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?

It's Object, essentially. There's no other common supertype between A and B. — ernest_k
– ernest_k, Commented Jul 13, 2021 at 8:54
Perhaps you are looking for an explanation of IntersectionType? — Hulk
– Hulk, Commented Jul 13, 2021 at 8:56
@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. — marstran
– marstran, Commented Jul 13, 2021 at 9:00

marstran · Accepted Answer · 2021-07-13 09:16:21Z

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$$Lambda$1/1642360923@1376c05c

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