Unsafe downcasting of Object to Comparable type causes trouble [duplicate]

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How to create a generic array in Java? (31 answers)

How to create a generic array? [duplicate] (4 answers)

Closed 8 years ago.

I'm implementing a set of data structures and decided to try an implementation of a maxheap through an array, since it is one of the common implementations for maxheaps. To do that, I have an interface called MaxHeap<T> which accepts Comparable types T and the following class signature:

 public class ArrayMaxHeap<T extends Comparable<T>> implements MaxHeap<T> {

T has to be Comparable, or else I won't be able to compare the elements with one another when doing additions and removals from the heap. The problem lies with the constructor of the class:

public class ArrayMaxHeap<T extends Comparable<T>> implements MaxHeap<T> {

    private T[] data;
    private int last;
    private static final int INIT_CAPACITY = 10;

    /**
     * Creates an empty ArrayMaxHeap with the default capacity.
     */
    public ArrayMaxHeap(){
        data = (T[])(new Object[INIT_CAPACITY]);
        last = 0;
    }

The type casting of data is throwing a ClassCastException, because the downcasting from Object, which is not Comparable, is unsafe. I've hit a wall with this and am not sure how I would go about implementing the constructor. Any help would be appreciated.

Answer 1

This is kind of a limitation of generics. Instead just declare your array as Object[] and cast the element you try to return. Something similar to what ArrayList does.

public E get(int index) {
    rangeCheck(index);

    return elementData(index);
}

E elementData(int index) {
    return (E) elementData[index];
}

where elementData is

private transient Object[] elementData;

If you control what is going in, there's no problem.

I'm going to link the question and answer in Rohit's comment because it's brilliant.

Answer 2

Just change it to

data = (T[])new Comparable[INIT_CAPACITY];