Object array not getting cast to a generic type

Question

I intend to create a simple Dictionary ADT using a linked list. I have got little problem with the getKeys() method, here is my code:

@Override
public K[] getKeys() 
{
    if(head==null)
    return null;
    else
    {
        Vector<K> v = new Vector();
        ListNode<K,V> temp= head;
        while(temp!=null)
        {
            v.add(temp.key);
            temp=temp.next;
        }
        //K keys[] = new O[v.size()];

        return (K[])v.toArray();//run time error
    }
}

I am having the following error:

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable;
    at Dictionary.ListDictionary.getKeys(ListDictionary.java:17)
    at Dictionary.DictionaryDriver.test(DictionaryDriver.java:83)
    at Dictionary.DictionaryDriver.main(DictionaryDriver.java:107)
Java Result: 1

Here is the interface i am trying to implement:

public interface DictionaryInterface <K extends Comparable<K>, V>
{
    public void insert(K key, V value);
    public V getValue(K str);
    public void remove(K key);
    public K[] getKeys();   
}

I understand that we cannot create an array of generics, but i never had problem with casting generic types to Objecttype.Does it have something to do with the generic type K extending Comparable? How do i work around this??

dupe of stackoverflow.com/questions/12137097/…

Ben Schulz
– Ben Schulz

2012-09-13 07:32:27 +00:00
Commented Sep 13, 2012 at 7:32 — Ben Schulz
– Ben Schulz, Commented Sep 13, 2012 at 7:32

Costi Ciudatu · Accepted Answer · 2012-09-13 08:00:26Z

3

Try something like:

return (K[])v.toArray(new Comparable[v.size()]);

As a side note, however, mixing generics and arrays is not the best idea.

To do this safely (and be able to accept types other than Comparable), you would need to get the type reference, so your constructor could look like this:

public Dictionary(Class<K> keyType) {
    this.keyType = keyType;
}

Later on, when instantiating that array, call:

return (K[]) v.toArray( Array.newInstance(keyType, v.size()) );

edited Sep 13, 2012 at 8:00

answered Sep 13, 2012 at 7:32

Costi Ciudatu

38.5k7 gold badges58 silver badges95 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

sasidhar Over a year ago

When i do this, i could eliminate the run time error from that point, but later when i create an instance of this class using <String> and getting the getKeys() i am getting this error

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Comparable; cannot be cast to [Ljava.lang.String; 	at Dictionary.DictionaryDriver.test(DictionaryDriver.java:83) 	at Dictionary.DictionaryDriver.main(DictionaryDriver.java:107) Java Result: 1

Costi Ciudatu Over a year ago

That's why mixing generics and arrays is a bad idea. You need a reference to the actual type class in order to do this safely (I'll edit my answer).

sasidhar Over a year ago

String implements Comparable then why is this failing??

sasidhar Over a year ago

i quite didn't get the edit you made to your answer.. little help?

Costi Ciudatu Over a year ago

It's failing because Comparable[] arr = new String[0] works fine but String[] arr = new Comparable[0] doesn't. You're trying to cast comparables to strings (which is your actual key type).

|

Collectives™ on Stack Overflow

Object array not getting cast to a generic type

1 Answer 1

7 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

7 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related