ClassCastException using Generics Object[] cannot be cast

Question

So I'm trying to create a class that implements a heap using an array in order to make a prioritized list. In my constructor I want to create an array of Entry objects. Is this possible to do? I have done some generic casting before and I have tried everything that usually works for me. I feel like It should be possible but i can't figure it out.

This is the exception I get when I run it:

[Ljava.lang.Object; cannot be cast to [Llab09.Entry;

`public class ArrayHeap<K,V> implements PriorityQueue<K,V> {
    `private Comparator<K> comp;
     private Entry<K,V>[] data;
     private int heapSize;

     @SuppressWarnings({"unchecked"})
     public ArrayHeap(int size, Comparator<K> c){
        data = (Entry<K,V>[]) new Object[size]; // type casting array
        heapSize = 0;
        comp = c;
    }
 }

Also I'll throw in my nested Entry class to look at aswell.

protected static class AHEntry<K,V> implements Entry<K,V> {
    private K k;
    private V v;

    public AHEntry(K key, V value){
        k = key;
        v = value;
    }
    public K getKey(){ return k;}
    public V getValue(){ return v;}
    public void setKey(K key){ k = key;}
    public void setValue(V value){ v = value;}
}

I think this applies here: stackoverflow.com/questions/4862960/… — Cong Hui
– Cong Hui, Commented May 1, 2015 at 6:24

Tim Biegeleisen · Accepted Answer · 2015-05-01 14:53:36Z

1

The line data = (Entry<K,V>[]) new Object[size] is resulting in a type cast error because an Object array cannot be cast to a Map.Entry array. The following code makes use of the custom AHEntry class which you provided:

public class ArrayHeap<K,V> implements PriorityQueue<K,V> {
    private Comparator<K> comp;
    private Entry<K,V>[] data;
    private int heapSize;

    @SuppressWarnings({"unchecked"})
    public ArrayHeap(int size, Comparator<K> c){
        data = new (AHEntry<K, V>)new AHEntry<?, ?>[size];
        heapSize = 0;
        comp = c;
    }
}

edited May 1, 2015 at 14:53

answered May 1, 2015 at 6:26

Tim Biegeleisen

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5 Comments

Jesper Over a year ago

It's the other way around: he's trying to cast an Object[] to a Map.Entry[].

Tim Biegeleisen Over a year ago

Right, and since this cannot be done we either have to try something else or change the logic of the code.

Nick1995Carp Over a year ago

Alright thanks that helps me out. I'm going to change it up a bit.

Jesper Over a year ago

@TimBiegeleisen I meant this: in your answer you say "... because a Map.Entry array cannot be cast to an Object array", but it's the other way around: because an Object array cannot be cast to a Map.Entry array.

Radiodef Over a year ago

Need to do new AHEntry[size] or (AHEntry<K, V>)new AHEntry<?, ?>[size].

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