NullPointerException when adding object in a Collection

Question

Hello,
I have this code:
----code----
Group group = new Group("test");
Collection col = null;
col.add(group);
----code----
The last line produces a NullPointerException.
But group is a valid object.
I don't understand why.
Could you help me?
Thanks
Sylvain

Answer 1

As a side note,
If you are fairly new to Java, then its worth reading about the Collections API as you'll find yourself using it a lot. Its also a create example of how powerful interfaces are.
In your code, try and make the compile time object a Collection and always pass the Collection interface around. (i.e. make the return type and parameter type of your methods Collection). This should let you do all the stuff you need e.g. getting objects out, putting them in and getting an iterator.
Only tie the Colection to a particular implementation at Runtime, e.g.
Collection c = new ArrayList();
is a lot better than
ArrayList c = new ArrayList();
That way, if you decide later that a HashSet() is better than an ArrayList() for your purpose, you only need to change one line of code. HashSet still implements Collection so all your methods will still work.
Hurrah for maintainable code!
As you can tell, I like interfaces