edu.emory.mathcs.backport.java.util
public interface Deque extends Queue
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 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:
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 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 Java Collections Framework.
Since: 1.6
Method Summary | |
---|---|
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.
|
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.
|
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.
|
boolean | contains(Object o)
Returns true if this deque contains the specified element.
|
Iterator | descendingIterator()
Returns an iterator over the elements in this deque in reverse
sequential order. |
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).
|
Object | getFirst()
Retrieves, but does not remove, the first element of this deque.
|
Object | getLast()
Retrieves, but does not remove, the last element of this deque.
|
Iterator | iterator()
Returns an iterator over the elements in this deque in proper sequence.
|
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. |
boolean | offerFirst(Object e)
Inserts the specified element at the front of this deque unless it would
violate capacity restrictions. |
boolean | offerLast(Object e)
Inserts the specified element at the end of this deque unless it would
violate capacity restrictions. |
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.
|
Object | peekFirst()
Retrieves, but does not remove, the first element of this deque,
or returns null if this deque is empty.
|
Object | peekLast()
Retrieves, but does not remove, the last element of this deque,
or returns null if this deque is empty.
|
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.
|
Object | pollFirst()
Retrieves and removes the first element of this deque,
or returns null if this deque is empty.
|
Object | pollLast()
Retrieves and removes the last element of this deque,
or returns null if this deque is empty.
|
Object | pop()
Pops an element from the stack represented by this deque. |
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.
|
Object | remove()
Retrieves and removes the head of the queue represented by this deque
(in other words, the first element of this deque).
|
boolean | remove(Object o)
Removes the first occurrence of the specified element from this deque.
|
Object | removeFirst()
Retrieves and removes the first element of this deque. |
boolean | removeFirstOccurrence(Object o)
Removes the first occurrence of the specified element from this deque.
|
Object | removeLast()
Retrieves and removes the last element of this deque. |
boolean | removeLastOccurrence(Object o)
Removes the last occurrence of the specified element from this deque.
|
int | size()
Returns the number of elements in this deque.
|
offer
.
This method is equivalent to Deque.
Parameters: e the element to add
Returns: true (as specified by java.util.Collection#add)
Throws: IllegalStateException if the element cannot be added at this time due to capacity restrictions ClassCastException if the class of the specified element prevents it from being added to this deque NullPointerException if the specified element is null and this deque does not permit null elements IllegalArgumentException if some property of the specified element prevents it from being added to this deque
Parameters: e the element to add
Throws: IllegalStateException if the element cannot be added at this time due to capacity restrictions ClassCastException if the class of the specified element prevents it from being added to this deque NullPointerException if the specified element is null and this deque does not permit null elements IllegalArgumentException if some property of the specified element prevents it from being added to this deque
This method is equivalent to Deque.
Parameters: e the element to add
Throws: IllegalStateException if the element cannot be added at this time due to capacity restrictions ClassCastException if the class of the specified element prevents it from being added to this deque NullPointerException if the specified element is null and this deque does not permit null elements IllegalArgumentException if some property of the specified element prevents it from being added to this deque
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 type 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)
Returns: an iterator over the elements in this deque in reverse sequence
peek
only in that it throws an
exception if this deque is empty.
This method is equivalent to getFirst.
Returns: the head of the queue represented by this deque
Throws: NoSuchElementException if this deque is empty
peekFirst
only in that it
throws an exception if this deque is empty.
Returns: the head of this deque
Throws: NoSuchElementException if this deque is empty
peekLast
only in that it
throws an exception if this deque is empty.
Returns: the tail of this deque
Throws: NoSuchElementException if this deque is empty
Returns: an iterator over the elements in this deque in proper sequence
This method is equivalent to Deque.
Parameters: e the element to add
Returns: true if the element was added to this deque, else false
Throws: ClassCastException if the class of the specified element prevents it from being added to this deque NullPointerException if the specified element is null and this deque does not permit null elements IllegalArgumentException if some property of the specified element prevents it from being added to this deque
Parameters: e the element to add
Returns: true if the element was added to this deque, else false
Throws: ClassCastException if the class of the specified element prevents it from being added to this deque NullPointerException if the specified element is null and this deque does not permit null elements IllegalArgumentException if some property of the specified element prevents it from being added to this deque
Parameters: e the element to add
Returns: true if the element was added to this deque, else false
Throws: ClassCastException if the class of the specified element prevents it from being added to this deque NullPointerException if the specified element is null and this deque does not permit null elements IllegalArgumentException if some property of the specified element prevents it from being added to this deque
This method is equivalent to peekFirst.
Returns: the head of the queue represented by this deque, or null if this deque is empty
Returns: the head of this deque, or null if this deque is empty
Returns: the tail of this deque, or null if this deque is empty
This method is equivalent to pollFirst.
Returns: the first element of this deque, or null if this deque is empty
Returns: the head of this deque, or null if this deque is empty
Returns: the tail of this deque, or null if this deque is empty
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
This method is equivalent to Deque.
Parameters: e the element to push
Throws: IllegalStateException if the element cannot be added at this time due to capacity restrictions ClassCastException if the class of the specified element prevents it from being added to this deque NullPointerException if the specified element is null and this deque does not permit null elements IllegalArgumentException if some property of the specified element prevents it from being added to this deque
poll
only in that it throws an
exception if this deque is empty.
This method is equivalent to removeFirst.
Returns: the head of the queue represented by this deque
Throws: NoSuchElementException if this deque is empty
This method is equivalent to Deque.
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)
pollFirst
only in that it throws an
exception if this deque is empty.
Returns: the head of this deque
Throws: NoSuchElementException if this deque is empty
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)
pollLast
only in that it throws an
exception if this deque is empty.
Returns: the tail of this deque
Throws: NoSuchElementException if this deque is empty
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)
Returns: the number of elements in this deque