Java循环队列原理与用法详解

package Queue;

public interface Queue<E> {
  //获取队列中元素个数
  int getSize();

  //队列中元素是否为空
  boolean isEmpty();

  //入队列
  void enqueue(E e);

  //出队列
  E dequeue();

  //获取队首元素
  E getFront();
}

package Queue;

//循环队列
public class LoopQueue<E> implements Queue<E> {
  private E[] data;
  private int front, tail;
  private int size;//队列中元素个数

  //构造函数，传入队列的容量capacity构造函数
  public LoopQueue(int capacity) {
    data = (E[]) new Object[capacity + 1];//浪费与一个空间
    front = 0;
    tail = 0;
    size = 0;
  }

  //无参构造函数，默认队列的容量capacity=10
  public LoopQueue() {
    this(10);
  }

  //真正容量
  public int getCapacity() {
    return data.length - 1;
  }

  //队列是否为空
  @Override
  public boolean isEmpty() {
    return front == tail;
  }

  //队列中元素个数
  @Override
  public int getSize() {
    return size;
  }

  //入队列操作
  @Override
  public void enqueue(E e) {
    if ((tail + 1) % data.length == front) {//队列已满，需要扩容
      resize(getCapacity() * 2);
    }
    data[tail] = e;
    tail = (tail + 1) % data.length;
    size++;
  }

  //出队操作

  @Override
  public E dequeue() {
    if (isEmpty()) {
      throw new IllegalArgumentException("队列为空");
    }

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

  //获取队首元素
  @Override
  public E getFront() {
    if (isEmpty()) {
      throw new IllegalArgumentException("队列为空");
    }
    return data[front];
  }

  //改变容量
  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];//循环数组防止越界
    }
    data = newData;
    front = 0;
    tail = size;
  }


  @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();
  }



}

 data = (E[]) new Object[capacity + 1];//浪费与一个空间

data.length - 1;

 tail = (tail + 1) % data.length;

  newData[i] = data[(front + i) % data.length];//循环数组防止越界

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

      if(i%3==2){//每添加3个元素出队列一个
        queue.dequeue();
        System.out.println(queue);
      }

    }

  }