（二）Java数据结构之基于数组实现的环形队列

package top.baikunlong.queue;
import java.util.Scanner;
/**
 * @author baikunlong
 * @date 2020/10/7 11:53
 */
public class ArrayQueue {
    private int maxSize;//最大容量，实际存储只有maxSize-1，有一个空间为预留空间。
    private int front;//队头
    private int rear;//队尾
    private int[] arr;
    public ArrayQueue(int maxSize) {
        this.maxSize = maxSize;
        arr = new int[maxSize];
    }
    public boolean isFull() {
        //这里要预留一个空间，也就是有一个空间永远是利用不了的，不然rear==front了，区别不了是空队列还是满队列。。。
        return (rear + 1) % maxSize == front;
    }
    public boolean isEmpty() {
        return rear == front;
    }
    /**
     * 入队
     *
     * @param n
     */
    public void add(int n) {
        if (isFull()) {
            System.out.println("队列已满");
        } else {
            arr[rear] = n;
            rear = (rear + 1) % maxSize;
        }
    }
    /**
     * 出队，会移除数据
     *
     * @return
     */
    public int poll() {
        if (isEmpty()) {
            throw new RuntimeException("队列为空");
        } else {
            int n = arr[front];
            front = (front + 1) % maxSize;
            return n;
        }
    }
    /**
     * 打印队列
     */
    public void show() {
        //从front开始遍历，遍历到front+size
        for (int i = front; i < front+size(); i++) {
            System.out.printf("arr[%d]=%d\n", i % maxSize, arr[i % maxSize]);
        }
    }
    /**
     * 当前队列有效个数
     *
     * @return
     */
    public int size() {
        return (rear + maxSize - front) % maxSize;
    }
    public static void main(String[] args) {
        ArrayQueue queue = new ArrayQueue(4);
        Scanner scanner = new Scanner(System.in);
        while (true) {
            System.out.println("a(add): 添加数据到队列");
            System.out.println("p(poll): 从队列取出数据");
            System.out.println("s(show): 显示队列");
            System.out.println("e(exit): 退出程序");
            String s = scanner.nextLine();
            switch (s) {
                case "a":
                    queue.add(scanner.nextInt());
                    break;
                case "p":
                    try {
                        System.out.println("出队得到："+queue.poll());
                    } catch (Exception e) {
                        System.out.println(e.getMessage());
                    }
                    break;
                case "s":
                    queue.show();
                    break;
                case "e":
                    System.exit(0);
                    break;
            }
        }
    }
}