Null Pointer Exception upon referencing variable

Question

I am attempting to reference a variable in a method in my class and keep running into a NullPointerException. I know it is happening at the variable pbook when it is referenced from the addPerson method. Why is this happening and how could I go about fixing it?

public class Phonebook <T> {

  private LinkedList<T> pbook;

  public T findPerson(T person) {
    for (int i = 0; i < pbook.size(); i++) {
      if (pbook.get(i).equals(person)) {
        return person;
      }
      else
      {
        i++;
      }
    }
    return null;
  }

  public void addPerson(T person) {
    pbook.addFirst(person);
  }

  public void deletePerson(T person) {
    for (int i = 0; i < pbook.size(); i++) {
      if (pbook.get(i).equals(person)) {
        pbook.remove(i);
      }
      else
      {
        i++;
      }
    }
  }


  /**
   * @param args
   */
  public static void main(String[] args){

    try{
     Phonebook<Integer> sspb = new Phonebook<Integer>();
     Phonebook<String> idpb = new Phonebook<String>();
     sspb.addPerson(1234567890);
     idpb.addPerson("Bob");

    }
    catch (Exception e){
      e.printStackTrace();
    }
  }
}

Note that while in some languages LinkedList<T> pbook; does create a new object, in Java it does not. — Radiodef
– Radiodef, Commented Jan 28, 2014 at 22:03

Radiodef · Accepted Answer · 2014-01-28 22:00:44Z

4

You must add a constructor to instantiate your LinkedList:

public Phonebook() {
    pbook = new LinkedList<T>();
}

edited Jan 28, 2014 at 22:00

Radiodef

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answered Jan 28, 2014 at 21:58

Happy

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Comments

barak manos · Accepted Answer · 2014-01-28 21:58:22Z

3

Change:

private LinkedList<T> pbook;

To:

private LinkedList<T> pbook = new LinkedList<T>();

answered Jan 28, 2014 at 21:58

barak manos

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Comments

Salih Erikci · Accepted Answer · 2014-01-28 21:59:11Z

1

private LinkedList<T> pbook; You don't create a list.

Try this.

private LinkedList<T> pbook = new LinkedList<T>()

answered Jan 28, 2014 at 21:59

Salih Erikci

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Comments

peter.petrov · Accepted Answer · 2014-01-28 22:05:21Z

1

1) You can define a constructor e.g. like this.

public Phonebook(LinkedList<T> pbook){
     this.pbook = pbook;
}

Then the calling code will have to set the
pbook when instantiating the Phonebook.

2) You can initialize pbook where you declare it.

private LinkedList<T> pbook = new LinkedList<T>();

edited Jan 28, 2014 at 22:05

answered Jan 28, 2014 at 21:59

peter.petrov

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