Is it okay for a class to have a field of its own type

Question

I did the following, i got a stackoverflow error after some time, and I understand why it was.

public class Cat {
    String name="default name";
    Cat insideCat;

    public Cat(){
         System.out.println("Cat constructor");
         insideCat = new Cat();
    }

}

But what if I don't create a new Cat object within the constructor, but take a parameter of Cat type and assign it to the insideCat field.

public class Cat {
    String name="default name";
    Cat insideCat;

    public Cat(Cat insideCat){
         System.out.println("Cat constructor");
         this.insideCat = insideCat;
    }

}

I am just playing around with the code, just trying to find out what Java can do and cannot. In the second code, everything looked normal, until I started to test this class. I need a Cat object to create a Cat object (and to create this Cat object I need another Cat object...and it goes on). So technically I cannot test this class.

So my question is WHY does java allow to create an instance variable of its own type? I guess the whole purpose of a constructor is to initialize it's instance variables. So either I have to create a new object to initialize the insideCat or else I have to take Cat object from outside. Both doesn't seem to work.

What am I missing here. Is there any occurrence where instance variables of its own types can become useful, and can be used without any problem? Is it bad OOP practice to come up with classes like this?

Answer 1

Classes like this exist all the time.

Consider linked lists or trees, e.g.,

class ListNode {
  ListNode next;
  // Etc.
}

class TreeNode {
  TreeNode left;
  TreeNode right;
  // Etc.
}

You wouldn't initialize the "child" objects in the constructor, you'd add them later.

In your example you'd need to have a method that created the insideCat at a later time. In general you wouldn't create child objects that had the exact same state, there'd be something to differentiate them either at construction time, in which case you could have a "oh god stop creating these now" condition, or while they were being added, e.g., you'd add them via a method and not in a constructor.