Java Generics restrict to interface

generics, interface, java

Solution

It's impossible to impose such a compile-time restriction. Generic type parameters are stand-ins for reference types; they make no distinction between class types and interface types. The fact that additional bounds in a type parameter's declaration must be interface types is merely incidental - your strategy to leverage this as a means to impute a type as an interface was clever, but it's defeated by the limitation that type parameters can't be used in multiple bounds.

Your only options are to settle for a runtime check using `Class.isInterface()` like Louis Wasserman pointed out, or to leave it up to the caller to be responsible with what it passes in. Either way, make sure to clearly document the method's expectations and behavior.

`B` is intended to be a placeholder for a wildcard, so that the client can get a single object that is both a `SomeClass` and an `A` without having to do casting based on trust. The client will not have access to the name of the actual class that implements `SomeClass` and `A`

This seems like a contradiction to me. There's no point to declaring `B` if the caller can't possibly know what it evaluates to. Remember: the caller of a generic method provides its type arguments. So a caller deciding `B` without anything to base it on can only be guessing - and that can never be type-safe.

It seems like what you really want your method to return is some type that is both a `SomeClass` and an `A`, but this is tricky because they don't share a common supertype:

public static <A> SomeClass&A makeSomeClass(A thing) {...}

(this is nonsensical syntax for demonstration purposes only)

As a workaround, consider alternative ways to represent both a `SomeClass` and some interface type. For example the candidate interfaces could have a common method for returning a `SomeClass`:

public interface IsSomeClass {
    SomeClass asSomeClass();
}

public interface Foo extends IsSomeClass { }

The implementation of `asSomeClass` would in fact just return `this`. Then you could do:

public static <A extends IsSomeClass> A makeSomeClass(Class<A> type) {...}

And the caller of that method would be able to use the returned object as either type:

final Foo foo = makeSomeClass(Foo.class);
final SomeClass someClass = foo.asSomeClass();

If the interfaces themselves can't be modified, then another option is to use a wrapper class and composition instead:

final class SomeClassWrapper<A> {

    private final SomeClass someClass;
    private final A a;

    //constructor and getters, etc.
}

And your method would return a wrapper instance instead, assigning the implementation instance to both `someClass` and `a`:

public static <A> SomeClassWrapper<A> makeSomeClass(Class<A> type) {...}

Problem

Original source

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