Java队列-Queue详细分析

Queue

Queue 是 Collection 接口的子接口，和List，Set 出于同一层级。

除了基本Collection操作外，队列还提供其他插入，提取和检查操作。这些方法中的每一种都以两种形式存在：一种在操作失败时引发异常，另一种返回一个特殊值（取决于操作，为null或false）。插入操作的后一种形式是专为与容量受限的Queue 实现一起使用而设计的；在大多数实现中，插入操作不会失败。

操作	引发异常	返回特殊值
插入	add(e)	offer(e)
移除	remove()	poll()
检查	element()	peek()

队列通常但不一定以FIFO（先进先出）的方式对元素进行排序。例外情况包括优先级队列（根据提供的比较器 comparator 对元素进行排序或元素的自然排序）和LIFO队列（或堆栈），对LIFO进行排序（后进先出）。无论使用哪种顺序，可以通过调用remove()或poll() 删除该队列的开头的元素(head of the queue)。在FIFO队列中，所有新元素都插入队列的尾部(the tail of queue)。其他种类的队列可能使用不同的放置规则。每个Queue实现必须指定其排序属性。

add(E e) 和 offer(E e)

可以看看AbstractQueue的源代码：

    public boolean add(E var1) {
        if (this.offer(var1)) {
            return true;
        } else {
            throw new IllegalStateException("Queue full");
        }
    }

可以看到，抽象类里，add()方法在执行成功后会返回true,执行失败会抛出IllegalStateException(“Queue full”);
而offer(E e) 方法没有实现。会在具体的子类中实现，比如ArrayBlockingQueue.java

    /**
     * Inserts the specified element at the tail of this queue if it is
     * possible to do so immediately without exceeding the queue's capacity,
     * returning {@code true} upon success and {@code false} if this queue
     * is full.  This method is generally preferable to method {@link #add},
     * which can fail to insert an element only by throwing an exception.
     *
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        Objects.requireNonNull(e);
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (count == items.length)
                return false;
            else {
                enqueue(e);
                return true;
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * Inserts element at current put position, advances, and signals.
     * Call only when holding lock.
     */
    private void enqueue(E e) {
        // assert lock.isHeldByCurrentThread();
        // assert lock.getHoldCount() == 1;
        // assert items[putIndex] == null;
        final Object[] items = this.items;
        items[putIndex] = e;
        if (++putIndex == items.length) putIndex = 0;
        count++;
        notEmpty.signal();
    }

如果e为null, 就会抛 NullPointerException().

     /**
     * Checks that the specified object reference is not {@code null}. This
     * method is designed primarily for doing parameter validation in methods
     * and constructors, as demonstrated below:
     * <blockquote><pre>
     * public Foo(Bar bar) {
     *     this.bar = Objects.requireNonNull(bar);
     * }
     * </pre></blockquote>
     *
     * @param obj the object reference to check for nullity
     * @param <T> the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     */
    public static <T> T requireNonNull(T obj) {
        if (obj == null)
            throw new NullPointerException();
        return obj;
    }

这里的NullPointerException 是运行时异常，继承 RuntimeException. 在java.lang包下。
还有一个NullPointerException.c在java.base/share/native/libjava/下。是C文件，需要翻翻虚拟机代码了：
https://github.com/kaffe/kaffe

扯远了。可以看到， ArrayBlockingQueue 在添加前会加锁。这也应正了它的名字。

可以参考 https://docs.oracle.com/javase/7/docs/technotes/guides/collections/index.html 了解更多。

LinkedList

先看看LinkedList.java的实现：
可以看到它有关键的first Node 和 last Node.

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    transient int size = 0;

    /**
     * Pointer to first node.
     */
    transient Node<E> first;

    /**
     * Pointer to last node.
     */
    transient Node<E> last;
    //...................
}

而一个Node元素会有一个prev，指向它的前一个Node，有一个next，指向它的后一个Node.还有一个泛型属性E，用于保存Node节点的值。

    private static class Node<E> {
        E item;
        Node<E> next;
        Node<E> prev;

        Node(Node<E> prev, E element, Node<E> next) {
            this.item = element;
            this.next = next;
            this.prev = prev;
        }
    }

对于LinkedList.它的 index 查询是怎么实现的呢？

    /**
     * Returns the (non-null) Node at the specified element index.
     */
    Node<E> node(int index) {
        // assert isElementIndex(index);

        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

先把size扩大两倍，然后用index和它比较。其实就是看index是在中间以前，还是在中间以后，如果在中间以前，就从前半部分遍历，直到找到该Node，如果是在后半部分，就从末尾开始遍历。

再看看它的各种操作：

   /**
     * Links e as first element.
     */
    private void linkFirst(E e) {
        final Node<E> f = first;
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
        modCount++;
    }

    /**
     * Links e as last element.
     */
    void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }

    /**
     * Inserts element e before non-null Node succ.
     */
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        final Node<E> pred = succ.prev;
        final Node<E> newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }

    /**
     * Unlinks non-null first node f.
     */
    private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;
        f.next = null; // help GC
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    }

    /**
     * Unlinks non-null last node l.
     */
    private E unlinkLast(Node<E> l) {
        // assert l == last && l != null;
        final E element = l.item;
        final Node<E> prev = l.prev;
        l.item = null;
        l.prev = null; // help GC
        last = prev;
        if (prev == null)
            first = null;
        else
            prev.next = null;
        size--;
        modCount++;
        return element;
    }

    /**
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        x.item = null;
        size--;
        modCount++;
        return element;
    }
    
   /**
     * Links e as first element.
     */
    private void linkFirst(E e) {
        final Node<E> f = first;
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
        modCount++;
    }

    /**
     * Links e as last element.
     */
    void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }

    /**
     * Inserts element e before non-null Node succ.
     */
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        final Node<E> pred = succ.prev;
        final Node<E> newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }

    /**
     * Unlinks non-null first node f.
     */
    private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;
        f.next = null; // help GC
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    }

    /**
     * Unlinks non-null last node l.
     */
    private E unlinkLast(Node<E> l) {
        // assert l == last && l != null;
        final E element = l.item;
        final Node<E> prev = l.prev;
        l.item = null;
        l.prev = null; // help GC
        last = prev;
        if (prev == null)
            first = null;
        else
            prev.next = null;
        size--;
        modCount++;
        return element;
    }

    /**
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        x.item = null;
        size--;
        modCount++;
        return element;
    }
   /**
     * Links e as first element.
     * 添加头部元素
     */
    private void linkFirst(E e) {
        // 新建一个Node保存原来老的first
        final Node<E> f = first;
        // 创建一个新节点 newNode, 新加的 e 作为保存之， 老的 first 节点作为这个新节点的next
        final Node<E> newNode = new Node<>(null, e, f);
        // 把创建的新节点 newNode 作为 first.
        first = newNode;
        if (f == null)
            last = newNode;
        else  
            f.prev = newNode;
        size++;
        modCount++;
    }

    /**
     * Links e as last element.
     */
    void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }

    /**
     * Inserts element e before non-null Node succ.
     */
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        final Node<E> pred = succ.prev;
        final Node<E> newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }

    /**
     * Unlinks non-null first node f.
     */
    private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;
        f.next = null; // help GC
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    }

    /**
     * Unlinks non-null last node l.
     */
    private E unlinkLast(Node<E> l) {
        // assert l == last && l != null;
        final E element = l.item;
        final Node<E> prev = l.prev;
        l.item = null;
        l.prev = null; // help GC
        last = prev;
        if (prev == null)
            first = null;
        else
            prev.next = null;
        size--;
        modCount++;
        return element;
    }

    /**
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        x.item = null;
        size--;
        modCount++;
        return element;
    }

PriorityQueue

PriorityQueue 继承了 AbstractQueue,看看源码：

public class PriorityQueue<E> extends AbstractQueue<E> implements Serializable {
    private static final long serialVersionUID = -7720805057305804111L;
    private static final int DEFAULT_INITIAL_CAPACITY = 11;
    transient Object[] queue;
    private int size;
    private final Comparator<? super E> comparator;
    transient int modCount;
    private static final int MAX_ARRAY_SIZE = 2147483639;
    // ......省略
}

可以看到他是以 Object[] 数组来保存元素的。这和LinkedList不一样。
它还有一个 final Comparator 属性。