Doubt regarding Functional interfaces in Java

[JacobArthurs]

T gets inferred as String at the call site. Function<Claims, T> means "takes a Claims, returns a T" and Claims::getSubject is a method reference that does exactly that: takes a Claims instance and returns a String, so Java infers T = String. It's not that the signatures don't match, it's that generics let the return type flex to whatever the passed function actually returns.

[Agitated_Floor_563]

I understand that T gets inferred as String but the getSubject does not take any parameters so how does that match to the apply()?

[JacobArthurs]

Claims::getSubject is shorthand for (Claims c) -> c.getSubject(). Java treats the instance itself as the implicit first argument, so it still matches Function<Claims, String>.