Ndtyjky

so I've been wracking my brains over why this isn't working for an hour now. I have two classes, class One containing an ArrayList of ArrayLists and a method to add another element to that list, and class Two, from which I'm trying to access that method.

 public class One

{

    private ArrayList<Element> myarraylist;



    public One()

    {

        myarraylist = new ArrayList<Element>();

    }



    public void addElement(String name)

    {

        myarraylist.add(new Element(name));

    }

}



//Element being another class











public class Two

{

    One database;



    public static void main(String args)

    {

        Two two = new Two();

        two.startMenu();

    }



    public Two()

    {

        One database = new One();

    }



    public void addElem()

    {

        Scanner keyboard = new Scanner(System.in);

        String name = keyboard.next();

        database.addElement(name);

    }

}



//where startMenu is just a small multiple choice menu thingy

Problem is, when I try to run through it and I get to the last line, I get the message: java.lang.NullPointerException

I tried inspecting the objects (I use BlueJ), and the ArrayList is initialized when I just make an instance of class One, but when I make an instance of class Two, the database instance is null.

Thanks in advance for your answers :D

The problem is at the following lines

public class Two

{

     One database;



     public Two()

     {

        One database = new One(); 

        //this variable is not the same as the one declared outside the constructor

     }

When declaring a variable in a method which has the same name as the variable declared in the class, the modifications which happen on the variable declared inside the method will not be seen in the variable outside the method(because the 2 variables are different). The variable in the method is shadowing the variable from the class.
To differentiate between the two variables you must use the this keyword

this.database = new One();

The final solution should be

public class Two

{

     One database;



     public Two()

     {

        this.database = new One(); 

     }

The reason you get the NullPointerException when doing database.addElement(name); is because in your example the database is not instatiated beacause the object created is stored in a different variable named database and not the one declared as a class attribute.