循环队列的Java简单实现

基本概念

队列：一种先进先出（First In First Out，FIFO）的数据结构。其来自于我们日常生活中的排队问题，排在前面的先获得服务。
循环队列：长度固定的队列。其优势是循环利用内存资源，入队和出队都是O（1）的时间复杂度。通常，我们使用一个数组来实现循环队列，对头通过front指针标记，对尾通过tail指针标记。当front = tail时，队列为空；当tail + 1 = head时，队列为满。
基于这些约定，就可以编程实现循环队列了。

Java实现

抽象出队列接口。

/**
 * A First In Fist Out (FIFO) data structure.
 * 
 * @author Xingjian
 * @since 2019/09/20
 * @param <T> element type
 */
public interface Queue<T> {
	/**
	 * add element to Queue.
	 * 
	 * @param element
	 * @return true, element added successfully. Otherwise,false.
	 */
	boolean enQueue(T element);

	/**
	 * get element from Queue.
	 * 
	 * @return element, removed from Queue.Or null if Queue is empty.
	 */
	T deQueue();

	/**
	 * test whether this Queue is empty.
	 * 
	 * @return true, if Queue is empty. Otherwise,false.
	 */
	boolean isEmpty();
}

循环队列实现。

/**
 * Circular Queue implementation.
 * 
 * @author Xingjian
 * @since 2019/09/20
 * @param <T> element type
 */
public class CircularQueue<T> implements Queue<T> {
	/**
	 * element stored in this array.
	 */
	private T elements[];

	/***
	 * front pointer.
	 */
	private int front;

	/**
	 * tail pointer.
	 */
	private int tail;

	/**
	 * construct a Circular Queue with given length.
	 * 
	 * @param length
	 */
	@SuppressWarnings("unchecked")
	public CircularQueue(int length) {
		if (length <= 0) {
			throw new IllegalArgumentException("length must be a positive integer.");
		}
		this.elements = (T[]) new Object[length];
		this.front = 0;
		this.tail = 0;
	}

	@Override
	public boolean enQueue(T element) {
		if ((tail + 1) % elements.length == front) {
			// there is no space to add element.
			return false;
		}
		elements[tail] = element;
		tail = (tail + 1) % elements.length;
		return true;
	}

	@Override
	public T deQueue() {
		if (isEmpty()) {
			return null;
		}
		T e = elements[front];
		front = (front + 1) % elements.length;
		return e;
	}

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