Implementing a Queue, getting an error when making it generic

Question

I'm confused as to what I am doing wrong with this implementation of AbstractQueue.

This code is what is causing the error:

public static void main(String[] args) {
    Stack<Integer> intStack = new Stack();
    MyQueue<Integer> realQueue = new MyQueue();
    PriorityQueue<Integer> intQueue = new PriorityQueue();
}

Eclipse is saying I need to remove the generic typing from MyQueue.

Here is the class as implemented:

import java.util.AbstractQueue;
import java.util.Iterator;

public class MyQueue extends AbstractQueue<Object> {

    private T[] arr;
    private int headPos;
    private int tailPos;
    private int size = 0;

    @Override
    public boolean offer(T e) {
        if (size < arr.length)
            size++;
        else if (headPos == tailPos)
            headPos = nextPos(headPos);
        arr[tailPos] = e;
        tailPos = nextPos(tailPos);
        return true;
    }

    private int nextPos(int pos) {
        return (pos + 1) % arr.length;
    }

    @Override
    public T peek() {
        if (size == 0)
            return null;
        return arr[headPos];
    }

    @Override
    public T poll() {
        if (size == 0)
            return null;
        size--;
        T res = arr[headPos];
        headPos = nextPos(headPos);
        return res;
    }

    @Override
    public Iterator iterator() {
        // TODO Auto-generated method stub
        return null;
    }

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

This is both my first time implementing a queue, and my first time creating a class to work with generics, so I am thoroughly lost as to what to do. When I follow Eclipse's advice, I get a null pointer exception, and when I leave the generic code in, it does not compile.

Your are extending AbstractQueue<Object>, there is no generic — Shivam
– Shivam, Commented May 30, 2013 at 1:03
You should declare your class public class MyQueue<T> extends AbstractQueue<T>. — fge
– fge, Commented May 30, 2013 at 1:03
changed that, still gives me a null pointer exception when I try to add something to the queue. — Marshall Tigerus
– Marshall Tigerus, Commented May 30, 2013 at 1:07
Your class declaration should be public class MyQueue<T> extends AbstractQueue<T>. Your variable declarations need to specify the generic parameter type on the RHS - Stack<Integer> intStack = new Stack<Integer>();. And your NPE is coming from somewhere else in your code. — Perception
– Perception, Commented May 30, 2013 at 1:08

Jatin Sehgal · Accepted Answer · 2013-05-30 05:18:44Z

1

I think you were not using generics properly so I have modified your class. Please check if it works for you.....
UPDATE: I have changed the code a bit to instantiate the arr[] array. At the bottom main method and a constructor have been added.

import java.util.AbstractQueue;
import java.util.Iterator;

public class MyQueue<T> extends AbstractQueue<T> {

    private T[] arr;
    private int headPos;
    private int tailPos;
    private int size = 0;

    @Override
    public boolean offer(T e) {
        if (size < arr.length)
            size++;
        else if (headPos == tailPos)
            headPos = nextPos(headPos);
        arr[tailPos] = e;
        tailPos = nextPos(tailPos);
        return true;
    }

    private int nextPos(int pos) {
        return (pos + 1) % arr.length;
    }

    @Override
    public T peek() {
        if (size == 0)
            return null;
        return arr[headPos];
    }

    @Override
    public T poll() {
        if (size == 0)
            return null;
        size--;
        T res = arr[headPos];
        headPos = nextPos(headPos);
        return res;
    }

    @Override
    public Iterator<T> iterator() {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public int size() {
        // TODO Auto-generated method stub
        return 0;
    }

    public static void main(String[] args) {
        MyQueue<Integer> realQueue = new MyQueue<Integer>(Integer[].class, 10);
        System.out.println(realQueue.offer(20));
    }
    public MyQueue(Class<T[]> clazz, int length) {
        arr = clazz.cast(Array.newInstance(clazz.getComponentType(), length));
    }

}

edited May 30, 2013 at 5:18

answered May 30, 2013 at 1:21

Jatin Sehgal

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2 Comments

Jatin Sehgal Over a year ago

Thts probably because you hv not instantiated your arr[] array. Instantiate it somewhere in the constructor or do it in offer(T e) before the if(size < arr.length).

Jatin Sehgal Over a year ago

@MarshallTigerus I have updated the above code to add a constructor in which the array is instantiated.

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