java队列的实现

还是拿之前的自己打造的ArrayList进行编写

Queue接口

public interface Queue<E> {
    int getSize();
    boolean isEmpty();
    //入队
    void enqueue(E e);
    //出队
    E dequeue();
    //获得队首
    E getFront();
}

ArrayQueue队列的实现

public class ArrayQueue<E> implements Queue<E> {
    private Array<E> array;

    public ArrayQueue(int capacity) {
        array = new Array<>(capacity);
    }

    public ArrayQueue() {
        array=new Array<>();
    }

    @Override
    public int getSize() {
        return array.getSize();
    }

    @Override
    public boolean isEmpty() {
        return array.isEmpty();
    }

    @Override
    public void enqueue(E e) {
     array.addLast(e);
    }

    @Override
    public E dequeue() {
        return array.removeFirst();
    }

    @Override
    public E getFront() {
        return array.getFirst();
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("Queue:").append("front [");
        for (int i = 0; i < array.getSize(); i++) {
            res.append(array.get(i));
            if (i != array.getSize()) {
                res.append(",");
            }
        }
        res.append("] tail");
        return res.toString();
    }

    public static void main(String[] args) {
        ArrayQueue<Integer >queue=new ArrayQueue<>();
        for (int i = 0; i < 10; i++) {
            queue.enqueue(i);
            System.out.println(queue);
            if(i%3==2){
                queue.dequeue();
                System.out.println(queue);
            }
        }
    }
}

LoopQueue循环队列

public class LoopQueue<E> implements Queue<E> {
    private E[] data;
    private int size;
    private int front, tail;//如果队首等于对于队尾，则代表数据为空

    public LoopQueue(int capacity) {
        data = (E[]) new Object[capacity + 1];//真如刚才所说，当队首等于队尾的时候数据为空，所以我们需要浪费一个空间
        size = 0;
        front = 0;
        tail = 0;
    }

    public LoopQueue() {
        this(10);
    }

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

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

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

    //入队
    @Override
    public void enqueue(E e) {
        if ((tail + 1) % data.length == front) {
            //此时队列为满了。进行扩容
            resize(getCapacity() * 2);
        }
        data[tail] = e;
        tail = (tail + 1) % data.length;
        size++;
    }

    //出队
    @Override
    public E dequeue() {
        if(isEmpty()){
            throw new IllegalArgumentException("cannot dequeue from an empty queue.");
        }
        E ret = data[front];//出队的队首
        data[front]=null;
        front=(front+1)%data.length;
        size--;
        //缩容
        if(size==getCapacity()/4&&getCapacity()!=0){
            resize(getCapacity()/2);
        }
        return ret;
    }

    //队首
    @Override
    public E getFront() {
        if(isEmpty()){
            throw new IllegalArgumentException("Queue is Empty");
        }

        return data[front];
    }
    //扩缩容
    private void resize(int newCapacity) {
        E[] newData = (E[]) new Object[newCapacity + 1];
        for (int i = 0; i < size; i++) {
            //将旧的数据的队列指向新的数据的0的位置
            newData[i] = data[(i + front) % data.length];
        }
        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();
    }
    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);
            }
        }
    }
}

测试循环队列和队列的效率

public class TestQueue {
    //测试使用q运行opCount和enqueue和dequeue操作所需要的时间，单位：s
    private static double testQueue(Queue<Integer> q, int opCount) {
        long startTime = System.nanoTime();
        Random random = new Random();
        for (int i = 0; i < opCount; i++) {
            q.enqueue(random.nextInt(Integer.MAX_VALUE));
        }
        for (int i = 0; i < opCount; i++) {
            q.dequeue();
        }
        long endTime = System.nanoTime();
        return (endTime - startTime) / 1000000000.0;
    }

    public static void main(String[] args) {
        int opCount = 100000;
        ArrayQueue<Integer> arrayQueue = new ArrayQueue<>();
        double time1 = testQueue(arrayQueue, opCount);
        System.out.println("ArrayQueue,time:" + time1 + "s");
        LoopQueue<Integer> loopQueue = new LoopQueue<>();
        double time2 = testQueue(loopQueue, opCount);
        System.out.println("LoopQueue,time:" + time2 + "s");
    }
}

循环队列的出栈复杂度是0(1)的，而队列的出栈复杂度是0(n)