not a supertype of the runtime type of every element in this collection.
add
public boolean add(E e) o)
-
Ensures that this collection contains the specified element (optional operation). Returns
true
if
this
the
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
the
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
.
-
-
Specified by:
-
add
in interface
Collection
<
E
>
-
-
Parameters:
-
e
o
- element whose presence in this collection is to be
ensured
ensured.
-
Returns:
-
true
if
this
the
collection changed as a result of the
call
call.
-
Throws:
-
UnsupportedOperationException
- if the
add
operation
method
is not supported by this
collection
collection.
-
ClassCastException
NullPointerException
- if the class of the specified element prevents it from being added to this collection
- if this collection does not permit
null
elements, and the specified element is
null
.
-
NullPointerException
ClassCastException
- if the specified element is null and this collection does not permit null elements
- if the class of the specified element prevents it from being added to this collection.
-
IllegalArgumentException
- if some
property
aspect
of
the
this
element prevents it from being added to this
collection
-
IllegalStateException
- if the element cannot be added at this time due to insertion restrictions
collection.
remove
public boolean remove(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))
(o==null ? e==null : o.equals(e))
, if
this
the
collection contains one or more such elements. Returns
true
if
this
the
collection contained the specified element (or equivalently, if
this
the
collection changed as a result of the call).
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.
-
-
Specified by:
-
remove
in interface
Collection
<
E
>
-
-
Parameters:
-
o - element to be removed from this collection, if
present
present.
-
Returns:
-
true
if an element was removed as a result of this call
if the collection contained the specified element.
-
Throws:
-
UnsupportedOperationException
- if the
remove
operation is not supported by this collection
-
ClassCastException
- 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)
method is not supported by this collection.
containsAll
public boolean containsAll(Collection<?> c)
-
Returns
true
if this collection contains all of the elements in the specified collection.
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
.
-
-
Specified by:
-
containsAll
in interface
Collection
<
E
>
-
-
Parameters:
-
c - collection to be checked for containment in this
collection
collection.
-
Returns:
-
true
if this collection contains all of the elements in the specified
collection
collection.
-
Throws:
-
ClassCastException
- 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
- if the specified collection is null.
-
See Also:
-
contains(Object)
addAll
public boolean addAll(Collection<? extends E> 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.)
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).
-
-
Specified by:
-
addAll
in interface
Collection
<
E
>
-
-
Parameters:
-
c - collection
containing
whose
elements
are
to be added to this
collection
collection.
-
Returns:
-
true
if this collection changed as a result of the
call
call.
-
Throws:
-
UnsupportedOperationException
- if
this collection does not support
the
addAll
operation is not supported by this collection
method.
-
ClassCastException
- if the class of an element of the specified collection prevents it from being added to this collection
-
NullPointerException
- if the specified collection contains a null element and this collection does not permit null elements, or if the specified collection is null
-
IllegalArgumentException
- if some property of an element of the specified collection prevents it from being added to this collection
-
IllegalStateException
- if not all the elements can be added at this time due to insertion restrictions
- if the specified collection is null.
-
See Also:
-
add(Object)
removeAll
public boolean removeAll(Collection<?> c)
-
Removes
from this collection
all of
this collection's
its
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.
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.
-
-
Specified by:
-
removeAll
in interface
Collection
<
E
>
-
-
Parameters:
-
c -
collection containing
elements to be removed from this
collection
collection.
-
Returns:
-
true
if this collection changed as a result of the
call
call.
-
Throws:
-
UnsupportedOperationException
- if the
removeAll
method is not supported by this
collection
collection.
-
ClassCastException
- 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 null
- if the specified collection is null.
-
See Also:
-
remove(Object)
,
contains(Object)
retainAll
public boolean retainAll(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.
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.
-
-
Specified by:
-
retainAll
in interface
Collection
<
E
>
-
-
Parameters:
-
c -
collection containing
elements to be retained in this
collection
collection.
-
Returns:
-
true
if this collection changed as a result of the
call
call.
-
Throws:
-
UnsupportedOperationException
- if the
retainAll
operation
method
is not supported by this
collection
Collection.
-
ClassCastException
- 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 null
- if the specified collection is null.
-
See Also:
-
remove(Object)
,
contains(Object)
clear
public void clear()
-
Removes all of the elements from this collection (optional operation). The collection will be empty after this
method returns.
call returns (unless it throws an exception).
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.
-
-
Specified by:
-
clear
in interface
Collection
<
E
>
-
-
Throws:
-
UnsupportedOperationException
- if the
clear
operation
method
is not supported by this
collection
collection.
toString
public String toString()
-
Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets (
"[]"
). Adjacent elements are separated by the characters
", "
(comma and space). Elements are converted to strings as by
String.valueOf(Object)
String.valueOf(Object)
.
This implementation creates an empty string buffer, appends a left square bracket, and iterates over the collection appending the string representation of each element in turn. After appending each element except the last, the string
", "
is appended. Finally a right bracket is appended. A string is obtained from the string buffer, and returned.
-
-
Overrides:
-
toString
in class
Object
-
-
Returns:
-
a string representation of this
collection
collection.