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E
- the type of elements held in this collectionpublic interface Deque<E>
A linear collection that supports element insertion and removal at both ends. The name deque is short for "double ended queue" and is usually pronounced "deck". Most 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 precisely 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 indicated 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.
Method Summary | |
---|---|
boolean |
add(E e)
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, returning true upon success and throwing an IllegalStateException if no space is currently available. |
void |
addFirst(E e)
Inserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions. |
void |
addLast(E e)
Inserts the specified element at the end of this deque if it is possible to do so immediately without violating capacity restrictions. |
boolean |
contains(java.lang.Object o)
Returns true if this deque contains the specified element. |
java.util.Iterator<E> |
descendingIterator()
Returns an iterator over the elements in this deque in reverse sequential order. |
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). |
E |
getFirst()
Retrieves, but does not remove, the first element of this deque. |
E |
getLast()
Retrieves, but does not remove, the last element of this deque. |
java.util.Iterator<E> |
iterator()
Returns an iterator over the elements in this deque in proper sequence. |
boolean |
offer(E e)
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, returning true upon success and false 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 |
offerLast(E e)
Inserts the specified element at the end of this deque unless it would violate capacity restrictions. |
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 returns null if this deque is empty. |
E |
peekFirst()
Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty. |
E |
peekLast()
Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty. |
E |
poll()
Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returns null if this deque is empty. |
E |
pollFirst()
Retrieves and removes the first element of this deque, or returns null if this deque is empty. |
E |
pollLast()
Retrieves and removes the last element of this deque, or returns null if this deque is empty. |
E |
pop()
Pops an element from the stack represented by this deque. |
void |
push(E e)
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, returning true upon success and throwing an IllegalStateException if no space is currently available. |
E |
remove()
Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque). |
boolean |
remove(java.lang.Object o)
Removes the first occurrence of the specified element from this deque. |
E |
removeFirst()
Retrieves and removes the first element of this deque. |
boolean |
removeFirstOccurrence(java.lang.Object o)
Removes the first occurrence of the specified element from this deque. |
E |
removeLast()
Retrieves and removes the last element of this deque. |
boolean |
removeLastOccurrence(java.lang.Object o)
Removes the last occurrence of the specified element from this deque. |
int |
size()
Returns the number of elements in this deque. |
Methods inherited from interface java.util.Collection |
---|
addAll, clear, containsAll, equals, hashCode, isEmpty, removeAll, retainAll, toArray, toArray |
Method Detail |
---|
void addFirst(E e)
offerFirst(E)
.
e
- the element to add
java.lang.IllegalStateException
- if the element cannot be added at this
time due to capacity restrictions
java.lang.ClassCastException
- if the class of the specified element
prevents it from being added to this deque
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements
java.lang.IllegalArgumentException
- if some property of the specified
element prevents it from being added to this dequevoid addLast(E e)
offerLast(E)
.
This method is equivalent to add(E)
.
e
- the element to add
java.lang.IllegalStateException
- if the element cannot be added at this
time due to capacity restrictions
java.lang.ClassCastException
- if the class of the specified element
prevents it from being added to this deque
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements
java.lang.IllegalArgumentException
- if some property of the specified
element prevents it from being added to this dequeboolean offerFirst(E e)
addFirst(E)
method,
which can fail to insert an element only by throwing an exception.
e
- the element to add
java.lang.ClassCastException
- if the class of the specified element
prevents it from being added to this deque
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements
java.lang.IllegalArgumentException
- if some property of the specified
element prevents it from being added to this dequeboolean offerLast(E e)
addLast(E)
method,
which can fail to insert an element only by throwing an exception.
e
- the element to add
java.lang.ClassCastException
- if the class of the specified element
prevents it from being added to this deque
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements
java.lang.IllegalArgumentException
- if some property of the specified
element prevents it from being added to this dequeE removeFirst()
pollFirst
only in that it throws an
exception if this deque is empty.
java.util.NoSuchElementException
- if this deque is emptyE removeLast()
pollLast
only in that it throws an
exception if this deque is empty.
java.util.NoSuchElementException
- if this deque is emptyE pollFirst()
E pollLast()
E getFirst()
peekFirst
only in that it
throws an exception if this deque is empty.
java.util.NoSuchElementException
- if this deque is emptyE getLast()
peekLast
only in that it
throws an exception if this deque is empty.
java.util.NoSuchElementException
- if this deque is emptyE peekFirst()
E peekLast()
boolean removeFirstOccurrence(java.lang.Object o)
o
- element to be removed from this deque, if present
java.lang.ClassCastException
- if the class of the specified element
is incompatible with this deque (optional)
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements (optional)boolean removeLastOccurrence(java.lang.Object o)
o
- element to be removed from this deque, if present
java.lang.ClassCastException
- if the class of the specified element
is incompatible with this deque (optional)
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements (optional)boolean add(E e)
offer
.
This method is equivalent to addLast(E)
.
add
in interface java.util.Collection<E>
add
in interface java.util.Queue<E>
e
- the element to add
Collection.add(E)
)
java.lang.IllegalStateException
- if the element cannot be added at this
time due to capacity restrictions
java.lang.ClassCastException
- if the class of the specified element
prevents it from being added to this deque
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements
java.lang.IllegalArgumentException
- if some property of the specified
element prevents it from being added to this dequeboolean offer(E e)
add(E)
method, which can fail to
insert an element only by throwing an exception.
This method is equivalent to offerLast(E)
.
offer
in interface java.util.Queue<E>
e
- the element to add
java.lang.ClassCastException
- if the class of the specified element
prevents it from being added to this deque
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements
java.lang.IllegalArgumentException
- if some property of the specified
element prevents it from being added to this dequeE remove()
poll
only in that it throws an
exception if this deque is empty.
This method is equivalent to removeFirst()
.
remove
in interface java.util.Queue<E>
java.util.NoSuchElementException
- if this deque is emptyE poll()
This method is equivalent to pollFirst()
.
poll
in interface java.util.Queue<E>
E element()
peek
only in that it throws an
exception if this deque is empty.
This method is equivalent to getFirst()
.
element
in interface java.util.Queue<E>
java.util.NoSuchElementException
- if this deque is emptyE peek()
This method is equivalent to peekFirst()
.
peek
in interface java.util.Queue<E>
void push(E e)
This method is equivalent to addFirst(E)
.
e
- the element to push
java.lang.IllegalStateException
- if the element cannot be added at this
time due to capacity restrictions
java.lang.ClassCastException
- if the class of the specified element
prevents it from being added to this deque
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements
java.lang.IllegalArgumentException
- if some property of the specified
element prevents it from being added to this dequeE pop()
This method is equivalent to removeFirst()
.
java.util.NoSuchElementException
- if this deque is emptyboolean remove(java.lang.Object o)
This method is equivalent to removeFirstOccurrence(java.lang.Object)
.
remove
in interface java.util.Collection<E>
o
- element to be removed from this deque, if present
java.lang.ClassCastException
- if the class of the specified element
is incompatible with this deque (optional)
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements (optional)boolean contains(java.lang.Object o)
contains
in interface java.util.Collection<E>
o
- element whose presence in this deque is to be tested
java.lang.ClassCastException
- if the type of the specified element
is incompatible with this deque (optional)
java.lang.NullPointerException
- if the specified element is null and this
deque does not permit null elements (optional)int size()
size
in interface java.util.Collection<E>
java.util.Iterator<E> iterator()
iterator
in interface java.util.Collection<E>
iterator
in interface java.lang.Iterable<E>
java.util.Iterator<E> descendingIterator()
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