Why does functional interface with void return-type method accept any return-type method? [duplicate]

Answer 1

I am not sure what exactly is confusing you so here is some other way how you can look at your example.

---

J j = Test::foo;

can be rewritten as

J j = () -> Test.foo();

since it provides body to m() method from J functional interface and that method doesn't require any arguments (which is why it starts with () ->).

But that can be seen as shorter version of

J j = new J(){
    public void m(){
        Test.foo(); //correct despite `foo` returning value    
    }
};

which is correct, because Java allows us to ignore returned value of called method like in case of List#add(E element) which returns boolean value, but we still are allowed to write code like list.add(1) without having to handle returned value.

Answer 2

Although interface J declares it has a void function, Test::foo returns an Object.

It's inaccurate to say that Test::foo returns something. In different contexts, it could mean different things.

Supplier<Object>  a = Test::foo;
J                 b = Test::foo;
Runnable          c = Test::foo;

It's more accurate to say that Test::foo can represent a target type the functional method of which returns either void or Object.

It's an example of expression statement (jls-14.8).

If the target type's function type has a void return, then the lambda body is either a statement expression (§14.8) or a void-compatible block (§15.27.2).
...
An expression statement is executed by evaluating the expression; if the expression has a value, the value is discarded.