What is difference between null and empty list?

Question

List> pcList = null;
Map pcMap = new HashMap();
ComputerConfigurations tempPC = null;

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Answer 1

In Java, collections won't magically spring into existence just by adding something to them. You have to initialize pcList by creating an empty collection first:

List> pcList = new ArrayList<>();

An empty collection isn't the same as null. An empty collection is actually a collection, but there aren't any elements in it yet. null means no collection exists at all.

Note that an object can't be of type List, because that's an interface; therefore, you have to tell Java what kind of List you really want (such as an ArrayList, as I've shown above, or a LinkedList, or some other class that implements List).