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.
Collection
protected AbstractCollection()
Sole constructor. (For invocation by subclass constructors, typically implicit.)
public abstract Iterator<E> iterator()
Returns an iterator over the elements contained in this collection.
iterator
in interface Iterable<E>
iterator
in interface Collection<E>
public abstract int size()
Description copied from interface: Collection
Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE
elements, returns Integer.MAX_VALUE
.
size
in interface Collection<E>
public boolean isEmpty()
Returns true
if this collection contains no elements.
This implementation returns size() == 0
.
isEmpty
in interface Collection<E>
true
if this collection contains no elementspublic 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))
.
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 testedtrue
if this collection contains the specified elementClassCastException
- 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()
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 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)
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.
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)
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.
This implementation always throws an UnsupportedOperationException
.
add
in interface Collection<E>
e
- element whose presence in this collection is to be ensuredtrue
if this collection changed as a result of the callUnsupportedOperationException
- 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)
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))
, 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).
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 presenttrue
if an element was removed as a result of this callUnsupportedOperationException
- 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)
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
.
containsAll
in interface Collection<E>
c
- collection to be checked for containment in this collectiontrue
if this collection contains all of the elements in the specified 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)
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).
addAll
in interface Collection<E>
c
- collection containing elements to be added to this collectiontrue
if this collection changed as a result of the callUnsupportedOperationException
- 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)
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.
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 collectiontrue
if this collection changed as a result of the callUnsupportedOperationException
- 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)
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.
retainAll
in interface Collection<E>
c
- collection containing elements to be retained in this collectiontrue
if this collection changed as a result of the callUnsupportedOperationException
- 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()
Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns.
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()
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)
.
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