-
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 {@code 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 {@code null} or {@code false}, depending on
-
the operation). The latter form of the insert operation is
-
designed specifically for use with capacity-restricted
-
{@code Deque} implementations; in most implementations, insert
-
operations cannot fail.
-
The twelve methods described above are summarized in the
-
following table:
* <td><b>Insert</b></td>
* <td>{@link Deque#addFirst addFirst(e)}</td>
* <td>{@link Deque#offerFirst offerFirst(e)}</td>
* <td>{@link Deque#addLast addLast(e)}</td>
* <td>{@link Deque#offerLast offerLast(e)}</td>
* </tr>
* <tr>
* <td><b>Remove</b></td>
* <td>{@link Deque#removeFirst removeFirst()}</td>
* <td>{@link Deque#pollFirst pollFirst()}</td>
* <td>{@link Deque#removeLast removeLast()}</td>
* <td>{@link Deque#pollLast pollLast()}</td>
* </tr>
* <tr>
* <td><b>Examine</b></td>
* <td>{@link Deque#getFirst getFirst()}</td>
* <td>{@link Deque#peekFirst peekFirst()}</td>
* <td>{@link Deque#getLast getLast()}</td>
* <td>{@link Deque#peekLast peekLast()}</td>
可以通过注释看到Deque可以实现类似双端队列的效果
可以支持先进先出 以及 后进先出
它继承的是Queue接口
它们都支持的操作如
insertion
extraction
inspection
Queue的注释:
in most implementations, insert operations cannot fail.
-
Queues typically, but do not necessarily, order elements in a
- FIFO (first-in-first-out) manner. Among the exceptions are
- priority queues, which order elements according to a supplied
- comparator, or the elements' natural ordering, and LIFO queues (or
- stacks) which order the elements LIFO (last-in-first-out).
- Whatever the ordering used, the head of the queue is that
- element which would be removed by a call to {@link #remove() } or
- {@link #poll()}. In a FIFO queue, all new elements are inserted at
- the tail of the queue. Other kinds of queues may use
- different placement rules. Every {@code Queue} implementation
- must specify its ordering properties.
这里讲了说这个Queue有的是FIFO 有的是LIFO (stack)
一般理解Queue是FIFO, Stack是LIFO
-
The {@link #remove()} and {@link #poll()} methods remove and
- return the head of the queue.
双端队列其实看方法就可以看出来,
那么先看Queue
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
/*
*
*
*
*
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util;
/**
* A collection designed for holding elements prior to processing.
* Besides basic {@link java.util.Collection Collection} operations,
* queues provide additional insertion, extraction, and inspection
* operations. Each of these methods exists in two forms: one throws
* an exception if the operation fails, the other returns a special
* value (either {@code null} or {@code false}, depending on the
* operation). The latter form of the insert operation is designed
* specifically for use with capacity-restricted {@code Queue}
* implementations; in most implementations, insert operations cannot
* fail.
*
* <table BORDER CELLPADDING=3 CELLSPACING=1>
* <caption>Summary of Queue methods</caption>
* <tr>
* <td></td>
* <td ALIGN=CENTER><em>Throws exception</em></td>
* <td ALIGN=CENTER><em>Returns special value</em></td>
* </tr>
* <tr>
* <td><b>Insert</b></td>
* <td>{@link Queue#add add(e)}</td>
* <td>{@link Queue#offer offer(e)}</td>
* </tr>
* <tr>
* <td><b>Remove</b></td>
* <td>{@link Queue#remove remove()}</td>
* <td>{@link Queue#poll poll()}</td>
* </tr>
* <tr>
* <td><b>Examine</b></td>
* <td>{@link Queue#element element()}</td>
* <td>{@link Queue#peek peek()}</td>
* </tr>
* </table>
*
* <p>Queues typically, but do not necessarily, order elements in a
* FIFO (first-in-first-out) manner. Among the exceptions are
* priority queues, which order elements according to a supplied
* comparator, or the elements' natural ordering, and LIFO queues (or
* stacks) which order the elements LIFO (last-in-first-out).
* Whatever the ordering used, the <em>head</em> of the queue is that
* element which would be removed by a call to {@link #remove() } or
* {@link #poll()}. In a FIFO queue, all new elements are inserted at
* the <em>tail</em> of the queue. Other kinds of queues may use
* different placement rules. Every {@code Queue} implementation
* must specify its ordering properties.
*
* <p>The {@link #offer offer} method inserts an element if possible,
* otherwise returning {@code false}. This differs from the {@link
* java.util.Collection#add Collection.add} method, which can fail to
* add an element only by throwing an unchecked exception. The
* {@code offer} method is designed for use when failure is a normal,
* rather than exceptional occurrence, for example, in fixed-capacity
* (or "bounded") queues.
*
* <p>The {@link #remove()} and {@link #poll()} methods remove and
* return the head of the queue.
* Exactly which element is removed from the queue is a
* function of the queue's ordering policy, which differs from
* implementation to implementation. The {@code remove()} and
* {@code poll()} methods differ only in their behavior when the
* queue is empty: the {@code remove()} method throws an exception,
* while the {@code poll()} method returns {@code null}.
*
* <p>The {@link #element()} and {@link #peek()} methods return, but do
* not remove, the head of the queue.
*
* <p>The {@code Queue} interface does not define the <i>blocking queue
* methods</i>, which are common in concurrent programming. These methods,
* which wait for elements to appear or for space to become available, are
* defined in the {@link java.util.concurrent.BlockingQueue} interface, which
* extends this interface.
*
* <p>{@code Queue} implementations generally do not allow insertion
* of {@code null} elements, although some implementations, such as
* {@link LinkedList}, do not prohibit insertion of {@code null}.
* Even in the implementations that permit it, {@code null} should
* not be inserted into a {@code Queue}, as {@code null} is also
* used as a special return value by the {@code poll} method to
* indicate that the queue contains no elements.
*
* <p>{@code Queue} implementations generally do not define
* element-based versions of methods {@code equals} and
* {@code hashCode} but instead inherit the identity based versions
* from class {@code Object}, because element-based equality is not
* always well-defined for queues with the same elements but different
* ordering properties.
*
*
* <p>This interface is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @see java.util.Collection
* @see LinkedList
* @see PriorityQueue
* @see java.util.concurrent.LinkedBlockingQueue
* @see java.util.concurrent.BlockingQueue
* @see java.util.concurrent.ArrayBlockingQueue
* @see java.util.concurrent.LinkedBlockingQueue
* @see java.util.concurrent.PriorityBlockingQueue
* @since 1.5
* @author Doug Lea
* @param <E> the type of elements held in this collection
*/
public interface Queue<E> extends Collection<E> {
/**
* Inserts the specified element into this queue if it is possible to do so
* immediately without violating capacity restrictions, returning
* {@code true} upon success and throwing an {@code IllegalStateException}
* if no space is currently available.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Collection#add})
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null and
* this queue does not permit null elements
* @throws IllegalArgumentException if some property of this element
* prevents it from being added to this queue
*/
boolean add(E e);
/**
* Inserts the specified element into this queue if it is possible to do
* so immediately without violating capacity restrictions.
* When using a capacity-restricted queue, this method is generally
* preferable to {@link #add}, which can fail to insert an element only
* by throwing an exception.
*
* @param e the element to add
* @return {@code true} if the element was added to this queue, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null and
* this queue does not permit null elements
* @throws IllegalArgumentException if some property of this element
* prevents it from being added to this queue
*/
boolean offer(E e);
/**
* Retrieves and removes the head of this queue. This method differs
* from {@link #poll poll} only in that it throws an exception if this
* queue is empty.
*
* @return the head of this queue
* @throws NoSuchElementException if this queue is empty
*/
E remove();
/**
* Retrieves and removes the head of this queue,
* or returns {@code null} if this queue is empty.
*
* @return the head of this queue, or {@code null} if this queue is empty
*/
E poll();
/**
* Retrieves, but does not remove, the head of this queue. This method
* differs from {@link #peek peek} only in that it throws an exception
* if this queue is empty.
*
* @return the head of this queue
* @throws NoSuchElementException if this queue is empty
*/
E element();
/**
* Retrieves, but does not remove, the head of this queue,
* or returns {@code null} if this queue is empty.
*
* @return the head of this queue, or {@code null} if this queue is empty
*/
E peek();
}
其中这些方法是类似的,只不过有的抛出了重要异常,而有的没有抛出异常
而双端队列很明显的对队列首部队列尾部同时都有方法操作
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
/*
*
*
*
*
*
* Written by Doug Lea and Josh Bloch with assistance from members of
* JCP JSR-166 Expert Group and released to the public domain, as explained
* at http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util;
/**
* A linear collection that supports element insertion and removal at
* both ends. The name <i>deque</i> is short for "double ended queue"
* and is usually pronounced "deck". Most {@code 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.
*
* <p>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 {@code null} or {@code false}, depending on
* the operation). The latter form of the insert operation is
* designed specifically for use with capacity-restricted
* {@code Deque} implementations; in most implementations, insert
* operations cannot fail.
*
* <p>The twelve methods described above are summarized in the
* following table:
*
* <table BORDER CELLPADDING=3 CELLSPACING=1>
* <caption>Summary of Deque methods</caption>
* <tr>
* <td></td>
* <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td>
* <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td>
* </tr>
* <tr>
* <td></td>
* <td ALIGN=CENTER><em>Throws exception</em></td>
* <td ALIGN=CENTER><em>Special value</em></td>
* <td ALIGN=CENTER><em>Throws exception</em></td>
* <td ALIGN=CENTER><em>Special value</em></td>
* </tr>
* <tr>
* <td><b>Insert</b></td>
* <td>{@link Deque#addFirst addFirst(e)}</td>
* <td>{@link Deque#offerFirst offerFirst(e)}</td>
* <td>{@link Deque#addLast addLast(e)}</td>
* <td>{@link Deque#offerLast offerLast(e)}</td>
* </tr>
* <tr>
* <td><b>Remove</b></td>
* <td>{@link Deque#removeFirst removeFirst()}</td>
* <td>{@link Deque#pollFirst pollFirst()}</td>
* <td>{@link Deque#removeLast removeLast()}</td>
* <td>{@link Deque#pollLast pollLast()}</td>
* </tr>
* <tr>
* <td><b>Examine</b></td>
* <td>{@link Deque#getFirst getFirst()}</td>
* <td>{@link Deque#peekFirst peekFirst()}</td>
* <td>{@link Deque#getLast getLast()}</td>
* <td>{@link Deque#peekLast peekLast()}</td>
* </tr>
* </table>
*
* <p>This interface extends the {@link 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 {@code Queue} interface are precisely equivalent to
* {@code Deque} methods as indicated in the following table:
*
* <table BORDER CELLPADDING=3 CELLSPACING=1>
* <caption>Comparison of Queue and Deque methods</caption>
* <tr>
* <td ALIGN=CENTER> <b>{@code Queue} Method</b></td>
* <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td>
* </tr>
* <tr>
* <td>{@link java.util.Queue#add add(e)}</td>
* <td>{@link #addLast addLast(e)}</td>
* </tr>
* <tr>
* <td>{@link java.util.Queue#offer offer(e)}</td>
* <td>{@link #offerLast offerLast(e)}</td>
* </tr>
* <tr>
* <td>{@link java.util.Queue#remove remove()}</td>
* <td>{@link #removeFirst removeFirst()}</td>
* </tr>
* <tr>
* <td>{@link java.util.Queue#poll poll()}</td>
* <td>{@link #pollFirst pollFirst()}</td>
* </tr>
* <tr>
* <td>{@link java.util.Queue#element element()}</td>
* <td>{@link #getFirst getFirst()}</td>
* </tr>
* <tr>
* <td>{@link java.util.Queue#peek peek()}</td>
* <td>{@link #peek peekFirst()}</td>
* </tr>
* </table>
*
* <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This
* interface should be used in preference to the legacy {@link 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
* {@code Deque} methods as indicated in the table below:
*
* <table BORDER CELLPADDING=3 CELLSPACING=1>
* <caption>Comparison of Stack and Deque methods</caption>
* <tr>
* <td ALIGN=CENTER> <b>Stack Method</b></td>
* <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td>
* </tr>
* <tr>
* <td>{@link #push push(e)}</td>
* <td>{@link #addFirst addFirst(e)}</td>
* </tr>
* <tr>
* <td>{@link #pop pop()}</td>
* <td>{@link #removeFirst removeFirst()}</td>
* </tr>
* <tr>
* <td>{@link #peek peek()}</td>
* <td>{@link #peekFirst peekFirst()}</td>
* </tr>
* </table>
*
* <p>Note that the {@link #peek 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.
*
* <p>This interface provides two methods to remove interior
* elements, {@link #removeFirstOccurrence removeFirstOccurrence} and
* {@link #removeLastOccurrence removeLastOccurrence}.
*
* <p>Unlike the {@link List} interface, this interface does not
* provide support for indexed access to elements.
*
* <p>While {@code Deque} implementations are not strictly required
* to prohibit the insertion of null elements, they are strongly
* encouraged to do so. Users of any {@code Deque} implementations
* that do allow null elements are strongly encouraged <i>not</i> to
* take advantage of the ability to insert nulls. This is so because
* {@code null} is used as a special return value by various methods
* to indicated that the deque is empty.
*
* <p>{@code Deque} implementations generally do not define
* element-based versions of the {@code equals} and {@code hashCode}
* methods, but instead inherit the identity-based versions from class
* {@code Object}.
*
* <p>This interface is a member of the <a
* href="{@docRoot}/../technotes/guides/collections/index.html"> Java Collections
* Framework</a>.
*
* @author Doug Lea
* @author Josh Bloch
* @since 1.6
* @param <E> the type of elements held in this collection
*/
public interface Deque<E> extends Queue<E> {
/**
* Inserts the specified element at the front of this deque if it is
* possible to do so immediately without violating capacity restrictions,
* throwing an {@code IllegalStateException} if no space is currently
* available. When using a capacity-restricted deque, it is generally
* preferable to use method {@link #offerFirst}.
*
* @param e the element to add
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
void addFirst(E e);
/**
* Inserts the specified element at the end of this deque if it is
* possible to do so immediately without violating capacity restrictions,
* throwing an {@code IllegalStateException} if no space is currently
* available. When using a capacity-restricted deque, it is generally
* preferable to use method {@link #offerLast}.
*
* <p>This method is equivalent to {@link #add}.
*
* @param e the element to add
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
void addLast(E 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 {@link #addFirst} method,
* which can fail to insert an element only by throwing an exception.
*
* @param e the element to add
* @return {@code true} if the element was added to this deque, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
boolean offerFirst(E 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 {@link #addLast} method,
* which can fail to insert an element only by throwing an exception.
*
* @param e the element to add
* @return {@code true} if the element was added to this deque, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
boolean offerLast(E e);
/**
* Retrieves and removes the first element of this deque. This method
* differs from {@link #pollFirst pollFirst} only in that it throws an
* exception if this deque is empty.
*
* @return the head of this deque
* @throws NoSuchElementException if this deque is empty
*/
E removeFirst();
/**
* Retrieves and removes the last element of this deque. This method
* differs from {@link #pollLast pollLast} only in that it throws an
* exception if this deque is empty.
*
* @return the tail of this deque
* @throws NoSuchElementException if this deque is empty
*/
E removeLast();
/**
* Retrieves and removes the first element of this deque,
* or returns {@code null} if this deque is empty.
*
* @return the head of this deque, or {@code null} if this deque is empty
*/
E pollFirst();
/**
* Retrieves and removes the last element of this deque,
* or returns {@code null} if this deque is empty.
*
* @return the tail of this deque, or {@code null} if this deque is empty
*/
E pollLast();
/**
* Retrieves, but does not remove, the first element of this deque.
*
* This method differs from {@link #peekFirst peekFirst} only in that it
* throws an exception if this deque is empty.
*
* @return the head of this deque
* @throws NoSuchElementException if this deque is empty
*/
E getFirst();
/**
* Retrieves, but does not remove, the last element of this deque.
* This method differs from {@link #peekLast peekLast} only in that it
* throws an exception if this deque is empty.
*
* @return the tail of this deque
* @throws NoSuchElementException if this deque is empty
*/
E getLast();
/**
* Retrieves, but does not remove, the first element of this deque,
* or returns {@code null} if this deque is empty.
*
* @return the head of this deque, or {@code null} if this deque is empty
*/
E peekFirst();
/**
* Retrieves, but does not remove, the last element of this deque,
* or returns {@code null} if this deque is empty.
*
* @return the tail of this deque, or {@code null} if this deque is empty
*/
E peekLast();
/**
* 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 {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>
* (if such an element exists).
* Returns {@code true} if this deque contained the specified element
* (or equivalently, if this deque changed as a result of the call).
*
* @param o element to be removed from this deque, if present
* @return {@code 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
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean removeFirstOccurrence(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 {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>
* (if such an element exists).
* Returns {@code true} if this deque contained the specified element
* (or equivalently, if this deque changed as a result of the call).
*
* @param o element to be removed from this deque, if present
* @return {@code 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
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean removeLastOccurrence(Object o);
// *** Queue methods ***
/**
* 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
* {@code true} upon success and throwing an
* {@code IllegalStateException} if no space is currently available.
* When using a capacity-restricted deque, it is generally preferable to
* use {@link #offer(Object) offer}.
*
* <p>This method is equivalent to {@link #addLast}.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Collection#add})
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
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
* {@code true} upon success and {@code false} if no space is currently
* available. When using a capacity-restricted deque, this method is
* generally preferable to the {@link #add} method, which can fail to
* insert an element only by throwing an exception.
*
* <p>This method is equivalent to {@link #offerLast}.
*
* @param e the element to add
* @return {@code true} if the element was added to this deque, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
boolean offer(E e);
/**
* 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 {@link #poll poll} only in that it throws an
* exception if this deque is empty.
*
* <p>This method is equivalent to {@link #removeFirst()}.
*
* @return the head of the queue represented by this deque
* @throws NoSuchElementException if this deque is empty
*/
E remove();
/**
* Retrieves and removes the head of the queue represented by this deque
* (in other words, the first element of this deque), or returns
* {@code null} if this deque is empty.
*
* <p>This method is equivalent to {@link #pollFirst()}.
*
* @return the first element of this deque, or {@code null} if
* this deque is empty
*/
E poll();
/**
* 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 {@link #peek peek} only in that it throws an
* exception if this deque is empty.
*
* <p>This method is equivalent to {@link #getFirst()}.
*
* @return the head of the queue represented by this deque
* @throws NoSuchElementException if this deque is empty
*/
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), or
* returns {@code null} if this deque is empty.
*
* <p>This method is equivalent to {@link #peekFirst()}.
*
* @return the head of the queue represented by this deque, or
* {@code null} if this deque is empty
*/
E peek();
// *** Stack methods ***
/**
* 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 an
* {@code IllegalStateException} if no space is currently available.
*
* <p>This method is equivalent to {@link #addFirst}.
*
* @param e the element to push
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this deque
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this deque
*/
void push(E e);
/**
* Pops an element from the stack represented by this deque. In other
* words, removes and returns the first element of this deque.
*
* <p>This method is equivalent to {@link #removeFirst()}.
*
* @return 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
*/
E pop();
// *** Collection methods ***
/**
* 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 {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>
* (if such an element exists).
* Returns {@code true} if this deque contained the specified element
* (or equivalently, if this deque changed as a result of the call).
*
* <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
*
* @param o element to be removed from this deque, if present
* @return {@code 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
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean remove(Object o);
/**
* Returns {@code true} if this deque contains the specified element.
* More formally, returns {@code true} if and only if this deque contains
* at least one element {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*
* @param o element whose presence in this deque is to be tested
* @return {@code true} if this deque contains the specified element
* @throws ClassCastException if the type of the specified element
* is incompatible with this deque
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean contains(Object o);
/**
* Returns the number of elements in this deque.
*
* @return the number of elements in this deque
*/
public int size();
/**
* 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).
*
* @return an iterator over the elements in this deque in proper sequence
*/
Iterator<E> iterator();
/**
* 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).
*
* @return an iterator over the elements in this deque in reverse
* sequence
*/
Iterator<E> descendingIterator();
}
看其实现类LinkedList的其中两个方法:
/**
* Returns the first element in this list.
*
* @return the first element in this list
* @throws NoSuchElementException if this list is empty
*/
public E getFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return f.item;
}
/**
* Returns the last element in this list.
*
* @return the last element in this list
* @throws NoSuchElementException if this list is empty
*/
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
}
而 first,last 从何而来呢?
它们是单个Node
/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
* (first.prev == null && first.item != null)
*/
transient Node<E> first;
/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
* (last.next == null && last.item != null)
*/
transient Node<E> last;
从addAll时判定首位得到并赋值了
/**
* Inserts all of the elements in the specified collection into this
* list, starting at the specified position. Shifts the element
* currently at that position (if any) and any subsequent elements to
* the right (increases their indices). The new elements will appear
* in the list in the order that they are returned by the
* specified collection's iterator.
*
* @param index index at which to insert the first element
* from the specified collection
* @param c collection containing elements to be added to this list
* @return {@code true} if this list changed as a result of the call
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index);
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
Node<E> pred, succ;
if (index == size) {
succ = null;
pred = last;
} else {
succ = node(index);
pred = succ.prev;
}
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
Node<E> newNode = new Node<>(pred, e, null);
if (pred == null)
first = newNode;
else
pred.next = newNode;
pred = newNode;
}
if (succ == null) {
last = pred;
} else {
pred.next = succ;
succ.prev = pred;
}
size += numNew;
modCount++;
return true;
}
在实现类ArrayDeque也一样,只不过换了head和tail标识首尾
/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E getFirst() {
@SuppressWarnings("unchecked")
E result = (E) elements[head];
if (result == null)
throw new NoSuchElementException();
return result;
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E getLast() {
@SuppressWarnings("unchecked")
E result = (E) elements[(tail - 1) & (elements.length - 1)];
if (result == null)
throw new NoSuchElementException();
return result;
}