Java之--Queue隊列

一、ArrayDeque數組隊列
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【先進先出】結構

  形象圖
買火車票(網上訂的不算)，排隊打飯(插隊的不講究)都是這種狀態。

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;//數組

    //The index of the element at the head of the deque
    private transient int head;//隊列頭

    //The index at which the next element would be added to the tail of the deque
    private transient int tail;//隊列尾
    
    //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;//初始化數組的最小容量
}

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

數組結構
想象兩個指針,一個指向隊頭,一個指向隊尾,人隊是從隊尾入的，出隊是從隊頭出的。

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();//擴容，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;//相當於*2,擴容一倍
        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;
    }

擴容

擴容時會消除已經出隊(remove掉)的元素佔據的空間,如上圖:
入隊(a,b,c,d,e)，然後出隊(a,b,c)，再入隊(f直到p),add(p)後，隊列滿了，會擴容doubleCapacity()，
System.arraycopy(elements, p, a, 0, r);顯示爲上圖
elements數組的3--15(數組下標index)複製到a數組的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);//頭指針後移
        return result;
    }