Java Singleton.getInstance() returns null?

This is not about your singleton pattern which looks fine to me. It is the Integer.getInteger(Singleton.K_LEVEL); method that is returning null. I bet the "level" system property has not been set and is null.

java A -Dlevel=1

You need to put the -Dlevel=1 before the A class on the command-line. If you debug your code or print out the system property you will see that it is null.

java -Dlevel=1 A

You get a NPE when you try to pass the null into addToLevel(int x) and it tries to auto-unbox the null to be int x.

As an aside, if this class is used by multiple threads, you should consider using an AtomicInteger inside of your Singleton class which is reentrant.

There is nothing wrong with your Singleton. There are no concurrency problems because this is not multithreaded code.

You were thinking s was null, but it is really i that was null.

Since addToLevel takes an int as a parameter, the Integer i was autounboxed (implicitly converted from Integer to int), but since i was null, NullPointerException was thrown. Autounboxing throws NullPointerException when the value being coverted is null.

The reason Integer.getInteger(Singleton.K_LEVEL) returned null is because you did java A -Dlevel=1 as opposed to java -Dlevel=1 A. The latter is the correct syntax.

java -Dlevel=1 A should suit your needs.

From the doc, the syntax is java [ options ] class [ argument ... ], and -Dlevel=1 is considered as an option (see the options section).