can't explain NullPointerException

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
        {
   开发者_如何转开发         rezultat.add(persoane.get(i));
        }

    }

    return rezultat;
}

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.

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){
    ...
}

The getData() or getZodie() returns null.

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;
}

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.

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

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;
}