Class AbstractCollection<E>
- java.lang.Object
-
- java.util.AbstractCollection<E>
- All Implemented Interfaces:
-
Iterable<E>
,Collection<E>
- Direct Known Subclasses:
-
AbstractList
,AbstractQueue
,AbstractSet
,ArrayDeque
,ConcurrentLinkedDeque
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 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.
- Since:
- 1.2
- See Also:
Collection
Constructor Summary
Modifier | Constructor | Description |
---|---|---|
protected | AbstractCollection() | Sole constructor. |
Method Summary
Modifier and Type | Method | 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 |
boolean | containsAll(Collection<?> c) | Returns |
boolean | isEmpty() | Returns |
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). |
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. |
Methods declared in class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
Methods declared in interface java.util.Collection
equals, hashCode, parallelStream, removeIf, size, spliterator, stream, toArray
Methods declared in interface java.lang.Iterable
forEach
Constructor Detail
AbstractCollection
protected AbstractCollection()
Sole constructor. (For invocation by subclass constructors, typically implicit.)
Method Detail
iterator
public abstract Iterator<E> iterator()
Returns an iterator over the elements contained in this collection.
- Specified by:
-
iterator
in interfaceCollection<E>
- Specified by:
-
iterator
in interfaceIterable<E>
- Returns:
- an iterator over the elements contained in this collection
isEmpty
public boolean isEmpty()
Returns true
if this collection contains no elements.
- Specified by:
-
isEmpty
in interfaceCollection<E>
- Implementation Requirements:
- This implementation returns
size() == 0
. - Returns:
-
true
if this collection contains no 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 Objects.equals(o, e)
.
- Specified by:
-
contains
in interfaceCollection<E>
- Implementation Requirements:
- This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
- Parameters:
-
o
- element whose presence in this collection is to be tested - 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)
toArray
public Object[] toArray()
Returns an array containing all of the elements in this collection. 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. The returned array's runtime component type is Object
.
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.
- Specified by:
-
toArray
in interfaceCollection<E>
- Implementation Requirements:
- 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. Thesize
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();
- Returns:
- an array, whose runtime component type is
Object
, containing all of the elements in this 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 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.
- Specified by:
-
toArray
in interfaceCollection<E>
- Implementation Requirements:
- 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. Thesize
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);
- Type Parameters:
-
T
- the component type of the array to contain the collection - Parameters:
-
a
- 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. - Returns:
- an array containing all of the elements in this collection
- Throws:
-
ArrayStoreException
- if the runtime type of any element in this collection is not assignable to the runtime component type of the specified array -
NullPointerException
- if the specified array is null
add
public boolean add(E e)
Ensures that this collection contains the specified element (optional operation). Returns true
if this 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 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.
- Specified by:
-
add
in interfaceCollection<E>
- Implementation Requirements:
- This implementation always throws an
UnsupportedOperationException
. - Parameters:
-
e
- element whose presence in this collection is to be ensured - Returns:
-
true
if this collection changed as a result of the call - Throws:
-
UnsupportedOperationException
- if theadd
operation is not supported by this collection -
ClassCastException
- if the class of the specified element prevents it from being added to this collection -
NullPointerException
- if the specified element is null and this collection does not permit null elements -
IllegalArgumentException
- if some property of the element prevents it from being added to this collection -
IllegalStateException
- if the element cannot be added at this time due to insertion restrictions
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 Objects.equals(o, e)
, if this collection contains one or more such elements. Returns true
if this collection contained the specified element (or equivalently, if this collection changed as a result of the call).
- Specified by:
-
remove
in interfaceCollection<E>
- Implementation Requirements:
- 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 theremove
method and this collection contains the specified object. - Parameters:
-
o
- element to be removed from this collection, if present - Returns:
-
true
if an element was removed as a result of this call - Throws:
-
UnsupportedOperationException
- if theremove
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)
containsAll
public boolean containsAll(Collection<?> c)
Returns true
if this collection contains all of the elements in the specified collection.
- Specified by:
-
containsAll
in interfaceCollection<E>
- Implementation Requirements:
- 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, otherwisefalse
. - Parameters:
-
c
- collection to be checked for containment in this collection - Returns:
-
true
if this collection contains all of the elements in the specified 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. - 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.)
- Specified by:
-
addAll
in interfaceCollection<E>
- Implementation Requirements:
- 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
unlessadd
is overridden (assuming the specified collection is non-empty). - Parameters:
-
c
- collection containing elements to be added to this collection - Returns:
-
true
if this collection changed as a result of the call - Throws:
-
UnsupportedOperationException
- if theaddAll
operation is not supported by this collection -
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 - See Also:
add(Object)
removeAll
public boolean removeAll(Collection<?> c)
Removes all of this collection's 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.
- Specified by:
-
removeAll
in interfaceCollection<E>
- Implementation Requirements:
- 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 theiterator
method does not implement theremove
method and this collection contains one or more elements in common with the specified collection. - Parameters:
-
c
- collection containing elements to be removed from this collection - Returns:
-
true
if this collection changed as a result of the call - Throws:
-
UnsupportedOperationException
- if theremoveAll
method is not supported by this 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 - 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.
- Specified by:
-
retainAll
in interfaceCollection<E>
- Implementation Requirements:
- 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 theiterator
method does not implement theremove
method and this collection contains one or more elements not present in the specified collection. - Parameters:
-
c
- collection containing elements to be retained in this collection - Returns:
-
true
if this collection changed as a result of the call - Throws:
-
UnsupportedOperationException
- if theretainAll
operation is not supported by this 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 - 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.
- Specified by:
-
clear
in interfaceCollection<E>
- Implementation Requirements:
- 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'siterator
method does not implement theremove
method and this collection is non-empty. - Throws:
-
UnsupportedOperationException
- if theclear
operation is not supported by this 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)
.