Returning a HashMap<K, M> from a method of a class typed<M>

843793

So, this is what I have. I have a class parameterized for a bean type of a collection it receives, and in a method of the class, I need (would like) to return a HashMap typed as <K, M> as in:

public class MyClass<M> {
   private Collection<M> beanList;

   public HashMap<K, M> getMap(  /* what do I do here */ );
}

I tried to cheat from looking at the Collections.sort method, knowing it uses the List type in its return type as in:

public static <T extends Comparable<? super T>> void sort(List<T> list)

So, I am trying:

public HashMap<K, M> getMap(Class<K> keyType)

But, it is telling me I need to create class K.

So... how does the Collections.sort method get away with it and I can't????

I suppose I could write the method as:

public HashMap<Object, M> getMap()

but became somewhat challenged by my first guess.

Any ideas... or just go with the HashMap with the untyped key?

EJP

You can do

public <K> HashMap<K, M> getMap(  /* what do I do here */ );

provided K is inferable from the arguments.

Otherwise you have to parameterise the class on <K, M>.

843793

Oh how very nice! Thank you. I, for some reason, cannot fully get my head around generics, though I do everything I can to utilize them to their fullest.

I didn't want to type the class because on each call of getMap, I might need a different type key, and didn't want to create singletons for my purpose.

Thanks again!

Joe

843793

ejp wrote:
provided K is inferable from the arguments.

Or from the assignment context:

Set<String> empty = Collections.emptySet();

Otherwise you have to parameterise the class on <K, M>.

You may also specifically provide K when you use the method if the context doesn't give the compiler enough information or if you want to override it.

Example:

Set<Object> singleton1 = Collections.singleton("Hello"); //error
Set<Object> singleton2 = Collections.<Object>singleton("Hello"); //fine

Edited by: endasil on 14-May-2010 4:13 PM