ClassCastException when casting from Object[] to Comparable[] [duplicate]

Question

Edit: Noticed the post was flagged as duplicate; I read through the other, similar, question but it really didn't treat the same problem as mine. I'm working with a class that is guaranteed to be of the type Comparable, yet I can't cast it.

I have the following class, which is an implementation of an interface Queue. I make sure that the type which HeapPriorityQueue object works with implements Comparable, so that the method E.compareTo(E other) is available.

I create an instance of HeapPriorityQueue in my main method. Where Patient is a class that implements Comparable

HeapPriorityQueue<Patient> queue = new HeapPriorityQueue<>();

At compilation however, I get an exception ClassCastException which points towards the last line in the code below

public class HeapPriorityQueue<E extends Comparable<E>> implements Queue<E>
{

    private final int DEFAULT_CAPACITY = 10;

    private int size;
    private E[] array;

    public HeapPriorityQueue()
    {
        size = 0;
        array = (E[]) new Object[DEFAULT_CAPACITY + 1];
    }
}

The exception:

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable;

Is there some reason (e.g. teacher said so) that it needs to be an array specifically? There are other data structures (e.g. List<E>) that might better suit you... — dcsohl
– dcsohl, Commented May 14, 2015 at 17:33
@dcsohl I am limited in my choices. It's a course about data structures, so I imagine my choices are either singly linked lists or arrays for the heap structure. — B. Lee
– B. Lee, Commented May 14, 2015 at 17:38
I would definitely go for some sort of List here, even if you have to write it yourself. As you have found, you cannot do new E[size]; and you cannot cast from an Object[] to an E[] (which you would not want to do, even if you could, since your new Object[] is full of Objects and not Es!). Your best bet is probably ArrayList. Whether or not that's allowed is a different question. Although I do have to ask... if it's a HeapPriorityQueue, shouldn't you be setting up a Heap and not an Array/List? — dcsohl
– dcsohl, Commented May 14, 2015 at 17:48
@dcsohl I'm under the impression that a heap can be built up from an array. That you could place a bunch of elements in an array and say that k:th's left child node is 2*k, and k:th's right child node is 2*k + 1. Is this not how it works? — B. Lee
– B. Lee, Commented May 14, 2015 at 18:00
Huh. Ok, yeah, sure, I suppose you could do that, if you felt like being masochistic (or had a sadistic teacher making you do that). I'd personally create a quick little Node<E> class that had a value and right and left children. I think the data structure manipulations would be easier that way. But this is veering off-topic for this question... — dcsohl
– dcsohl, Commented May 14, 2015 at 18:07

Jihed Amine · Accepted Answer · 2015-05-14 18:13:07Z

1

I'm working with a class that is guaranteed to be of the type Comparable, yet I can't cast it.

new Object[DEFAULT_CAPACITY + 1]; doesn't guarantee anything instead of the fact that it's an object array. An object array can't be safely cast to a Comparable array.

You'd have to use something like:

Class<E> clazz;
array = (E[]) Array.newInstance(clazz, size);

edited May 14, 2015 at 18:13

answered May 14, 2015 at 18:01

Jihed Amine

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