Is it legal to make a type parameter recursive for an already generic class?

Question

If I have a class:

class Foo<T> {}

Is it legal / does it make sense to do this?

class Foo<T, R extends Foo<T, R>> {}

For context, I am trying to implement this pattern for a Builder that already has one generic parameter.

The second declaration is valid, but changing from the first to second is not a source-compatible change. — Brian Goetz, Dec 16 '18 at 22:46

score 1 · Answer 1 · answered Nov 07 '18 at 13:02

1

Yes, it is legal to have recursive generic types. The canonical example is

package java.lang;
public abstract class Enum<E extends Enum<E>>

from the JRE.

answered Nov 07 '18 at 13:02

memo

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My question is, what if the class has a generic parameter that it's not recursive on? – Aidenhjj Nov 07 '18 at 13:03
It shouldn't matter. And your code compiles just fine. Now if it makes sense, that I can't tell right away – memo Nov 07 '18 at 13:04

Ondřej Fischer · Accepted Answer · 2018-12-16T23:01:13.590

I`m not sure, if I'm properly getting your additional question in your comment. But let's try to explain.

The class with what you call "recursive parameter" is definitely legal and it makes sense.

It's used in "extendable" builders. Let's see following example:

public interface BaseBuilder<T, B extends BaseBuilder<T,B> {
    B add(String field, String value);

    default B person(String firstName, String lastName) {
        // This chaining is only possible thanks to that recursive parameter
        return add("firstName", firstName).add("lastName", lastName);
    }

    T build();

    default T buildPerson(String firstName, String lastName) {
        return person(firstName, lastName).build();
    }
}

You re-use this e.g. like this:

 public interface MyBuilder<T> extends BaseBuilder<T, MyBuilder<T> {}

Is it legal to make a type parameter recursive for an already generic class?

2 Answers2