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:
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:
Deques can also be used as LIFO (Last-In-First-Out) stacks. This
interface should be used in preference to the legacy
java.util.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:
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
java.util.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
../../../../../../../guide/collections/index.html"> Java Collections
Framework.
add
public boolean add(Object 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.
When using a capacity-restricted deque, it is generally preferable to
use
offer
.
This method is equivalent to
addLast(Object)
.
- add in interface Queue
- true (as specified by
java.util.Collection.add
)
addFirst
public void addFirst(Object e)
Inserts the specified element at the front of this deque if it is
possible to do so immediately without violating capacity restrictions.
When using a capacity-restricted deque, it is generally preferable to
use method
offerFirst(Object)
.
addLast
public void addLast(Object e)
Inserts the specified element at the end of this deque if it is
possible to do so immediately without violating capacity restrictions.
When using a capacity-restricted deque, it is generally preferable to
use method
offerLast(Object)
.
This method is equivalent to
add(Object)
.
contains
public boolean contains(Object o)
Returns true if this deque contains the specified element.
More formally, returns true if and only if this deque contains
at least one element e such that
(o==null ? e==null : o.equals(e)).
o
- element whose presence in this deque is to be tested
- true if this deque contains the specified element
descendingIterator
public Iterator descendingIterator()
Returns an iterator over the elements in this deque in reverse
sequential order. The elements will be returned in order from
last (tail) to first (head).
- an iterator over the elements in this deque in reverse
sequence
element
public Object 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 from
peek
only in that it throws an
exception if this deque is empty.
This method is equivalent to
getFirst()
.
- element in interface Queue
- the head of the queue represented by this deque
getFirst
public Object getFirst()
Retrieves, but does not remove, the first element of this deque.
This method differs from
peekFirst
only in that it
throws an exception if this deque is empty.
getLast
public Object getLast()
Retrieves, but does not remove, the last element of this deque.
This method differs from
peekLast
only in that it
throws an exception if this deque is empty.
iterator
public Iterator 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).
- an iterator over the elements in this deque in proper sequence
offer
public boolean offer(Object 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. When using a capacity-restricted deque, this method is
generally preferable to the
add(Object)
method, which can fail to
insert an element only by throwing an exception.
This method is equivalent to
offerLast(Object)
.
- offer in interface Queue
- true if the element was added to this deque, else
false
offerFirst
public boolean offerFirst(Object e)
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 the
addFirst(Object)
method,
which can fail to insert an element only by throwing an exception.
- true if the element was added to this deque, else
false
offerLast
public boolean offerLast(Object e)
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 the
addLast(Object)
method,
which can fail to insert an element only by throwing an exception.
- true if the element was added to this deque, else
false
peek
public Object 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.
This method is equivalent to
peekFirst()
.
- peek in interface Queue
- the head of the queue represented by this deque, or
null if this deque is empty
peekFirst
public Object peekFirst()
Retrieves, but does not remove, the first element of this deque,
or returns null if this deque is empty.
- the head of this deque, or null if this deque is empty
peekLast
public Object peekLast()
Retrieves, but does not remove, the last element of this deque,
or returns null if this deque is empty.
- the tail of this deque, or null if this deque is empty
poll
public Object 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.
This method is equivalent to
pollFirst()
.
- poll in interface Queue
- the first element of this deque, or null if
this deque is empty
pollFirst
public Object pollFirst()
Retrieves and removes the first element of this deque,
or returns null if this deque is empty.
- the head of this deque, or null if this deque is empty
pollLast
public Object pollLast()
Retrieves and removes the last element of this deque,
or returns null if this deque is empty.
- the tail of this deque, or null if this deque is empty
pop
public Object 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()
.
- the element at the front of this deque (which is the top
of the stack represented by this deque)
push
public void push(Object 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.
This method is equivalent to
addFirst(Object)
.
remove
public Object 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 from
poll
only in that it throws an
exception if this deque is empty.
This method is equivalent to
removeFirst()
.
- remove in interface Queue
- the head of the queue represented by this deque
remove
public boolean remove(Object o)
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 element
e such that
(o==null ? e==null : o.equals(e))
(if such an element exists).
Returns
true 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)
.
o
- element to be removed from this deque, if present
- true if an element was removed as a result of this call
removeFirst
public Object removeFirst()
Retrieves and removes the first element of this deque. This method
differs from
pollFirst
only in that it throws an
exception if this deque is empty.
removeFirstOccurrence
public boolean removeFirstOccurrence(Object o)
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 element e such that
(o==null ? e==null : o.equals(e))
(if such an element exists).
Returns true if this deque contained the specified element
(or equivalently, if this deque changed as a result of the call).
o
- element to be removed from this deque, if present
- true if an element was removed as a result of this call
removeLast
public Object removeLast()
Retrieves and removes the last element of this deque. This method
differs from
pollLast
only in that it throws an
exception if this deque is empty.
removeLastOccurrence
public boolean removeLastOccurrence(Object o)
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 element e such that
(o==null ? e==null : o.equals(e))
(if such an element exists).
Returns true if this deque contained the specified element
(or equivalently, if this deque changed as a result of the call).
o
- element to be removed from this deque, if present
- true if an element was removed as a result of this call
size
public int size()
Returns the number of elements in this deque.
- the number of elements in this deque