循环数组队列中的 toString 方法

Question

我需要创建一个允许用户“环绕”数组的循环数组队列。我的老师检查了入队和出队方法，但我无法正确打印队列。

public class MyArrayQueue<E> implements QueueInterface<E> {

    public static final int CAPACITY = 10;  
    private int capacity; 
    private E q[]; //array of E 
    private int f; //front 
    private int r; //rear

    //constructor
    public MyArrayQueue() {
         this (CAPACITY);
    }

    //constructor
    public MyArrayQueue(int n) {
        capacity = n;
        q = (E[]) new Object[capacity];
    }

    public void enqueue(E obj) throws FullQueueException {
        if(size() == capacity - 1) { //cannot hold more than n-1
            throw new FullQueueException("Full queue exception.");
    }

        q[r] = obj; //insert object in end of the queue
        r = (r + 1) % capacity; //wrap around r

    }

    public E dequeue() throws EmptyQueueException {
        if (isEmpty())
            throw new EmptyQueueException("Empty queue exception.");

        E temp = q[f]; //retrieve the front object
        q[f] = null; //good programming practice

        f = (f + 1) % capacity; //wrap around f

        return temp;
  }

  public E front() throws EmptyQueueException {
      if (isEmpty()) 
          throw new EmptyQueueException("Empty queue exception.");

      return q[f]; //return the front object without removing it
  }

  public boolean isEmpty() {
      return (f == r);
  }

  public int size() {
      return (capacity - f + r) % capacity;
  }

  //fix this loop for homework assignment, make it wrap around
  public String toString() {
      if (isEmpty())
          return "[]";

      String result = "[";

      result += q[f];

      for (int i = (f+1) % capacity; i != r; i = (i+1) % capacity) {
          result += " " +q[i];
      }
      return result + "]";
  }

} //end class

这也是我的客户端类。我将 toString 方法更改为用户 Jyr 建议的方法，但它仍然无法正确打印。我在客户端类中实现它时可能会出错。

public static void main(String[] args) {

    Scanner console = new Scanner(System.in);

    MyArrayQueue<String> list = new MyArrayQueue<>();

    int capacity;
    int CAPACITY = 10;
    String q[];

    for (int i = 0; i < CAPACITY; i++) {
        capacity = i;
        q = new String[i];
    }

    String name;

    boolean flag = true;
    int num;

    while (flag) {
        System.out.println();
        System.out.println("1 ----- enqueue");
        System.out.println("2 ----- dequeue");
        System.out.println("3 ----- front");
        System.out.println("4 ----- ouput elements in the queue");
        System.out.println("5 ----- isEmpty");
        System.out.println("6 ----- size");
        System.out.println("0 ----- exit");
        System.out.println();

        System.out.println("Enter a command: ");
        num = console.nextInt();
        System.out.println();

        switch(num) {
            case 1:
                try {
                    System.out.println("Enter a word to enqueue: ");
                    name = console.next();
                    list.enqueue(name); 
                }
                catch (FullQueueException e) {
                    System.out.println("Full Queue");
                }
                break;
            case 2:
                try  {
                    System.out.println(list.dequeue());
                }
                catch (EmptyQueueException e){
                    System.out.println("Empty Queue");
                }
                break;
            case 3:
                try {
                    System.out.println(list.front());
                }
                catch (EmptyQueueException e) {
                    System.out.println("Empty Queue");
                }
                break;
            case 4:
                System.out.println(list.toString());
                break;
            case 5:
                System.out.println(list.isEmpty());
                break;
            case 6:
                System.out.println(list.size());
                break;
            case 0:
                flag = false;
                System.out.println("Thank you for using this program. ");
                break;
            default:
                System.out.println("Invalid input.");
                break;
        }
    }

}

Answer 1

我相信你把事情复杂化了。你想要的是从f（例如i = f）开始，然后向上移动一个，但是既然我们可以环绕，我们应该向上移动(i + 1) % capacity。我们继续这样做直到我们到达终点（例如r）。在我们进入这个循环之前，我们应该验证队列不为空（以防止错误）。如果它实际上是空的，我们只需返回一个带有两个括号的String。所以你的代码看起来有点像这样；

public String toString() {

    if (isEmpty()) return "[]";
    String result = "[";

    for (int i = f; i != r; i = (i+1)%capacity) {
        result += q[i] + ", ";
    }   

    return result.substring(0,result.length()-2) + "]";
    // a bit dirty but gets the job done
}

或不同的方法（关于括号和逗号的打印）；

public String toString() {

    if (isEmpty()) return "[]";
    String result = "[";

    result += q[f];

    for (int i = (f+1)%capacity; i != r; i = (i+1)%capacity) {
        result += ", " + q[i];
    }   

    return result + "]";

}