Recursive toString()

Question

I have a doubleLinkedList and need to implement a recursive toString() method without parameters. Here is what I have, and it throws a NullPointerException

//Print forwards
public String recursiveToString(){
  return first.rTS();
}

//Print backwards
public String recursiveBackwardsString(){
  return last.rBS();
}


    //Recursive helper in the node class
    public String rBS(){
      if (this!=null)
        return info+" "+back.rBS();
      return "";
    }

    //Recursive helper in the node class
    public String rTS(){
      if (this!=null)
        return info+" "+next.rTS();
      return "";
    }

Where are `first`, `last`, `back`, and `next` defined? One of them is `null`. — Mike B, May 05 '14 at 18:44
possible duplicate of [What is a Null Pointer Exception, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it) — Kevin Panko, May 05 '14 at 21:14

score 1 · Answer 1 · answered May 05 '14 at 18:46

1

this won't be null in any code, if some method in null is called, the NullPointerException is thrown before the body of method is reached, so, the protection from null here is incorrect:

if (this != null)
    return ...

You should write it in this way:

public String rBS(){
    if (back != null)
        return info+" "+back.rBS();
    return info;
}

and the same thing for rTS.

answered May 05 '14 at 18:46

Dmitry Ginzburg

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if back is null this will throw a NullPointerException, the check should be if(back != null) – Martin May 05 '14 at 18:47
@Martin Oh, that's exactly what I mean to write, thanks! – Dmitry Ginzburg May 05 '14 at 18:48

Recursive toString()

1 Answers1