Question

In the past, I’ve been able to implement a generic ArrayList by casting Object[] to E[]:

public class ArrayList<E>
{
    private E[] elements;

    @SuppressWarnings("unchecked")
    public ArrayList()
    {
        elements = (E[])new Object[16];
    }
 }

and I could use the generic array normally, like so:

public void add(E element)
{
    ...
    elements[size++] = element;
}

I thought I could implement a trie similarly:

public class Trie<V>
{
    private static class Node<V>
    {
        public V value;
        @SuppressWarnings("unchecked")
        public Node<V>[] next = (Node<V>[])new Object[26];
    }

    private Node<V> root = new Node<V>();
}

However, the line public Node<V>[] next = (Node<V>[])new Object[26]; in Trie.Node causes a ClassCastException:

public class Main
{
    public static void main(String[] args)
    {
        ArrayList<Integer> a = new ArrayList<Integer>();
        Trie<Integer> t = new Trie<Integer>();
    }
}

java Main
Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [LTrie$Node;
    at Trie$Node.<init>(Trie.java:6)
    at Trie$Node.<init>(Trie.java:3)
    at Trie.<init>(Trie.java:10)
    at Main.main(Main.java:6)

So it’s saying I can’t cast Object[] to Trie.Node[].

What’s the difference between casting from Object[] to E[] and casting from Object[] to Trie.Node<V>[]? Why is the former permitted and not the latter?

Answer 1

The difference is that generics are implemented via erasure, and giving no bounds will erase that generic type into Object. E[] will become Object[], while Trie.Node<V>[] will become Trie.Node<Object>[]. Hence, (Node<V>[])new Object[26]; is a invalid cast as you’re trying to cast Object[] to Node<Object>[].

You can find more information about how erasure works at http://docs.oracle.com/javase/tutorial/java/generics/erasure.html.