can't explain NullPointerException

Question

In the following code, i have a method to get a Vector of persons with the same zodiac sign. persoane is a Vector<Persoana>. I keep getting a NullPointerException at the if condition (persoane is definetly not null). I am unable to see why. Any help would be greatly appreciated

public Vector<Persoana> cautaDupaZodie(String zodie)
{
    Vector<Persoana> rezultat= new Vector<Persoana>();

    for(int i=0; i<persoane.size(); i++)
    {
        if(persoane.get(i).getData().getZodie().equals(zodie)) //the exception occurs here
        {
            rezultat.add(persoane.get(i));
        }

    }

    return rezultat;
}

Answer 1

NullPointerException occurs, when you try to call a method on an Object that is null.

This means that one of the following returns null:

• get(i)
• getData()
• getZodie()

Add them one by one to find out what actually is causing your exception.

Answer 2

persoane.get(i).getData().getZodie()

break that down into several lines to see where the NPE occurs.

Also, consider using the for-each loop.

for (Persoana persoana: rezultat){
    ...
}

Answer 3

The getData() or getZodie() returns null.

Answer 4

You may add some "tracing" code:

public Vector<Persoana> cautaDupaZodie(String zodie)
{
    Vector<Persoana> rezultat= new Vector<Persoana>();

    for(int i=0; i<persoane.size(); i++)
    {
        System.err.println(persoane.get(i));
        System.err.println(persoane.get(i).getData());
        System.err.println(persoane.get(i).getData().getZodie());
        if(persoane.get(i).getData().getZodie().equals(zodie)) //the exception occurs here
        {
            rezultat.add(persoane.get(i));
        }

    }

    return rezultat;
}

Answer 5

It could be any of the following:

• persoane contains a null at any index
• Any of the elements of persoane returns null for getData()
• Any of the getData() results retruns null for getZodie()

To investigate further, you'd best break up that chain of methods and add a conditional breakpoint. Additionally, rethink your design - this kind of "method operating on deeply nested, behaviourless data structure" is bad for this and other reasons.

Answer 6

First of all, you should use List<Person> if possible. Then, I recommend breaking the method chain to a smaller steps to see which exact step is failing.

public List<Person> searchZodiac(String zodiac) {
    assert zodiac != null; // if it fails here, zodiac == null

    List<Person> result = new ArrayList<Person>();

    for (Person p : persons) {
        Data d = p.getData();      // if it fails here, p == null
        String z = d.getZodiac();  // if it fails here, d == null
        if (z.equals(zodiac)) {    // if it fails here, z == null
           result.add(p);
        }
    }

    return result;
}

Answer 7

if(persoane.get(i).getData().getZodie().equals(zodie))

break the above line into several parts.Here getData must be returning null so you are getting NullPointerException