Concept of functional interface

Question

When I'm shooting a glance at lambda expressions, the book touches on a functional interface that has only one abstract method. My issue addresses on that quiz question

/* Which of these interfaces are functional interfaces? */
public interface Adder{
   int add(int a, int b);
}
public interface SmartAdder extends Adder{
   int add(double a, double b);
}
public interface Nothing{
}

I know the last one is not, but I think the first and second ones should be functional interface. But the book says second one is not. Why? Doesn't it overrides add method? So even in second, isn't there only one abstract method?

"Doesn't it overrides add method?" - no. A method has to have the same signature as another method to override it. — user2357112, Jun 07 '17 at 23:51
If you annotate an interface with `@FunctionalInterface`, the compiler will tell you if it's a functional interface or not. — Radiodef, Jun 08 '17 at 00:20
Yup. You are right. It is absolutely my fail to notice that. @user2357112 — Soner from The Ottoman Empire, Jun 08 '17 at 00:20

dimo414 · Accepted Answer · 2017-06-08T06:03:13.007

An easy way to find out would be to try to define a class that implements SmartAdder. The compiler will tell you you need to implement both add(int, int) and add(double, double).

It's understandable that you thought add(double, double) would override add(int, int), but they are in fact separate methods, and could potentially have totally unrelated implementations.

If SmartAdder had defined a default implementation of add(int, int) it would be a functional interface still:

public interface SmartAdder extends Adder {
   int add(double a, double b);

   default int add(int a, int b) {
     return add((double)a, (double)b); // this calls the double method instead
  }
}

You may also have come across the @FunctionalInterface annotation - this can be placed on an interface to enforce at compile-time that the interface has exactly one abstract method. If SmartAdder was annotated with @FunctionalInterface the interface itself would not compile.

Thank you and all answerers +1. It is absolutely my fail to notice that. — Soner from The Ottoman Empire, Jun 08 '17 at 00:19

score 2 · Answer 2 · answered Jun 07 '17 at 23:56

2

SmartAdder has two methods. The method signatures are different. Functional Interface can have only one method.

answered Jun 07 '17 at 23:56

cosmos

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fps · Answer 3 · 2017-06-08T00:25:43.217

In Java, a method in a subtype overrides a method of a parent type when it has the same signature. Signature means both name and arguments of the method. In particular, arguments must be of the same exact type and must be declared in the same exact order in both methods, i.e. the types of the arguments of the method declared in the subtype can't be subtypes or types wider than the types of the arguments declared in the parent type's method.

So, in your SmartAdder interface, the method with signature add(double a, double b) does not override the method add(int a, int b) of your Adder interface, because double is wider than int. When a type has two or more methods with the same name but with different arguments, it is called method overloading, and it's totally different than method overriding.

This is why SmartAdder ends up having two abstract methods, hence it's not a functional interface (which requires the type to have only one abstract method).

score 1 · Answer 4 · answered Jun 07 '17 at 23:56

interface SmartAdder overloads, not overrides interface Adder. That's because the name is the same name, but parameters types are different. Therefore, it has 2 functions. To be a functional interface it needs to have only one function.

==> Only interface Adder is a functional interface.

Concept of functional interface

4 Answers4

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