队列是一种特殊的线性表，它只允许在表的前端进行删除操作，而在表的后端进行插入操作。
先看下JDK中Queue接口的定义：
/*
* 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 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 class="striped">
* <caption>Summary of Queue methods</caption>
* <thead>
* <tr>
* <td></td>
* <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
* <th scope="col" style="font-weight:normal; font-style:italic">Returns special value</th>
* </tr>
* </thead>
* <tbody>
* <tr>
* <th scope="row">Insert</th>
* <td>{@link #add(Object) add(e)}</td>
* <td>{@link #offer(Object) offer(e)}</td>
* </tr>
* <tr>
* <th scope="row">Remove</th>
* <td>{@link #remove() remove()}</td>
* <td>{@link #poll() poll()}</td>
* </tr>
* <tr>
* <th scope="row">Examine</th>
* <td>{@link #element() element()}</td>
* <td>{@link #peek() peek()}</td>
* </tr>
* </tbody>
* </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}/java.base/java/util/package-summary.html#CollectionsFramework">
* Java Collections Framework</a>.
*
* @since 1.5
* @author Doug Lea
* @param <E> the type of elements held in this queue
*/
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();
}
方法签名
public interface Queue<E> extends Collection<E> {
boolean add(E e); 向队添加元素
boolean offer(E e); 添加元素
E remove(); // 移除对列头部元素
E poll(); // 移除对列头部元素
E element(); //
E peek(); // 查看对列头部元素
}主要/子接口/子类/抽象类/实现类：
AbstractQueue
ArrayDeque
CheckedQueue
AsLIFOQueue
Deque子接口
LinkedList
PriorityQueue
ArrayBlockingQueue
BlockingQueue
BlockingDeque
ConcurrentLinkedDeque
ConcurrentLinkedQueue
DelayQueue
DelayedWorkQueue
PriorityBlockingQueue
SynchronousQueue
TransferQueue
基本使用
LinkedList类实现了Queue接口，因此我们可以把LinkedList当成Queue来用。
以下实例演示了队列（Queue）的用法：
import java.util.LinkedList;
import java.util.Queue;
public class Main {
public static void main(String[] args) {
//add()和remove()方法在失败的时候会抛出异常(不推荐)
Queue<String> queue = new LinkedList<String>();
//添加元素
queue.offer("a");
queue.offer("b");
queue.offer("c");
queue.offer("d");
queue.offer("e");
for(String q : queue){
System.out.println(q);
}
System.out.println("===");
System.out.println("poll="+queue.poll()); //返回第一个元素，并在队列中删除
for(String q : queue){
System.out.println(q);
}
System.out.println("===");
System.out.println("element="+queue.element()); //返回第一个元素
for(String q : queue){
System.out.println(q);
}
System.out.println("===");
System.out.println("peek="+queue.peek()); //返回第一个元素
for(String q : queue){
System.out.println(q);
}
}
}输出结果：
a
b
c
d
e
===
poll=a
b
c
d
e
===
element=b
b
c
d
e
===
peek=b
b
c
d
e
来源：oschina
链接：https://my.oschina.net/Adven/blog/4254561