Java Singleton.getInstance() returns null?
autoboxing, java, singleton
Solution
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.
Problem
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) ```