Question

Whenever I run this method the print goes perfectly fine so the argument is passed and not null. Still it gives a NullPointerException when the argument is added to the children ArrayList (which is a part of the class). Why is this?

public void addChild(_Node n){
    System.out.println("Add " + n.getClass().getSimpleName() + " to " + this.getClass().getSimpleName());
    children.add(n);
}

Answer 1

Check to see if children has been instantiated correctly. This is a common mistake beginners make.