This line: is equivalent to this line: Declaring an object reference does not give it an object to reference. You must create and assign one to the field. The List class is an interface and cannot be directly instantiated; however, there are multiple classes offered in the java package that implement it with different behaviors. The most commonly used one is probably the ArrayList class, which creates a basic list. The following should work: You will need to import 'java.util.ArrayList' as well as 'java.util.List'.

The code that you have to add an object into a list: ... implies a misunderstanding about how lists and objects in general work. A List is an object like any other. If you have a List variable, and it has the value 'null', then it does not refer to a list. If it does not refer to a list, you cannot call the 'add' method (or any other method) on it. Indeed the list is still null, because you specifically allow that in the 'if' statement. If you take the second part of the condition out from your if statement you have effectively got this: This could never work. It effectively says: "if I have a null reference, call a method on that null reference". You can never call a method on a null reference, for a list or for any other type. Danpost has shown you how to properly instantiate a list in his post, but I wanted to elaborate on the issues in your code because you do seem quite confused. I hope this helped.