Recursive Class throws NullPointerException

[freeBatjko]

Hi all,

I literally just started with Java and I am experimenting with OOP, but this puzzles me:

I have something that resembles the following (plus getters, setters etc):

public class person {

public String name;
public person father;
public ArrayList<person> children;

}

Now I wanted to create relationships:

person p1 = new person();
p1.setName("Lucy");  

person p2 = new person();
p2.setName("Fred");

[B]p1.father= p2;
p2.children.add(p1);[/B]

However, here is where I get the NullPointerExceptions.
It doesn't seem to work this way, and I just assume I am not yet in the right mind set for OOP to understand what's wrong here.

btw, using a setter for the first bold line:

p1.setFather(p2);

doesn't work either. same exception.


Can anyone enlighten me, please?

[RamyaSivakanth]

Could u paste the complete code with code tag?

[r035198x]

public ArrayList<person> children;

does not create a list of person. It only creates a reference that is automatically initialized to null. Trying to add anything to it will throw the N.P.E
You need to do

ArrayList<person> children = new ArrayList<person>();

to initialize the list of children before you try to manipulate it.

[freeBatjko]

public ArrayList<person> children;

does not create a list of person. It only creates a reference that is automatically initialized to null. Trying to add anything to it will throw the N.P.E
You need to do

ArrayList<person> children = new ArrayList<person>();

to initialize the list of children before you try to manipulate it.

ah of course! makes sense.

can i use this as a field in my class though?

[hardwired]

can i use this as a field in my class though?
Yes, you can declare the variable and later initialize it in a constructor:

public class Person {
    public String name;
    public person father;
    public ArrayList<Person> children;

    public Person() {
        children = new ArrayList<Person>();
    }
}

or you can declare and initialize it in one step:

public class Person {
    public String name;
    public person father;
    public ArrayList<Person> children = new ArrayList<Person>();

[freeBatjko]

Great. Exactly what I needed.
Many thanks, guys. Much appreciated!