using reflection to set up inner class member

I have 2 class (i've simplified my sample, its not really my classes)

class Father {
    private Child child;

    public Father() {
          child = new Child();
    }
}

class Child {
    private int age;
}

In order to make unit tests, i need to set the value of age, ONLY from Father class, using reflection.

Basically, i have a problem setting the final value in age, and its generating an exception.

1st, i'm retrieving Father class, and then from it, taking child field, which works well. My problem is calling ageField.set(ageField, 15); since ageField is a field and not the actual age member.

Field field = Father.class.getDeclaredField("child");
field.setAccessible(true);
Object value = field.get("child");

Child child = (Child) value;
Field ageField = value.getClass().getDeclaredField("age");
ageField.setAccessible(true);
ageField.set(ageField, 15);

Fyi, i've looked at the following links, so this is not a duplicate.

• reflection1
• reflection2
• reflection3