java - 尝试调用方法时出现"invalid method declaration; return type required"

Question

我是 Java 新手，在整理列表时遇到一些问题。我必须编写 Iterable 类，以便当您迭代队列时它会以随机顺序返回项目。这就是为什么我编写了 shuffle 方法，然后在 RandomizedQueueIterator 类中调用它

import java.util.Iterator;
import java.util.NoSuchElementException;
public class RandomizedQueue<Item> implements Iterable<Item>
{
    // instance variables
    private Item[] queue;
    private int N = 0;
    private int R = 0;


    // constructor
    public RandomizedQueue()
    {
        queue = (Item[]) new Object[1];
    }


    // helper functions
    private void resize(int capacity)
    {
        Item[] copy = (Item[]) new Object[capacity];
        for (int i = 0; i < N; i++)
            copy[i] = queue[i];
        queue = copy;
    }

    private void shrink(int capacity)
    {
        Item[] copy = (Item[]) new Object[capacity];
        int M = 0;
        for (int i = 0; i < N; i++)
        {
            if (queue[i] != null)
                copy[M++] = queue[i];
        }
        queue = copy;
    }   

    // shuffle queue
    private void shuffle(Item[] a )
    {
        int N = a.length;
        for (int i = 0; i < N; i++)
        {
            int r = StdRandom.uniform(i + 1);
            Item temp = a[r];
            a[r] = a[i];
            a[i] = temp;
        }
    }


    // return wether queue is empty
    public boolean isEmpty()  {  return queue.length == 0;   }


    // return size of queue
    public int size()   {   return queue.length;   }


    // add item to the queue
    public void enqueue(Item item)
    {
        if (N == queue.length)   {   resize(N*2);   }   
        queue[N++] = item;
    }


    // remove and return a random item from the queue
    public Item deque()
    {   
        // generate a random index and store item
        int random_index;
        do  {
            random_index = StdRandom.uniform(0, N);                            
        } while (queue[random_index] == null);

        Item item = queue[random_index];

        // delete item from array and increment counter of null items
        queue[random_index] = null;
        R += 1;

        // shrink array if corresponds
        if (R == (3/4 * N))   {   shrink(N/2);   }

        // return item
        return item;
    }    

    // return a random item from the queue
    public Item sample()
    {   
        // generate a random index and store item
        int random_index;
        do  {
            random_index = StdRandom.uniform(0, N);                            
        } while (queue[random_index] == null);

        Item item = queue[random_index];

        // return item
        return item;       
    }


    // iteration
    public Iterator<Item> iterator()  
    {  
        return new RandomizedQueueIterator();      
    }

    private class RandomizedQueueIterator implements Iterator<Item>
    {
        private int i = 0;
        shuffle(queue);


        public boolean hasNext()   
        {  
            return i < N;  
        }

        public Item next()
        {   
            if (i >= N)
            {
                throw java.util.NoSuchElementException; 
            }
            return queue[i++];

        }

        public void remove()
        {
            throw new UnsupportedOperationException("Invalid operation for array");
        }    
    }


    // main method
    public static void main(String[] args)
    {
        RandomizedQueue example = new RandomizedQueue();
        example.enqueue(0);
        example.enqueue(1);
        example.enqueue(2);
        example.enqueue(3);
        example.enqueue(3);
        example.enqueue(3);
        example.enqueue(3);
    }   
}

尝试编译时的输出:

2 errors found:

[line: 114] Error: invalid method declaration; return type required

[line: 114] Error: expected

line:114 > shuffle(queue);

任何帮助将不胜感激。谢谢

Answer 1

如果你想运行该类，你需要将方法调用放在主方法中

public static void main(String...args) {
   shuffle(list_test);
}

如果您这样做，您可以将 list_test 定义为 static 变量，或者将其定义在 main 方法中。