循环队列的实现

package com.practice;

public class LoopQueue<E> implements Queue<E> {

    private E[] data;
    private int front,tail;
    private int size;

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

    public LoopQueue(){
        this(10);
    }

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

    @Override
    public boolean isEmpty(){
        return front == tail;
    }

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

    @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 IllegalArgumentException("cannot dequeue from an empty queue.");

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

    @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();
    }

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

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

            if(i % 3 == 2){
                queue.dequeue();
                System.out.println(queue);
            }
        }
    }
}

运行结果:

Queue:size= 1 , capacity = 10 
front [0] tail
Queue:size= 2 , capacity = 10 
front [0,1] tail
Queue:size= 3 , capacity = 10 
front [0,1,2] tail
Queue:size= 2 , capacity = 5 
front [1,2] tail
Queue:size= 3 , capacity = 5 
front [1,2,3] tail
Queue:size= 4 , capacity = 5 
front [1,2,3,4] tail
Queue:size= 5 , capacity = 5 
front [1,2,3,4,5] tail
Queue:size= 4 , capacity = 5 
front [2,3,4,5] tail
Queue:size= 5 , capacity = 5 
front [2,3,4,5,6] tail
Queue:size= 6 , capacity = 10 
front [2,3,4,5,6,7] tail
Queue:size= 7 , capacity = 10 
front [2,3,4,5,6,7,8] tail
Queue:size= 6 , capacity = 10 
front [3,4,5,6,7,8] tail
Queue:size= 7 , capacity = 10 
front [3,4,5,6,7,8,9] tail