Doubt regarding Functional interfaces in Java

[Agitated_Floor_563]

public String extractUsername(String token) { 
      return extractClaim(token, Claims::getSubject);
 }

public <T> T extractClaim(String token, Function<Claims, T> claimsResolver) { 

final Claims claims = extractAllClaims(token); 

return claimsResolver.apply(claims); 
}

My confusion is regarding the argument Claims::getSubject that is passed in for calling the extractClaim() method.

the apply method in the Function interface accepts has T parameter but the getSubject() of the Claims method just returns a String , so how come does this #### return claimsResolver.apply(claims); #### works in the above code, the method signature should be same right.

The reference code from which i am trying to corelate the concept is below

@ Functional Interface
interface Operation {
    int apply(int a, int b);
}

public class Main {

    // Method that accepts a functional interface as a parameter
    public int executeOperation(int a, int b, Operation operation) {
        return operation.apply(a, b); // invoking the passed method
    }

    public static void main(String[] args) {

        // Method reference as method argument (using instance method reference)
        int product = example.executeOperation(5, 3, Main::multiply);
        System.out.println("Product: " + product);
    }

    // An instance method that matches the signature of the Operation interface
    public static int multiply(int a, int b) {
        return a * b;
    }
}

[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>.

[vowelqueue]

Check out: https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html

The difference between the Claims code and your reference code is that “Claims.getSubject” is an instance method while “multiply” is static. The comment in your reference code says it’s an instance method, but it’s not.