使用堆栈反转队列时出现空指针异常

Question

我在练习如何使用数组实现队列。我已经很容易地实现了如何对队列中的元素进行入队和出队。但是我在使用堆栈

实现队列反向时遇到了异常

public class QueueImpl {
private int capacity;
int queueArr[];
int front = 0;
int rear = -1;
int currentSize = 0;
QueueImpl(int queueSize){
    this.capacity=queueSize;
    queueArr=new int[this.capacity];
}

public void enqueue(int data){
    if(isQueueFull()){
        System.out.println("Overflow");
        return;
    }
    else{
        rear=rear+1;
        if(rear==capacity-1)
        {
            rear=0;
        }
        queueArr[rear]=data;
        currentSize++;
        System.out.println("Element " + data+ " is pushed to Queue !");
    }

}


public int dequeue(){
    if(isQueueEmpty()){
        System.out.println("UnderFlow");
    }
    else{
        front=front+1;
        if(front == capacity-1){
            System.out.println("Pop operation done ! removed: "+queueArr[front-1]);
            front = 0;
        } else {
            System.out.println("Pop operation done ! removed: "+queueArr[front-1]);
        }
        currentSize--;
    }
    return queueArr[front-1];

}
private boolean isQueueEmpty() {
    boolean status=false;
    if(currentSize==0){
        status=true;
    }
    return status;
}

private boolean isQueueFull() {
    boolean status=false;
    if(currentSize==capacity){
        status=true;
    }
    return status;
}

public static void main(String arg[]) {
    QueueImpl queueImpl=new QueueImpl(5);
    queueImpl.enqueue(5);
    queueImpl.enqueue(2);
    queueImpl.enqueue(9);
    queueImpl.enqueue(1);
//  queueImpl.dequeue();
    queueImpl.printQueue(queueImpl);
    queueImpl.reverse(queueImpl);
}

private void printQueue(QueueImpl queueImpl) {
System.out.println(queueImpl.toString());       
}
 @Override
    public String toString() {
        return "Queue [front=" + front + ", rear=" + rear + ", size=" + currentSize
                + ", queue=" + Arrays.toString(queueArr) + "]";
    }
private QueueImpl reverse(QueueImpl queueImpl) throws ArrayIndexOutOfBoundsException {
    int i=0;
    Stack<Integer> stack=new Stack<Integer>();  
    while(!queueImpl.isQueueEmpty()){
        stack.push(queueImpl.dequeue());
    }
    while(!stack.isEmpty()){
        stack.get(i);
        i++;
    }
    while(!stack.isEmpty()){
        queueImpl.enqueue(stack.pop());
    }
    return queueImpl;
}

}

错误日志为-

 Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
 at com.tcs.QueueUsingAraay.QueueImpl.dequeue(QueueImpl.java:51)
 at com.tcs.QueueUsingAraay.QueueImpl.reverse(QueueImpl.java:93)
 at com.tcs.QueueUsingAraay.QueueImpl.main(QueueImpl.java:78)

Answer 1

您的出队方法有问题： 1. 你让 front=0 并试图访问 front-1 这是 -1 结果抛出了异常。

已删除 stack.get，因为不需要。

更正了工作代码。

public class QueueImpl {
        private int capacity;
        int queueArr[];
        int front = 0;
        int rear = -1;
        int currentSize = 0;

        QueueImpl(int queueSize) {
            this.capacity = queueSize;
            queueArr = new int[this.capacity];
        }

        public void enqueue(int data) {
            if (isQueueFull()) {
                System.out.println("Overflow");
                return;
            } else {
                rear = rear + 1;
                if (rear == capacity - 1) {
                    rear = 0;
                }
                queueArr[rear] = data;
                currentSize++;
                System.out.println("Element " + data + " is pushed to Queue !");
            }

        }

        public int dequeue() {
            int element=-1;
            if (isQueueEmpty()) {
                System.out.println("UnderFlow");
            } else {
                element = queueArr[front];
                front=front+1;
                if (front == capacity - 1) {
                    System.out.println("Pop operation done ! removed: "
                            + queueArr[front - 1]);
                    front = 0;
                } else {
                    System.out.println("Pop operation done ! removed: "
                            + queueArr[front - 1]);
                }
                currentSize--;
            }
            return element;

        }

        private boolean isQueueEmpty() {
            boolean status = false;
            if (currentSize == 0) {
                status = true;
            }
            return status;
        }

        private boolean isQueueFull() {
            boolean status = false;
            if (currentSize == capacity) {
                status = true;
            }
            return status;
        }

        public static void main(String arg[]) {
            QueueImpl queueImpl = new QueueImpl(5);
            queueImpl.enqueue(5);
            queueImpl.enqueue(2);
            queueImpl.enqueue(9);
            queueImpl.enqueue(1);

            queueImpl.printQueue(queueImpl);
            queueImpl.reverse(queueImpl);
            queueImpl.printQueue(queueImpl);
        }

        private void printQueue(QueueImpl queueImpl) {
            System.out.println(queueImpl.toString());
        }

        @Override
        public String toString() {
            return "Queue [front=" + front + ", rear=" + rear + ", size="
                    + currentSize + ", queue=" + Arrays.toString(queueArr) + "]";
        }

        private QueueImpl reverse(QueueImpl queueImpl)
                throws ArrayIndexOutOfBoundsException {
            Stack<Integer> stack = new Stack<Integer>();
            while (!queueImpl.isQueueEmpty()) {
                stack.push(queueImpl.dequeue());
            }
            while (!stack.isEmpty()) {
                queueImpl.enqueue(stack.pop());
            }
            return queueImpl;
        }

    }

使用堆栈反转队列时出现空指针异常

Null pointer Exception while reversing a Queue using stack

queue

stack

reverse

data-structures