Initialize Java Generic Array of Type Generic

Question

So I have this general purpose HashTable class I'm developing, and I want to use it generically for any number of incoming types, and I want to also initialize the internal storage array to be an array of LinkedList's (for collision purposes), where each LinkedList is specified ahead of time (for type safety) to be of the type of the generic from the HashTable class. How can I accomplish this? The following code is best at clarifying my intent, but of course does not compile.

public class HashTable<K, V>
{
    private LinkedList<V>[] m_storage;

    public HashTable(int initialSize)
    {
        m_storage = new LinkedList<V>[initialSize];
    }
}

Duplicate of stackoverflow.com/questions/529085/…

skaffman
– skaffman

2009-06-22 07:27:51 +00:00
Commented Jun 22, 2009 at 7:27 — skaffman
– skaffman, Commented Jun 22, 2009 at 7:27

Avi · Accepted Answer · 2009-06-22 07:37:46Z

15

Generics in Java doesn't allow creation of arrays with generic types. You can cast your array to a generic type, but this will generate an unchecked conversion warning:

public class HashTable<K, V>
{
    private LinkedList<V>[] m_storage;

    public HashTable(int initialSize)
    {
        m_storage = (LinkedList<V>[]) new LinkedList[initialSize];
    }
}

Here is a good explanation, without getting into the technical details of why generic array creation isn't allowed.

answered Jun 22, 2009 at 7:37

Avi

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4 Comments

Aaron Digulla Over a year ago

You can add @SuppressWarnings({"unchecked"}) to the assignment to keep the compiler quiet.

Paul Bellora Over a year ago

The page you linked to is actually not about the same thing. This section of Angelika Langer's Generics FAQ explains it well.

Daniel Alder Over a year ago

If you use @SuppressWarnings({"unchecked"}) before this command to keep the compiler quiet, you can also just do @SuppressWarnings({"unchecked"}) m_storage = new LinkedList[initialSize]; which is a bit simpler and technically no difference to @SuppressWarnings({"unchecked"}) m_storage = (LinkedList<V>[]) new LinkedList[initialSize];. But I have to agree: neither of those are really nice :-(

Daniel Alder Over a year ago

The link to the explanation is broken

Cornelius Dol · Accepted Answer · 2009-09-16 20:34:37Z

0

Also, you can suppress the warning on a method by method basis using annotations:

@SuppressWarnings("unchecked")
public HashTable(int initialSize) {
    ...
    }

answered Sep 16, 2009 at 20:34

Cornelius Dol

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