數據結構之循環隊列[Java]

/**
 * @ClassName: LoopQueue
 * @Author: Leo
 * @Description: 循環隊列
 * @Date: 4/4/2020 4:35 PM
 */
public class LoopQueue<E> implements Queue<E> {

    //存放元素的數組
    private E[] data;
    //隊首引用  隊尾引用
    private int front, tail;
    //隊列元素數量
    private int size;

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

    public LoopQueue() {
        this(10);
    }

    /**
     * 獲取隊列容積
     *
     * @return
     */
    public int getCapacity() {
        return data.length - 1;
    }

    /**
     * 獲取隊列元素數量
     *
     * @return
     */
    @Override
    public int getSize() {
        return size;
    }

    /**
     * 判斷隊列是否爲空
     *
     * @return
     */
    @Override
    public boolean isEmpty() {
        //首尾引用相等 證明隊列爲空 如果(tail + 1) % data.length == front 則隊列滿
        return front == tail;
    }

    /**
     * 入隊
     *
     * @param e
     */
    @Override
    public void enqueue(E e) {
        //如果隊列滿 則擴容兩倍
        if ((tail + 1) % data.length == front) {
            resize(getCapacity() * 2);
        }
        //把元素添加到隊尾
        data[tail] = e;
        //隊尾指針後移一位
        tail = (tail + 1) % data.length;
        size++;
    }

    /**
     * 擴容
     *
     * @param newCapacity
     */
    private void resize(int newCapacity) {
        //新建一個指定容量的數組
        E[] newData = (E[]) new Object[newCapacity + 1];
        //把之前數組的元素複製到新數組
        for (int i = 0; i < size; i++) {
            //偏移量爲front
            newData[i] = data[(i + front) % data.length];
        }
        data = newData;
        front = 0;
        tail = size;
    }

    /**
     * 出隊
     *
     * @return
     */
    @Override
    public E dequeue() {
        if (isEmpty()) {
            throw new IllegalArgumentException("Cannot dequeue from an empty queue.");
        }
        //暫存出隊元素
        E e = data[front];
        data[front] = null;
        //頭引用+1
        front = (front + 1) % data.length;
        size--;
        //如果元素數量爲容積的1/4 則縮容爲原來的1/2
        if (size == getCapacity() / 4 && getCapacity() / 2 != 0) {
            resize(getCapacity() / 2);
        }
        return e;
    }

    /**
     * 獲取隊首元素
     *
     * @return
     */
    @Override
    public E getFront() {
        if (isEmpty()) {
            throw new IllegalArgumentException("Queue is empty.");
        }
        return data[front];
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append(String.format("Queue: size = %d , capacity = %d\n", size, getCapacity()));
        res.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();
    }
}