Java Singleton.getInstance() returns null?

Question

I have this singleton I'm trying to use, but getInstance can apparently return null:

class Singleton {
    public static final String K_LEVEL = "level";
    static Singleton instance = new Singleton();
    private int level;

    static Singleton getInstance() {
        return instance;
    }

    int getLevel() {
        return level;
    }

    void incrementLevel() {
        System.out.println("LEVEL INCREASED TO " + ++level);
    }

    void addToLevel(int x) {
        for(int i=0;i<x;i++)
            incrementLevel();
    }

}

class A {
    public static void main(String[] args) {
        Singleton s = Singleton.getInstance();
        Integer i = Integer.getInteger(Singleton.K_LEVEL);
        s.addToLevel(i);
    }
}

I heard implementing singletons in Java is very hard and prone to race conditions. Is my singleton pattern implemented wrong? I recently changed my code to look like this, and now getInstance returns null sometimes. Why?

$ java A -Dlevel=1
Exception in thread "main" java.lang.NullPointerException
    at A.main(A.java:29)

Answer 1

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.