Java generics: get class of generic method's return type

Question

Background

I once wrote this method:

private  SortedSet createSortedSet() {
  return new TreeSet();
}

Answer 1

This doesn't work with all JVM!

You can first create a generic object and then retrieve its parameterized type:

private <T> T evaluate(String expression) {
  List<T> dummy = new ArrayList<>(0);
  Type[] actualTypeArguments = ((ParameterizedType) dummy.getClass().getGenericSuperclass()).getActualTypeArguments();
  Type clazz = actualTypeArguments[0];
  Class<T> theClass = (Class<T>) clazz.getClass();

  ExpressionEvaluator evaluator = pageContext.getExpressionEvaluator();
  return evaluator.evaluate(expression, theClass, null, pageContext.getVariableResolver());
}

Beware! You don't get a Class<> object, you get a TypeVariableImpl subclass object, which may behave differently.

Answer 2

Unfortunately, you will certainly have to change your method to:

private <T> T evaluate(Class<T> clazz, String expression)

and then pass clazz as your second parameter. Not as short as you expected.