'public' class exposes its 'public/*package*/' superType xxxClass

Question

In Java, here is the code

abstract class A {
    Code code;

    // constructor
    A (Code code) {
    ...
    }
}

In kotlin, I wanna inherit it

class B(code: Code): A(code) {
...
}

And the compiler give me this error:

 'public' class exposes its 'public/*package*/' superType xxxClass

Can anybody help me find out why this happen?