Java - Creating array of an object with generic parameters

Question

So I know normally to create a generic array you could do:

E[] e = (E[]) new Object[10]; 

However I have a class Entrant<K, V> which has two generic parameters. I can't seem to be able to cast an Object array to it.

Here is the full code and error at runtime

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [LHashTable.Entrant;
 at HashTable.HashTable.<init>(HashTable.java:10)
 at Mainy.map(Mainy.java:32)

line 32 in Mainy :

HashTable h = new HashTable();

Hashtable code:

public class HashTable<K, V> {
 Entrant<K, V>[] _entrants;
 private static final int N = 16;
 public HashTable() {
 _entrants = (Entrant<K, V>[]) new Object[N]; //line 10
 }
}

• Jesper: What you are trying to do does not work. An Object[] is not an Entrant<K, V>[], so you get a ClassCastException. See this question: stackoverflow.com/questions/1817524/generic-arrays-in-java?rq=1
• TheLostMind: also.. Please follow standard Java naming conventions - camelCase for instance

Answer 1

Casting Object[] to E[] is not guaranteed to work when you expose the array outside your class. Casting works in constructor because the type of the type parameter E is erased to Object, and the cast is effectively equivalent to:

Object[] e = (Object[]) new Object[10]; 

However, suppose your HashTable class provides a K[] array:

class HashTable<K, V> {
 K[] _entrants;
 private static final int N = 16;
 public HashTable() {
 _entrants = (K[]) new Object[N]; //line 10
 }
 public K[] getEntrants() {
 return _entrants;
 }
}

And you create instance of it, and gets the entrants from it:

HashTable<String, String> hashTable = new HashTable<>();
String[] entrants = hashTable.getEntrants();

That code will throw ClassCastException in the second assignment.

While in case of parameterized type array, casting would fail as it is erased to:

_entrants = (Entrant[]) new Object[N]; //line 10

Clearly an Object[] is not a Extrant[]. So that would fail to work. Rather than doing the cast, you can directly create an array of raw type:

_entrants = new Entrant[N];

and suppress the warning that comes.

Also See:

• How to create a generic array?
• How to create a generic array in Java?

• newacct: or new Entrant<?, ?>[N] if you want to avoid raw types

Answer 2

The logic behind this is :

Every Entrant is an Object, but every object is not an Entrant. See, you are casting Objects to Entrants which won't work.