LambdaExpressions.java

package pt.goncalo.freeroam.lambda;

public class LambdaExpressions {
   
    interface Payback<T>{
        T apply(T origin, T rate);
    }

    static void calculate(Double origin, Double rate, Payback<Double> func){
        double payback = func.apply(origin,rate);
        System.out.println(payback);
    }
    public static void methodReferenceDoubt(){
        double origin = 1000;
        double rate = 0.1;
        calculate(origin, rate, new Payback<Double>() {
            @Override
            public Double apply(Double origin, Double rate) {
                return origin * (1+rate);
            }
        }::apply);
    }

   
    public static void main(String [] args){

        new LambdaExpressions().methodReferenceDoubt();

    }
}

Why do we need the ::apply in line 18