Custom functional interface as method argument

Anton :

I have the code below that does produce expected result. Could somebody explain what's happening behind the curtains? I don't understand how does the compiler/JVM knows that it's needed to invoke store(String str) on the Storer object or how does it define the doSomething(Storer s, String str) implementation.

Storer.java

public class Storer {
    private List<String> storer = new ArrayList<>();

    public void store(String str) {
        storer.add(str);
    }

    @Override
    public String toString() {
        return "Storer [storer=" + storer + "]";
    }

}

MyInterface.java

@FunctionalInterface
public interface MyInterface {
    public abstract void doSomething(Storer s, String str);
}

Executor.java

public class Executor {
    public void doStore(Storer storer, String s, MyInterface p) {
        p.doSomething(storer, s);
    }
}

TestFunctionalInterfaces.java

public class TestFunctionalInterfaces {
    public static void main(String[] args) {
        Storer storer = new Storer();

        Executor test = new Executor();
        test.doStore(storer, "I've got added", Storer::store);

        System.out.println(storer);
    }
}

An output is:

Storer [storer=[I've got added]]

Thanks in advance.

Mathias :

The method reference Store::storer is equivalent to the lambda (s, str) -> s.store(str). In general, given a functional interface that expects args a1, a2, a3, ..., a method reference of this type is equivalent to a lambda that calls a1.namedMethod(a2, a3, ...). See this answer.