Android-Java Reflection Access Private Method From Interface Implementing class

I want to know that how can I invoke a private method from a class implementing a interface. Example:-

interface foo{
    void method();
}

class myClass implements foo {
    @Override
    public void method(){}
    private void methodInternal(){}
} 
class someClass {
    foo instan = new myClass();
}

Note that i dont have access to myClass(I Can't Cast To myClass becasue it is private class) but only have access to interface foo. Then how do i access method methodInternal() throught reflection. I Have This

foo service = someClass.instan;
// foo is actually a myClass then how do i access methodInternal

I wanna invoke method methodInternal after this. how Can I Do That