Resizable-array implementation of the
Deque
interface. Array
deques have no capacity restrictions; they grow as necessary to support
usage. They are not thread-safe; in the absence of external
synchronization, they do not support concurrent access by multiple threads.
Null elements are prohibited. This class is likely to be faster than
java.util.Stack
when used as a stack, and faster than
LinkedList
when used as a queue.
Most
ArrayDeque operations run in amortized constant time.
Exceptions include
remove
,
removeFirstOccurrence
,
removeLastOccurrence
,
contains
,
iterator.remove()
, and the bulk operations, all of which run in linear
time.
The iterators returned by this class's
iterator method are
fail-fast: If the deque is modified at any time after the iterator
is created, in any way except through the iterator's own
remove
method, the iterator will generally throw a
ConcurrentModificationException
. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than risking
arbitrary, non-deterministic behavior at an undetermined time in the
future.
Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Fail-fast iterators
throw
ConcurrentModificationException on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness:
the fail-fast behavior of iterators
should be used only to detect bugs.
This class and its iterator implement all of the
optional methods of the
Collection
and
Iterator
interfaces.
This class is a member of the
../../../../../../../guide/collections/index.html">
Java Collections Framework.
add
public boolean add(Object e)
Inserts the specified element at the end of this deque.
This method is equivalent to
addLast(Object)
.
- add in interface Deque
- add in interface Queue
- true (as specified by
Collection.add
)
addFirst
public void addFirst(Object e)
Inserts the specified element at the front of this deque.
- addFirst in interface Deque
addLast
public void addLast(Object e)
Inserts the specified element at the end of this deque.
This method is equivalent to
add(Object)
.
- addLast in interface Deque
clear
public void clear()
Removes all of the elements from this deque.
The deque will be empty after this call returns.
clone
public Object clone()
Returns a copy of this deque.
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.equals(e).
- contains in interface Deque
o
- object to be checked for containment in this deque
- 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).
- descendingIterator in interface Deque
- 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. 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 Deque
- element in interface Queue
- the head of the queue represented by this deque
isEmpty
public boolean isEmpty()
Returns true if this deque contains no elements.
- true if this deque contains no elements
iterator
public Iterator iterator()
Returns an iterator over the elements in this deque. The elements
will be ordered from first (head) to last (tail). This is the same
order that elements would be dequeued (via successive calls to
remove
or popped (via successive calls to
pop()
).
- iterator in interface Deque
- an iterator over the elements in this deque
offer
public boolean offer(Object e)
Inserts the specified element at the end of this deque.
This method is equivalent to
offerLast(Object)
.
- offer in interface Deque
- offer in interface Queue
offerFirst
public boolean offerFirst(Object e)
Inserts the specified element at the front of this deque.
- offerFirst in interface Deque
offerLast
public boolean offerLast(Object e)
Inserts the specified element at the end of this deque.
- offerLast in interface Deque
peek
public Object peek()
Retrieves, but does not remove, the head of the queue represented by
this deque, or returns
null if this deque is empty.
This method is equivalent to
peekFirst()
.
- peek in interface Deque
- 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.
- peekFirst in interface Deque
- 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.
- peekLast in interface Deque
- 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 Deque
- poll in interface Queue
- the head of the queue represented by 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.
- pollFirst in interface Deque
- 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.
- pollLast in interface Deque
- 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()
.
- pop in interface Deque
- 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, inserts the element at the front of this deque.
This method is equivalent to
addFirst(Object)
.
- push in interface Deque
remove
public Object remove()
Retrieves and removes the head of the queue represented by 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 Deque
- remove in interface Queue
- the head of the queue represented by this deque
remove
public boolean remove(Object o)
Removes a single instance 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.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)
.
- remove in interface Deque
o
- element to be removed from this deque, if present
- true if this deque contained the specified element
removeFirstOccurrence
public boolean removeFirstOccurrence(Object o)
Removes the first occurrence of the specified element in this
deque (when traversing the deque from head to tail).
If the deque does not contain the element, it is unchanged.
More formally, removes the first element e such that
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).
- removeFirstOccurrence in interface Deque
o
- element to be removed from this deque, if present
- true if the deque contained the specified element
removeLastOccurrence
public boolean removeLastOccurrence(Object o)
Removes the last occurrence of the specified element in this
deque (when traversing the deque from head to tail).
If the deque does not contain the element, it is unchanged.
More formally, removes the last element e such that
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).
- removeLastOccurrence in interface Deque
o
- element to be removed from this deque, if present
- true if the deque contained the specified element
size
public int size()
Returns the number of elements in this deque.
- size in interface Deque
- the number of elements in this deque
toArray
public Object[] toArray()
Returns an array containing all of the elements in this deque
in proper sequence (from first to last element).
The returned array will be "safe" in that no references to it are
maintained by this deque. (In other words, this method must allocate
a new array). The caller is thus free to modify the returned array.
This method acts as bridge between array-based and collection-based
APIs.
- toArray in interface AbstractCollection
- an array containing all of the elements in this deque
toArray
public Object[] toArray(Object[] a)
Returns an array containing all of the elements in this deque in
proper sequence (from first to last element); the runtime type of the
returned array is that of the specified array. If the deque 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 deque.
If this deque fits in the specified array with room to spare
(i.e., the array has more elements than this deque), the element in
the array immediately following the end of the deque is set to
null.
Like the
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 deque known to contain only strings.
The following code can be used to dump the deque 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().
- toArray in interface AbstractCollection
a
- the array into which the elements of the deque are to
be stored, if it is big enough; otherwise, a new array of the
same runtime type is allocated for this purpose
- an array containing all of the elements in this deque