java design: repetitive methods of sub-objects

Question

When I was working on a project of mine and had to use a lot of nested objects, I became very unsure of my design of structure in Java when I wanted to set an instance variable of a deeply nested object from the upper object. It feels like I miss a basic understanding perhaps of structured design in Java. Any insight in this is appreciated.

Imagine I have an object which has a logical structure of having nested sub objects as variables. For example:

Country
  Capital
     Palace
        King

If I now want to set a private String variable 'nameOfKing' in King from the main object (Country), I would have to define every method in all upper classes of King. So something like this:

public class Country {
   private Capital capital;

   public void setNameOfKing(String n) {
      capital.setNameOfKing(n);
   }
}

public class Capital{
   private Palace palace;

   public void setNameOfKing(String n) {
      palace.setNameOfKing(n);
   }
}

public class Palace{
   private King king;

   public void setNameOfKing(String n) {
      king.setNameOfKing(n);
   }
}

public class King{
   private String nameOfKing;

   public void setNameOfKing(String n) {
      this.nameOfKing = n;
   }
}

so that I can call country.setNameOfKing(n);. Now this is fine if you have just a few variables, but what if the classes King, Palace, Capital each have many other variables and methods defined and all of them have to be called from Country. What if Palace also contains other class objects like (just making stuff up now) ThroneRoom, Treasury, Queen, etc... It would mean that for almost all methods in the sub classes, one has to exist in the Country class, which could potentially mean you have to create a huge amount of methods simply for passing info to the sub objects resulting in a very large Country class.

Now I suppose you could technically say that King could also be defined in the Country itself, doing away with the extra methods in Palace and Capital, but it would still mean the methods have to exists in the Country class and what if this structure makes sense as in logic grouped together, what would be the best approach for this? I'm not sure there is a better way, but I can't shake the feeling I'm missing something here.

So you mean changing the methid in Country class like this? `capital.getPalace().setNameOfKing(n);` It would change the fact that you could potentially have a great amount of methods in the Country class — lokipoki, Feb 23 '17 at 20:18

Guillaume F. · Accepted Answer · 2017-02-23T20:31:28.877

This is a responsibility issue.

Ask yourself : "Who is responsible for knowing who the king is?". When you are a palace, are you responsible for the king? Obviously not. The king is not managed by your palace, but he is attached to your Country by law.

Querying palace.setNameOfKing() is similar to asking your house to give your newborn a name. Your house doesn't have this responbility, and should not be allowed to do it.

The correct way to do it is to retrieve the King instance, and set its name directly.

Now there is an issue with this approach : you may have to use many getters to find the King instance. But you can solve this issue with a functional approach of your Classes.

palace.getOwner() can return an interface Owner with King implements Owner.

The same rules can be applied to your other classes.

Yes, it is a responsibility issue and I'd go even a step further and not even allow setting the king's name from theoretically everywhere by supplying a public setter method. — Markus Benko, Feb 23 '17 at 20:54

java design: repetitive methods of sub-objects

1 Answers1