Why are implicit generics in classes not allowed in Java? [closed]

Question

I am trying to understand why Java does not allow using implicit types in generic classes as it does in generic methods.

I have scoured the webs for answers on this, but have yet to come across a reason as to why something like the following is not supported in Java:

// This will not compile:
public <T> interface ElementPicker<L extends List<T>> { ... }

// This on the other hand will
public interface ElementPicker<T, L extends List<T>> { ... }

And so we must explicitly mention the type T in the class generic arguments. This of course means that we must now always write:

ElementPicker<Integer, List<Integer>>
// instead of just:
ElementPicker<List<Integer>>

This leads to a constant head-ache in my code where I am attempting to balance using generics wisely, while making my types readable and short.

Unfortunately, in my current project I am dealing with a bunch of nested generic types, and their type arguments are bubbling to the top so that I have very long top-level classes that must include all generic type data.

To see how this can become a problem consider:

interface ScalarValue<T extends Number> {
  T toNumber();
}

interface ScalarAdder<T, U extends ScalarValue<T>> {
  T add(U v1, U v2);
}

class ScalarAccumulator<T, U extends ScalarValue<T>, 
                           A extends ScalarAdder<T, U>> {
  ...
}

// Assuming we have these implementations:
class RationalValue implements ScalarValue<Float> { .. }
class RationalAdder implements ScalarAdder<Float, RationalValue> { .. }

// A typical type could look like this monster:
ScalarAccumulator<Float, RationalValue, RationalAdder>

// Whereas an implicit declaration like this:
public <T extends Number, 
        U extends ScalarValue<T>,
        A extends ScalarAdder<T, U>
class ScalarAccumulator<A> { ... }

// ... would allow us to simply write
ScalarAccumulator<RationalAdder>
// ...as all other types can be inferred.

Again this is an example, but I encounter these kinds of things quite often in my work. And I have yet to find a reason for why this is not possible. Methods can this just fine (infer types from a single value and its class).

So why can Java support this in methods but not in classes? I cannot think of an example where it would be a problem for a class to not explicitly include the type. But I am probably missing something.

If anybody has any good tips on work-arounds to deal with such situations in general, I would also appreciate it.

Answer 1

In Java 10 you can use var to skip type declarations. Here's a (somewhat hacky) way to combine that with type inference so you can create an instance without declaring all the nested types:

static <T, U extends ScalarValue<T>, A extends ScalarAdder<T, U>> ScalarAccumulator<T, U, A> create(Class<A> adderClass) {
    return new ScalarAccumulator<>();
}

static void test() {
    var a = create(RationalAdder.class);
}