Why should I care that Java doesn't have reified generics?

Question

This came up as a question I asked in an interview recently as something the candidate wished to see added to the Java language. It\'s commonly-identified as a pain that Jav

Answer 1

My exposure to Java Geneircs is quite limited, and apart from the points other answers have already mentioned there is a scenario explained in the book Java Generics and Collections, by Maurice Naftalin and Philip Walder, where the reified generics are useful.

Since the types are not reifiable, it is not possible to have Parameterized exceptions.

For example the declaration of below form is not valid.

class ParametericException extends Exception // compile error

This is because the catch clause checks whether the thrown exception matches a given type. This check is same as the check performed by instance test and since the type is not reifiable the above form of statement is invalid.

If the above code was valid then exception handling in the below manner would have been possible:

try {
     throw new ParametericException(42);
} catch (ParametericException e) { // compile error
  ...
}

The book also mentions that if Java generics are defined similar to the way C++ templates are defined (expansion) it may lead to more efficient implementation as this offers more opportunities for optimization. But doesn't offer any explanation more than this, so any explanation (pointers) from the knowledgeable folks would be helpful.