环形队列

当队列尾部插入元素满了，头部又删掉了一些元素，这种情况下，就会误认为已满无法存入数据，而实际上头部删除了元素已经留出了空间。

这时候环形队列就解决了这样的一个问题，环形队列的 front 指针始终指向当前队列的第一个元素；rear 指针始终指向最后一个元素后一个的位置（也可以理解为第一个元素的前一个位置为）

队列满条件  ( rear + 1 ) % maxSize = font

队列空条件 rear = font

队列有效数据个数 ( rear + maxSize - front) % maxSize

package queue;

import java.util.Scanner;

public class CircleArrayQueueDemo {
    public static void main(String[] args) {
        CircleArray arrayQueue = new CircleArray(4);
        char key = ' ';
        Scanner scanner = new Scanner(System.in);
        boolean loop = true;
        while (loop) {
            System.out.println("s 显示队列");
            System.out.println("e 退出程序");
            System.out.println("a 添加数据到队列");
            System.out.println("g 从队列取出数据");
            System.out.println("h 查看队列头数据");
            key = scanner.next().charAt(0);
            switch (key) {
                case 's':
                    arrayQueue.showQueue();
                    break;
                case 'e':
                    scanner.close();
                    loop = false;
                    break;
                case 'a':
                    System.out.println("请输入");
                    int value = scanner.nextInt();
                    arrayQueue.addQueue(value);
                    break;
                case 'g':
                    try {
                        int res = arrayQueue.getQueue();
                        System.out.printf("取出数据为%d\n", res);
                    } catch (Exception e) {
                        System.out.println(e.getMessage());
                    }
                    break;
                case 'h':
                    try {
                        int res = arrayQueue.headQueue();
                        System.out.printf("队列头数据为%d\n", res);
                    } catch (Exception e) {
                        System.out.println(e.getMessage());
                    }
                    break;
            }
            System.out.println("程序退出");
        }
    }
}

class CircleArray {
    private int maxSize;
    private int front;
    private int rear;
    private int[] arr;

    public CircleArray(int arrMaxSize) {
        maxSize = arrMaxSize;
        arr = new int[maxSize];
    }

    public boolean isFull() {
        return (rear + 1) % maxSize == front;
    }

    public boolean isEmpty() {
        return rear == front;
    }

    public void addQueue(int n) {
        if (isFull()) {
            System.out.println("队列已满");
            return;
        }
        arr[rear] = n;
        rear = (rear + 1) % maxSize;
    }

    public int getQueue() {
        if (isEmpty()) {
            throw new RuntimeException("队列为空");
        }
        int value = arr[front];
        front = (front + 1) % maxSize;
        return value;
    }

    public void showQueue() {
        if (isEmpty()) {
            System.out.println("队列为空");
            return;
        }
        for (int i = front; i < front + size(); i++) {
            System.out.printf("arr[%d]=%d\n", i % maxSize, arr[i % maxSize]);
        }
    }

    public int size() {
        return (rear + maxSize - front) % maxSize;
    }

    public int headQueue() {
        if (isEmpty()) {
            throw new RuntimeException("队列为空");
        }
        return arr[front];
    }
}