Java之--Queue队列

最新推荐文章于 2023-04-05 16:09:04 发布

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#ArrayDeque数组队列 #java队列

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本文详细介绍了Java中的ArrayDeque数据结构，它是一个基于数组的双端队列，支持先进先出（FIFO）的操作。ArrayDeque在满时会进行扩容，扩容时会消除已出队元素的空间。主要操作包括add()用于入队，remove()用于出队，文章通过实例展示了这些操作的工作原理。

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一、ArrayDeque数组队列
=====================================================
【先进先出】结构

形象图
买火车票(网上订的不算)，排队打饭(插队的不讲究)都是这种状态。

public class ArrayDeque<E> extends AbstractCollection<E>
                           implements Deque<E>, Cloneable, Serializable{
    //The array in which the elements of the deque are stored
    private transient E[] elements;//数组

    //The index of the element at the head of the deque
    private transient int head;//队列头

    //The index at which the next element would be added to the tail of the deque
    private transient int tail;//队列尾
    
    //The minimum capacity that we'll use for a newly created deque.
    //Must be a power of 2.
    private static final int MIN_INITIAL_CAPACITY = 8;//初始化数组的最小容量
}

//初始化,容量为16
public ArrayDeque() {
elements = (E[]) new Object[16];
}

数组结构
想象两个指针,一个指向队头,一个指向队尾,人队是从队尾入的，出队是从队头出的。

1.add()

public boolean add(E e) {
        addLast(e);
        return true;
    }
public void addLast(E e) {
        if (e == null)
            throw new NullPointerException();
        elements[tail] = e;        //
        if ( (tail = (tail + 1) & (elements.length - 1)) == head)
            doubleCapacity();//扩容，增加一倍容量double
    }
private void doubleCapacity() {
        int p = head;
        int n = elements.length;
        int r = n - p; // number of elements to the right of p
        int newCapacity = n << 1;//相当于*2,扩容一倍
        if (newCapacity < 0)
            throw new IllegalStateException("Sorry, deque too big");
        Object[] a = new Object[newCapacity];
        System.arraycopy(elements, p, a, 0, r);
        System.arraycopy(elements, 0, a, r, p);
        elements = (E[])a;
        head = 0;
        tail = n;
    }

扩容

扩容时会消除已经出队(remove掉)的元素占据的空间,如上图:
入队(a,b,c,d,e)，然后出队(a,b,c)，再入队(f直到p),add(p)后，队列满了，会扩容doubleCapacity()，
System.arraycopy(elements, p, a, 0, r);显示为上图
elements数组的3--15(数组下标index)复制到a数组的0-12

2.remove()

public E remove() {
        return removeFirst();
    }
public E removeFirst() {
        E x = pollFirst();
        if (x == null)
            throw new NoSuchElementException();
        return x;
    }
public E pollFirst() {
        int h = head;
        E result = elements[h]; // Element is null if deque empty
        if (result == null)
            return null;
        elements[h] = null;     // Must null out slot
        head = (h + 1) & (elements.length - 1);//头指针后移
        return result;
    }