定义

队列是一种先进先出（First In First Out，FIFO）的数据结构。

实现

可以和栈一样，把队列看成是数组实现的一个子集。由于队列可以有多个底层数据结构的实现，因此，先定义队列的接口：

public interface Queue<E> {

    int size();

    boolean isEmpty();

    void enqueue(E e);

    E dequeue();

    E getFront();

}

实现由Array作为底层实现的队列：

public class ArrayQueue<E> implements Queue<E> {
    private final Array<E> array;

    public ArrayQueue(int capacity) {
        array = new Array<>(capacity);
    }

    public ArrayQueue() {
        array = new Array<>();
    }

    @Override
    public int size() {
        return array.getSize();
    }

    @Override
    public boolean isEmpty() {
        return array.isEmpty();
    }

    @Override
    public void enqueue(E e) {
        array.addLast(e);
    }

    @Override
    public E dequeue() {
        return array.removeLast();
    }

    @Override
    public E getFront() {
        return array.getFirst();
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("ArrayQueue ").append("front [");
        for (int i = 0; i < array.getSize(); i++) {
            res.append(array.get(i));
            if (i != array.getSize() - 1) {
                res.append(", ");
            }
        }
        res.append("] tail");
        return res.toString();
    }
}

这种实现对于出队，由于每次都在队首移除，因此对于dequeue来讲，算法复杂度为O(n)。使用循环队列可以使得出队操作的算法复杂度也为O(1)

循环队列

public class LoopQueue<E> implements Queue<E> {
    private E[] data;
    /**
     * 指向队首
     */
    private int front;
    /**
     * 指向下一个入队的位置
     */
    private int tail;
    /**
     * 队列长度
     */
    private int size;

    public LoopQueue(int capacity) {
        data = (E[]) new Object[capacity + 1];
        front = 0;
        tail = 0;
        size = 0;
    }

    public LoopQueue() {
        this(10);
    }

    @Override
    public int size() {
        return size;
    }

    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    public int getCapacity() {
        return data.length - 1;
    }

    @Override
    public void enqueue(E e) {
        if ((tail + 1) % data.length == front) {
            resize(getCapacity() * 2);
        }
        data[tail] = e;
        tail = (tail + 1) % data.length;
        size++;
    }

    @Override
    public E dequeue() {
        if (isEmpty()) {
            throw new IllegalStateException("Cannot dequeue in a empty queue!");
        }

        E e = data[front];
        data[front] = null;
        front = (front + 1) % data.length;
        size--;
        if (size == getCapacity() / 4 && getCapacity() / 2 != 0) {
            resize(getCapacity() / 2);
        }
        return e;
    }

    @Override
    public E getFront() {
        return data[front];
    }

    private void resize(int newCapacity) {
        E[] newData = (E[]) new Object[newCapacity + 1];
        for (int i = 0; i < size; i++) {
            newData[i] = data[(front + i) % data.length];
        }
        data = newData;
        front = 0;
        tail = size;
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("LoopQueue ").append("front [");
        for (int i = front; i != tail; i = (i + 1) % data.length) {
            res.append(data[i]);
            if ((i + 1) % data.length != tail) {
                res.append(", ");
            }
        }
        res.append("] tail");
        return res.toString();
    }

    public static void main(String[] args) {
        LoopQueue<Integer> queue = new LoopQueue<>();
        for (int i = 0; i < 10; i++) {
            queue.enqueue(i);
            if ((i + 1) % 3 == 0) {
                queue.dequeue();
            }
            System.out.println(queue);
        }
    }
}