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??

Answer 1

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()) );