在Java中访问没有公共访问者的私有成员

Question

I've been given a challenge, that I must get value of list item at given index, from outside Queue class (items is private). I'm not allowed to modify the class, and not allowed to use Reflection. Is it possible (in a real case, I would rather create public accessor to get items value) ?

class Queue {
    private List<Integer> items;

    private Queue() {
        items = new ArrayList<Integer>();
    }

    public static Queue create() {
        return new Queue();
    }

    public void push(int item) {
        items.add(item);
    }

    public int shift() {
        return items.remove(0);
    }

    public boolean isEmpty() {
        return items.size() == 0;
    }
}

Answer 1

You can :

1. remove all the elements from the Queue , using shift
2. add each removed element to your own ArrayList
3. iterate over the ArrayList and re-add the elements to the Queue in the same order using push in order to restore the Queue to its original state.
4. return the index 'th element of your ArrayList .

It's very inefficient, but it solves your challenge.

Answer 2

You may try this

public class TestQueue {
public static void main(String[] args){

    Queue q=  Queue.create();
    q.push(10);
    q.push(20);
    q.push(30);
    q.push(40);
    q.push(50);     
        System.out.println(q.shift());      
}}

Answer 3

Above source code is a basic implementation of Queue. From your question I understand that you want to extract item of a given index. I think you should iterate data to get the preferable index. If you come to end of array before finding that index then you can throw ArrayIndexOutOfBoundsException exception.

Here is a basic implementation.

public void dataRetrieve() throws ArrayIndexOutOfBoundsException {
        Queue queue =  Queue.create();
        queue.push(10);
        queue.push(20);
        queue.push(30);
        queue.push(40);
        queue.push(50);

        int indexToRetrieve = 5;

        int index = 0;
        while(!queue.isEmpty()) {
            int value = queue.shift();
            if (index == indexToRetrieve) {
                System.out.println(value);
                return;
            }
            index++;
        }

        throw new ArrayIndexOutOfBoundsException();
    }