Java generic function: how to return Generic type

Question

Here's a Java generic pattern:

public <T> T getResultData(Class<T> resultClass, other_args) { 
   ...
   return resultClass.cast(T-thing);
}

A typical call looks like:

   DoubleBuffer buffer;
   buffer = thing.getResultData(DoubleBuffer.class, args);

I've never been able to figure out how to use this pattern cleanly when the desired return type is, itself, generic. To be 'concrete', what if a function like this wants to return Map<String,String>? Since you can't get a class object for a generic, of course, the only option would be to pass Map.class, and then you need a cast and an @SuppressWarning after all.

One ends up with a call like:

Map<String, String> returnedMap;
returnedMap = thing.getResultData(Map.class, some_other_args);

Now one is back to needing casts and suppressing a warning.

I suppose that one could take something from the java.lang.reflect.Type family instead of the Class, but those aren't especially easy to concoct. That looks like:

class Dummy {
 Map<String, String> field;
}

...

Type typeObject = Dummy.class.getField("field").getGenericType();

Given this, the called function could extract the base type and use that for casting or newInstance-ing, but the dummy field business sure looks ugly.

Note that functions like this are not always calling newInstance. Obviously, if they do, they don't need to call resultClass.cast.

Answer 1

You cannot do that in the standard Java API. However, there are utility classes available which can get the Type out of a generic class. In for example Google Gson (which converts JSON to fullworthy Javabeans and vice versa) there's a TypeToken class. You can use it as follows:

List<Person> persons = new Gson().fromJson(json, new TypeToken<List<Person>>() {}.getType());

Such a construct is exactly what you need. You can find here the source sode, you may find it useful as well. It only requires an additional getResultData() method which can accept a Type.