How to get Constructor Method Reference using reflection in Java

Question

I have a library that I am using in a project. It has a class which can be instantiated as

A object = new A(new B(value), C::new, D::new);

None of the classes A, B, C, D are public, although the constructors are.

So, I want to use Reflection to achieve this. But, I can't figure out how to do method reference C::new, D::new using reflection.

Would appreciate any pointers on how I can do this.

any good reason why you are going against library design? – user902383 Jun 20 '17 at 17:12 — user902383, Jun 20 '17 at 17:12
`C.class.newInstance()`? I'm not getting your question ... – Seelenvirtuose Jun 20 '17 at 17:14 — Seelenvirtuose, Jun 20 '17 at 17:14
So you expect us to guess the parameter types? – Holger Jun 21 '17 at 09:09 — Holger, Jun 21 '17 at 09:09

score 2 · Answer 1 · answered Jun 20 '17 at 17:18

The fact that a constructor reference is used is not important. You just need to know the functional interface type into which it is converted, and then create an instance of that.

In the case of a default constructor, this would probably be Supplier<...>. You'd get something like this:

Class<C> cKlass = ...;
Constructor<C> cCons = cKlass.getDeclaredConstructor();

Supplier<C> cSupp = () -> { // similarly for class D
        try {
            return cCons.newInstance();
        } catch (Exception e) {
            throw new RuntimeException("Can not default construct " + cKlass, e);
        }
};

...

Class<A> aKlass = ...;
Constructor<A> aCons = aKlass.getDeclaredConstructor(B.class, Supplier.class, Supplier.class);
A a = aCons.newInstance(b, cSupp, dSupp);

How to get Constructor Method Reference using reflection in Java

1 Answers1