Interface Deque<E>
- Type Parameters:
E
- the type of elements held in this deque
- All Superinterfaces:
Collection<E>
,Iterable<E>
,Queue<E>
,SequencedCollection<E>
- All Known Subinterfaces:
BlockingDeque<E>
- All Known Implementing Classes:
ArrayDeque
,ConcurrentLinkedDeque
,LinkedBlockingDeque
,LinkedList
Deque
implementations place no fixed limits on the number of elements
they may contain, but this interface supports capacity-restricted
deques as well as those with no fixed size limit.
This interface defines methods to access the elements at both
ends of the deque. Methods are provided to insert, remove, and
examine the element. Each of these methods exists in two forms:
one throws an exception if the operation fails, the other returns a
special value (either null
or false
, depending on
the operation). The latter form of the insert operation is
designed specifically for use with capacity-restricted
Deque
implementations; in most implementations, insert
operations cannot fail.
The twelve methods described above are summarized in the following table:
First Element (Head) | Last Element (Tail) | |||
---|---|---|---|---|
Throws exception | Special value | Throws exception | Special value | |
Insert | addFirst(e) |
offerFirst(e) |
addLast(e) |
offerLast(e) |
Remove | removeFirst() |
pollFirst() |
removeLast() |
pollLast() |
Examine | getFirst() |
peekFirst() |
getLast() |
peekLast() |
This interface extends the Queue
interface. When a deque is
used as a queue, FIFO (First-In-First-Out) behavior results. Elements are
added at the end of the deque and removed from the beginning. The methods
inherited from the Queue
interface are precisely equivalent to
Deque
methods as indicated in the following table:
Queue Method |
Equivalent Deque Method |
---|---|
add(e) |
addLast(e) |
offer(e) |
offerLast(e) |
remove() |
removeFirst() |
poll() |
pollFirst() |
element() |
getFirst() |
peek() |
peekFirst() |
Deques can also be used as LIFO (Last-In-First-Out) stacks. This
interface should be used in preference to the legacy Stack
class.
When a deque is used as a stack, elements are pushed and popped from the
beginning of the deque. Stack methods are equivalent to Deque
methods as indicated in the table below:
Stack Method | Equivalent Deque Method |
---|---|
push(e) |
addFirst(e) |
pop() |
removeFirst() |
peek() |
peekFirst() |
Note that the peek
method works equally well when
a deque is used as a queue or a stack; in either case, elements are
drawn from the beginning of the deque.
This interface provides two methods to remove interior
elements, removeFirstOccurrence
and
removeLastOccurrence
.
Unlike the List
interface, this interface does not
provide support for indexed access to elements.
While Deque
implementations are not strictly required
to prohibit the insertion of null elements, they are strongly
encouraged to do so. Users of any Deque
implementations
that do allow null elements are strongly encouraged not to
take advantage of the ability to insert nulls. This is so because
null
is used as a special return value by various methods
to indicate that the deque is empty.
Deque
implementations generally do not define
element-based versions of the equals
and hashCode
methods, but instead inherit the identity-based versions from class
Object
.
This interface is a member of the Java Collections Framework.
- Since:
- 1.6
-
Method Summary
Modifier and TypeMethodDescriptionboolean
Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque) if it is possible to do so immediately without violating capacity restrictions, returningtrue
upon success and throwing anIllegalStateException
if no space is currently available.boolean
addAll
(Collection<? extends E> c) Adds all of the elements in the specified collection at the end of this deque, as if by callingaddLast(E)
on each one, in the order that they are returned by the collection's iterator.void
Inserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions, throwing anIllegalStateException
if no space is currently available.void
Inserts the specified element at the end of this deque if it is possible to do so immediately without violating capacity restrictions, throwing anIllegalStateException
if no space is currently available.boolean
Returnstrue
if this deque contains the specified element.Returns an iterator over the elements in this deque in reverse sequential order.element()
Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque).getFirst()
Retrieves, but does not remove, the first element of this deque.getLast()
Retrieves, but does not remove, the last element of this deque.iterator()
Returns an iterator over the elements in this deque in proper sequence.boolean
Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque) if it is possible to do so immediately without violating capacity restrictions, returningtrue
upon success andfalse
if no space is currently available.boolean
offerFirst
(E e) Inserts the specified element at the front of this deque unless it would violate capacity restrictions.boolean
Inserts the specified element at the end of this deque unless it would violate capacity restrictions.peek()
Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque), or returnsnull
if this deque is empty.Retrieves, but does not remove, the first element of this deque, or returnsnull
if this deque is empty.peekLast()
Retrieves, but does not remove, the last element of this deque, or returnsnull
if this deque is empty.poll()
Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returnsnull
if this deque is empty.Retrieves and removes the first element of this deque, or returnsnull
if this deque is empty.pollLast()
Retrieves and removes the last element of this deque, or returnsnull
if this deque is empty.pop()
Pops an element from the stack represented by this deque.void
Pushes an element onto the stack represented by this deque (in other words, at the head of this deque) if it is possible to do so immediately without violating capacity restrictions, throwing anIllegalStateException
if no space is currently available.remove()
Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).boolean
Removes the first occurrence of the specified element from this deque.Retrieves and removes the first element of this deque.boolean
Removes the first occurrence of the specified element from this deque.Retrieves and removes the last element of this deque.boolean
Removes the last occurrence of the specified element from this deque.reversed()
Returns a reverse-ordered view of this collection.int
size()
Returns the number of elements in this deque.Methods inherited from interface java.util.Collection
clear, containsAll, equals, hashCode, isEmpty, parallelStream, removeAll, removeIf, retainAll, spliterator, stream, toArray, toArray, toArray
-
Method Details
-
addFirst
Inserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions, throwing anIllegalStateException
if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use methodofferFirst(E)
.- Specified by:
addFirst
in interfaceSequencedCollection<E>
- Parameters:
e
- the element to add- Throws:
IllegalStateException
- if the element cannot be added at this time due to capacity restrictionsClassCastException
- if the class of the specified element prevents it from being added to this dequeNullPointerException
- if the specified element is null and this deque does not permit null elementsIllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
-
addLast
Inserts the specified element at the end of this deque if it is possible to do so immediately without violating capacity restrictions, throwing anIllegalStateException
if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use methodofferLast(E)
.This method is equivalent to
add(E)
.- Specified by:
addLast
in interfaceSequencedCollection<E>
- Parameters:
e
- the element to add- Throws:
IllegalStateException
- if the element cannot be added at this time due to capacity restrictionsClassCastException
- if the class of the specified element prevents it from being added to this dequeNullPointerException
- if the specified element is null and this deque does not permit null elementsIllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
-
offerFirst
Inserts the specified element at the front of this deque unless it would violate capacity restrictions. When using a capacity-restricted deque, this method is generally preferable to theaddFirst(E)
method, which can fail to insert an element only by throwing an exception.- Parameters:
e
- the element to add- Returns:
true
if the element was added to this deque, elsefalse
- Throws:
ClassCastException
- if the class of the specified element prevents it from being added to this dequeNullPointerException
- if the specified element is null and this deque does not permit null elementsIllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
-
offerLast
Inserts the specified element at the end of this deque unless it would violate capacity restrictions. When using a capacity-restricted deque, this method is generally preferable to theaddLast(E)
method, which can fail to insert an element only by throwing an exception.- Parameters:
e
- the element to add- Returns:
true
if the element was added to this deque, elsefalse
- Throws:
ClassCastException
- if the class of the specified element prevents it from being added to this dequeNullPointerException
- if the specified element is null and this deque does not permit null elementsIllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
-
removeFirst
E removeFirst()Retrieves and removes the first element of this deque. This method differs frompollFirst
only in that it throws an exception if this deque is empty.- Specified by:
removeFirst
in interfaceSequencedCollection<E>
- Returns:
- the head of this deque
- Throws:
NoSuchElementException
- if this deque is empty
-
removeLast
E removeLast()Retrieves and removes the last element of this deque. This method differs frompollLast
only in that it throws an exception if this deque is empty.- Specified by:
removeLast
in interfaceSequencedCollection<E>
- Returns:
- the tail of this deque
- Throws:
NoSuchElementException
- if this deque is empty
-
pollFirst
E pollFirst()Retrieves and removes the first element of this deque, or returnsnull
if this deque is empty.- Returns:
- the head of this deque, or
null
if this deque is empty
-
pollLast
E pollLast()Retrieves and removes the last element of this deque, or returnsnull
if this deque is empty.- Returns:
- the tail of this deque, or
null
if this deque is empty
-
getFirst
E getFirst()Retrieves, but does not remove, the first element of this deque. This method differs frompeekFirst
only in that it throws an exception if this deque is empty.- Specified by:
getFirst
in interfaceSequencedCollection<E>
- Returns:
- the head of this deque
- Throws:
NoSuchElementException
- if this deque is empty
-
getLast
E getLast()Retrieves, but does not remove, the last element of this deque. This method differs frompeekLast
only in that it throws an exception if this deque is empty.- Specified by:
getLast
in interfaceSequencedCollection<E>
- Returns:
- the tail of this deque
- Throws:
NoSuchElementException
- if this deque is empty
-
peekFirst
E peekFirst()Retrieves, but does not remove, the first element of this deque, or returnsnull
if this deque is empty.- Returns:
- the head of this deque, or
null
if this deque is empty
-
peekLast
E peekLast()Retrieves, but does not remove, the last element of this deque, or returnsnull
if this deque is empty.- Returns:
- the tail of this deque, or
null
if this deque is empty
-
removeFirstOccurrence
Removes the first occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the first elemente
such thatObjects.equals(o, e)
(if such an element exists). Returnstrue
if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).- Parameters:
o
- element to be removed from this deque, if present- Returns:
true
if an element was removed as a result of this call- Throws:
ClassCastException
- if the class of the specified element is incompatible with this deque (optional)NullPointerException
- if the specified element is null and this deque does not permit null elements (optional)
-
removeLastOccurrence
Removes the last occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the last elemente
such thatObjects.equals(o, e)
(if such an element exists). Returnstrue
if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).- Parameters:
o
- element to be removed from this deque, if present- Returns:
true
if an element was removed as a result of this call- Throws:
ClassCastException
- if the class of the specified element is incompatible with this deque (optional)NullPointerException
- if the specified element is null and this deque does not permit null elements (optional)
-
add
Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque) if it is possible to do so immediately without violating capacity restrictions, returningtrue
upon success and throwing anIllegalStateException
if no space is currently available. When using a capacity-restricted deque, it is generally preferable to useoffer
.This method is equivalent to
addLast(E)
.- Specified by:
add
in interfaceCollection<E>
- Specified by:
add
in interfaceQueue<E>
- Parameters:
e
- the element to add- Returns:
true
(as specified byCollection.add(E)
)- Throws:
IllegalStateException
- if the element cannot be added at this time due to capacity restrictionsClassCastException
- if the class of the specified element prevents it from being added to this dequeNullPointerException
- if the specified element is null and this deque does not permit null elementsIllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
-
offer
Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque) if it is possible to do so immediately without violating capacity restrictions, returningtrue
upon success andfalse
if no space is currently available. When using a capacity-restricted deque, this method is generally preferable to theadd(E)
method, which can fail to insert an element only by throwing an exception.This method is equivalent to
offerLast(E)
.- Specified by:
offer
in interfaceQueue<E>
- Parameters:
e
- the element to add- Returns:
true
if the element was added to this deque, elsefalse
- Throws:
ClassCastException
- if the class of the specified element prevents it from being added to this dequeNullPointerException
- if the specified element is null and this deque does not permit null elementsIllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
-
remove
E remove()Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque). This method differs frompoll()
only in that it throws an exception if this deque is empty.This method is equivalent to
removeFirst()
.- Specified by:
remove
in interfaceQueue<E>
- Returns:
- the head of the queue represented by this deque
- Throws:
NoSuchElementException
- if this deque is empty
-
poll
E poll()Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returnsnull
if this deque is empty.This method is equivalent to
pollFirst()
. -
element
E element()Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque). This method differs frompeek
only in that it throws an exception if this deque is empty.This method is equivalent to
getFirst()
.- Specified by:
element
in interfaceQueue<E>
- Returns:
- the head of the queue represented by this deque
- Throws:
NoSuchElementException
- if this deque is empty
-
peek
E peek()Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque), or returnsnull
if this deque is empty.This method is equivalent to
peekFirst()
. -
addAll
Adds all of the elements in the specified collection at the end of this deque, as if by callingaddLast(E)
on each one, in the order that they are returned by the collection's iterator.When using a capacity-restricted deque, it is generally preferable to call
offer
separately on each element.An exception encountered while trying to add an element may result in only some of the elements having been successfully added when the associated exception is thrown.
- Specified by:
addAll
in interfaceCollection<E>
- Parameters:
c
- the elements to be inserted into this deque- Returns:
true
if this deque changed as a result of the call- Throws:
IllegalStateException
- if not all the elements can be added at this time due to insertion restrictionsClassCastException
- if the class of an element of the specified collection prevents it from being added to this dequeNullPointerException
- if the specified collection contains a null element and this deque 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 deque- See Also:
-
push
Pushes an element onto the stack represented by this deque (in other words, at the head of this deque) if it is possible to do so immediately without violating capacity restrictions, throwing anIllegalStateException
if no space is currently available.This method is equivalent to
addFirst(E)
.- Parameters:
e
- the element to push- Throws:
IllegalStateException
- if the element cannot be added at this time due to capacity restrictionsClassCastException
- if the class of the specified element prevents it from being added to this dequeNullPointerException
- if the specified element is null and this deque does not permit null elementsIllegalArgumentException
- if some property of the specified element prevents it from being added to this deque
-
pop
E pop()Pops an element from the stack represented by this deque. In other words, removes and returns the first element of this deque.This method is equivalent to
removeFirst()
.- Returns:
- the element at the front of this deque (which is the top of the stack represented by this deque)
- Throws:
NoSuchElementException
- if this deque is empty
-
remove
Removes the first occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the first elemente
such thatObjects.equals(o, e)
(if such an element exists). Returnstrue
if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).This method is equivalent to
removeFirstOccurrence(Object)
.- Specified by:
remove
in interfaceCollection<E>
- Parameters:
o
- element to be removed from this deque, if present- Returns:
true
if an element was removed as a result of this call- Throws:
ClassCastException
- if the class of the specified element is incompatible with this deque (optional)NullPointerException
- if the specified element is null and this deque does not permit null elements (optional)
-
contains
Returnstrue
if this deque contains the specified element. More formally, returnstrue
if and only if this deque contains at least one elemente
such thatObjects.equals(o, e)
.- Specified by:
contains
in interfaceCollection<E>
- Parameters:
o
- element whose presence in this deque is to be tested- Returns:
true
if this deque contains the specified element- Throws:
ClassCastException
- if the class of the specified element is incompatible with this deque (optional)NullPointerException
- if the specified element is null and this deque does not permit null elements (optional)
-
size
int size()Returns the number of elements in this deque.- Specified by:
size
in interfaceCollection<E>
- Returns:
- the number of elements in this deque
-
iterator
Returns an iterator over the elements in this deque in proper sequence. The elements will be returned in order from first (head) to last (tail). -
descendingIterator
-
reversed
Returns a reverse-ordered view of this collection. The encounter order of elements in the returned view is the inverse of the encounter order of elements in this collection. The reverse ordering affects all order-sensitive operations, including those on the view collections of the returned view. If the collection implementation permits modifications to this view, the modifications "write through" to the underlying collection. Changes to the underlying collection might or might not be visible in this reversed view, depending upon the implementation.- Specified by:
reversed
in interfaceSequencedCollection<E>
- Implementation Requirements:
- The implementation in this interface returns a reverse-ordered Deque
view. The
reversed()
method of the view returns a reference to this Deque. Other operations on the view are implemented via calls to public methods on this Deque. The exact relationship between calls on the view and calls on this Deque is unspecified. However, order-sensitive operations generally behave as if they delegate to the appropriate method with the opposite orientation. For example, callinggetFirst
on the view might result in a call togetLast
on this Deque. - Returns:
- a reverse-ordered view of this collection, as a
Deque
- Since:
- 21
-