Queue AbstractQueue Deque 源代码
1、Queue 源代码
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
*
*
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* 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
*/
/*
在处理元素前用于保存元素的 collection。除了基本的 Collection 操作外，队列还提供其他的插入、
提取和检查操作。每个方法都存在两种形式：一种抛出异常（操作失败时），另一种返回一个特殊值
（null 或 false，具体取决于操作）。插入操作的后一种形式是用于专门为有容量限制的 Queue 实现设计的；
在大多数实现中，插入操作不会失败。
抛出异常 返回特殊值
插入 add(e) offer(e)
移除 remove() poll()
检查 element() peek()
队列通常（但并非一定）以 FIFO（先进先出）的方式排序各个元素。不过优先级队列和 LIFO 队列（或堆栈）例外，
前者根据提供的比较器或元素的自然顺序对元素进行排序，后者按 LIFO（后进先出）的方式对元素进行排序。
无论使用哪种排序方式，队列的头 都是调用 remove() 或 poll() 所移除的元素。在 FIFO 队列中，
所有的新元素都插入队列的末尾。其他种类的队列可能使用不同的元素放置规则。每个 Queue 实现必须指定其顺序属性。
如果可能，offer 方法可插入一个元素，否则返回 false。这与 Collection.add 方法不同，
该方法只能通过抛出未经检查的异常使添加元素失败。offer 方法设计用于正常的失败情况，
而不是出现异常的情况，例如在容量固定（有界）的队列中。
remove() 和 poll() 方法可移除和返回队列的头。到底从队列中移除哪个元素是队列排序策略的功能，
而该策略在各种实现中是不同的。remove() 和 poll() 方法仅在队列为空时其行为有所不同：remove()
方法抛出一个异常，而 poll() 方法则返回 null。
element() 和 peek() 返回，但不移除，队列的头。
Queue 接口并未定义阻塞队列的方法，而这在并发编程中是很常见的。BlockingQueue 接口定义了
那些等待元素出现或等待队列中有可用空间的方法，这些方法扩展了此接口。
Queue 实现通常不允许插入 null 元素，尽管某些实现（如 LinkedList）并不禁止插入 null。
即使在允许 null 的实现中，也不应该将 null 插入到 Queue 中，因为 null 也用作 poll 方法
的一个特殊返回值，表明队列不包含元素。
Queue 实现通常未定义 equals 和 hashCode 方法的基于元素的版本，而是从 Object 类继承了
基于身份的版本，因为对于具有相同元素但有不同排序属性的队列而言，基于元素的相等性并非总是定义良好的。
此接口是 Java Collections Framework 的成员
*/
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
*/
//将元素插入队列，如果没有容量，则抛出IllegalStateException
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
*/
//插入一个元素，如果没有容量，不会抛出异常
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
*/
//获取并移除队列头，如果队列为空，则抛出NoSuchElementException
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
*/
//获取并移除队列头，不会抛出异常，队列为空，则返回null
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
*/
//获取队列头，不移除，队列为空则抛出NoSuchElementException异常
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
*/
//获取队列头，不移除，队列为空则返回null
E peek();
}
2、AbstractQueue 源代码
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
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*
*
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/*
*
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*
* 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;
/**
* This class provides skeletal implementations of some {@link Queue}
* operations. The implementations in this class are appropriate when
* the base implementation does <em>not</em> allow <tt>null</tt>
* elements. Methods {@link #add add}, {@link #remove remove}, and
* {@link #element element} are based on {@link #offer offer}, {@link
* #poll poll}, and {@link #peek peek}, respectively, but throw
* exceptions instead of indicating failure via <tt>false</tt> or
* <tt>null</tt> returns.
*
* <p>A <tt>Queue</tt> implementation that extends this class must
* minimally define a method {@link Queue#offer} which does not permit
* insertion of <tt>null</tt> elements, along with methods {@link
* Queue#peek}, {@link Queue#poll}, {@link Collection#size}, and
* {@link Collection#iterator}. Typically, additional methods will be
* overridden as well. If these requirements cannot be met, consider
* instead subclassing {@link AbstractCollection}.
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @since 1.5
* @author Doug Lea
* @param <E> the type of elements held in this collection
*/
/*
此类提供某些 Queue 操作的骨干实现。此类中的实现适用于基本实现不 允许包含 null 元素时。add、remove
和 element 方法分别基于 offer、poll 和 peek 方法，但是它们通过抛出异常而不是返回 false 或 null 来指示失败。
扩展此类的 Queue 实现至少必须定义一个不允许插入 null 元素的 Queue.offer(E) 方法，该方法以及 Queue.peek()、
Queue.poll()、Collection.size() 和 Collection.iterator() 都支持 Iterator.remove() 方法。
通常还要重写其他方法。如果无法满足这些要求，那么可以转而考虑为 AbstractCollection 创建子类。
此类是 Java Collections Framework 的成员
*/
public abstract class AbstractQueue<E>
extends AbstractCollection<E>
implements Queue<E> {
/**
* Constructor for use by subclasses.
*/
protected AbstractQueue() {
}
/**
* Inserts the specified element into this queue if it is possible to do so
* immediately without violating capacity restrictions, returning
* <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
* if no space is currently available.
*
* <p>This implementation returns <tt>true</tt> if <tt>offer</tt> succeeds,
* else throws an <tt>IllegalStateException</tt>.
*
* @param e the element to add
* @return <tt>true</tt> (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
*/
public boolean add(E e) {
//调用Queue接口中的offer(e)
if (offer(e))
return true;
else
throw new IllegalStateException("Queue full");
}
/**
* 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.
*
* <p>This implementation returns the result of <tt>poll</tt>
* unless the queue is empty.
*
* @return the head of this queue
* @throws NoSuchElementException if this queue is empty
*/
public E remove() {
E x = poll();
if (x != null)
return x;
else
throw new NoSuchElementException();
}
/**
* 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.
*
* <p>This implementation returns the result of <tt>peek</tt>
* unless the queue is empty.
*
* @return the head of this queue
* @throws NoSuchElementException if this queue is empty
*/
public E element() {
E x = peek();
if (x != null)
return x;
else
throw new NoSuchElementException();
}
/**
* Removes all of the elements from this queue.
* The queue will be empty after this call returns.
*
* <p>This implementation repeatedly invokes {@link #poll poll} until it
* returns <tt>null</tt>.
*/
public void clear() {
//全部元素出栈
while (poll() != null)
;
}
/**
* Adds all of the elements in the specified collection to this
* queue. Attempts to addAll of a queue to itself result in
* <tt>IllegalArgumentException</tt>. Further, the behavior of
* this operation is undefined if the specified collection is
* modified while the operation is in progress.
*
* <p>This implementation iterates over the specified collection,
* and adds each element returned by the iterator to this
* queue, in turn. A runtime exception encountered while
* trying to add an element (including, in particular, a
* <tt>null</tt> element) may result in only some of the elements
* having been successfully added when the associated exception is
* thrown.
*
* @param c collection containing elements to be added to this queue
* @return <tt>true</tt> if this queue changed as a result of the call
* @throws ClassCastException if the class of an element of the specified
* collection prevents it from being added to this queue
* @throws NullPointerException if the specified collection contains a
* null element and this queue does not permit null elements,
* or if the specified collection is null
* @throws IllegalArgumentException if some property of an element of the
* specified collection prevents it from being added to this
* queue, or if the specified collection is this queue
* @throws IllegalStateException if not all the elements can be added at
* this time due to insertion restrictions
* @see #add(Object)
*/
public boolean addAll(Collection<? extends E> c) {
if (c == null)
throw new NullPointerException();
if (c == this)
throw new IllegalArgumentException();
boolean modified = false;
//添加容器c中元素
for (E e : c)
if (add(e))
modified = true;
return modified;
}
}
3、Deque源代码【双向队列】
/*
* 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
*/
/*
一个线性 collection，支持在两端插入和移除元素。名称 deque 是“double ended queue（双端队列）”
的缩写，通常读为“deck”。大多数 Deque 实现对于它们能够包含的元素数没有固定限制，
但此接口既支持有容量限制的双端队列，也支持没有固定大小限制的双端队列。
此接口定义在双端队列两端访问元素的方法。提供插入、移除和检查元素的方法。每种方法都存在两种形式：
一种形式在操作失败时抛出异常，另一种形式返回一个特殊值（null 或 false，具体取决于操作）。
插入操作的后一种形式是专为使用有容量限制的 Deque 实现设计的；在大多数实现中，插入操作不能失败。
下表总结了上述 12 种方法：
第一个元素（头部） 最后一个元素（尾部）
抛出异常 特殊值 抛出异常 特殊值
插入 addFirst(e) offerFirst(e) addLast(e) offerLast(e)
移除 removeFirst() pollFirst() removeLast() pollLast()
检查 getFirst() peekFirst() getLast() peekLast()
此接口扩展了 Queue 接口。在将双端队列用作队列时，将得到 FIFO（先进先出）行为。将元素添加到双端队列的末尾，从双端队列的开头移除元素。从 Queue 接口继承的方法完全等效于 Deque 方法，如下表所示：
Queue 方法 等效 Deque 方法
add(e) addLast(e)
offer(e) offerLast(e)
remove() removeFirst()
poll() pollFirst()
element() getFirst()
peek() peekFirst()
双端队列也可用作 LIFO（后进先出）堆栈。应优先使用此接口而不是遗留 Stack 类。在将双端队列用作堆栈时，元素被推入双端队列的开头并从双端队列开头弹出。堆栈方法完全等效于 Deque 方法，如下表所示：
堆栈方法 等效 Deque 方法
push(e) addFirst(e)
pop() removeFirst()
peek() peekFirst()
注意，在将双端队列用作队列或堆栈时，peek 方法同样正常工作；无论哪种情况下，都从双端队列的开头抽取元素。
此接口提供了两种移除内部元素的方法：removeFirstOccurrence 和 removeLastOccurrence。
与 List 接口不同，此接口不支持通过索引访问元素。
虽然 Deque 实现没有严格要求禁止插入 null 元素，但建议最好这样做。建议任何事实上允许 null 元素的 Deque 实现用户最好不 要利用插入 null 的功能。这是因为各种方法会将 null 用作特殊的返回值来指示双端队列为空。
Deque 实现通常不定义基于元素的 equals 和 hashCode 方法，而是从 Object 类继承基于身份的 equals 和 hashCode 方法。
此接口是 Java Collections Framework 的成员
*/
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
*/
//插入双向队列的开头，容量不够抛出异常
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
*/
//插入双向队列的末尾，容量不够抛出异常
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
*/
//插入双向队列的开头，容量不够返回false
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
*/
//插入双向队列末尾，容量不够返回false
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
*/
//移除双向队列的头，如果队列为空，则抛出异常
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
*/
//移除双向队列的末尾，如果队列为空，则抛出异常
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
*/
//返回双向队列的头元素，并移除
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
*/
//返回双向队列的尾元素，并移除
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
*/
//返回双向队列的头元素，不移除，如果队列为空，则抛出异常
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
*/
//返回双向队列的尾元素，不移除，如果队列为空，则抛出异常
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
*/
//返回双向队列的头元素，不移除，如果队列为空，则返回null
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
*/
//返回双向队列的尾元素，不移除，如果队列为空，则返回null
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>)
*/
//移除第一个出现的o
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>)
*/
//移除最后一个出现的o
boolean removeLastOccurrence(Object o);
// *** Queue methods ***
//队列方法
/**
* 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
*/
//添加一个元素，该方法等价与 addLast
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
*/
//入队一个元素，该方法等价于offerLast
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
*/
//移除掉头元素，该方法等价于removeFirst
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
*/
//出队，该方法等价于pollFirst
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
*/
//获取队列头元素，该方法等价于getFirst
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
*/
//返回队列头，该方法等价于peekFirst
E peek();
// *** Stack methods ***
//栈方法
/**
* 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
*/
//入栈，该方法等价于addFirst
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
*/
//出栈，该方法等价于removeFirst
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>)
*/
//移除掉o元素，该方法等价于removeFirstOccurrence(o)
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>)
*/
//返回是否包含o元素
boolean contains(Object o);
/**
* Returns the number of elements in this deque.
*
* @return the number of elements in this deque
*/
//双向队列的容量
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
*/
//返回该双向队列的迭代器
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
*/
//逆向迭代器
Iterator<E> descendingIterator();
}