Java interface &

Question

I can't understand what is it??

interface A{
 void foo();
}
interface B{
 void foo();
}

then what is the result of (A & B)

 System.out.println((A & B) () -> {
 System.out.println("1");
 System.out.println("2");
 });

Could someone help me?

• ernest_k: It's Object, essentially. There's no other common supertype between A and B.
• Hulk: Perhaps you are looking for an explanation of IntersectionType?
• Hulk: See also stackoverflow.com/questions/60148411/…
• marstran: @FedericoklezCulloca It's not a syntax error. The type A & B is an intersection type that also happens to be a functional "interface" because A and B's foo-method has the same signature. The System.out.println will just print the toString of the lambda-object that is created.
• Federico klez Culloca: @marstran didn't know that. Thanks for clarifying.

Answer 1

A & B is an intersection type. An object of this type can act as both an A and as a B.

This particular intersection type also happens to be a "functional interface" type, which means that it only has 1 abstract method. This lets you instantiate an object of this type using a lambda-expression. A & B is a "functional interface" type because both A and B have the foo-method with the exact same type signature.

So that is what this code means. You are instantiating an object of type A & B, implementing the foo-method with the lambda-function.

(A & B) () -> {
 System.out.println("1");
 System.out.println("2");
}

The System.out.println will just print out this lambda object using it's toString-method. Note that it will not call the actual foo-method. Your code will therefore just print out something similar to this: Main$$Lambda1ドル/1642360923@1376c05c