java - public interface - private menthods

.... warning - I'm more familiar with C++ but I'm learning Java for new
project. I'll try hard not to troll. ...

Say I have a class that exposes some interfaces. The intent is that
methods may be overridden by clients but only the class that defines the
interfaces are allowed to call them.

In C++ you would do it like so:

class X
{
   public:
   class Y
   {
     friend class X;
     private:
     virtual void = 0;
   };

   void YConsumer( Y & y )
   {
     y.FuncOnlyForX();
   }
};

class YClient : public X::Y
{
    public:
    void FuncOnlyForX() // override X::Y
    {
       std::cout << "THIS IS YClient\n";
    }
};

int main()
{
    X x;
    X::Y & y = YClient();

    y.FuncOnlyForX(); // error

    x.YConsumer( y ); // OK
}

I tried this in Java but the compiler keeps on getting in the way:

public class X
{
    public interface Y
    {
        private void FuncOnlyForX(); // javac saya illegal use of private
    }

    public void YConsumer( Y y )
    {
       y.FuncOnlyForX();
    }
}

public class Yclient extends X.Y
{
    void FuncOnlyForX()
    {
    }
}

So, how do you get java to enforce use visibility policy on interfaces ?