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)

What does System.out.println(i); right before s.addToLevel(i); print? — jlordo
– jlordo, Commented Mar 26, 2013 at 21:53

Harold R. Eason · Accepted Answer · 2013-03-27 19:23:54Z

3

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.

answered Mar 27, 2013 at 19:23

Harold R. Eason

5632 silver badges9 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Gray · Accepted Answer · 2019-11-26 19:52:12Z

3

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.

edited Nov 26, 2019 at 19:52

answered Mar 26, 2013 at 21:39

Gray

117k24 gold badges305 silver badges360 bronze badges

6 Comments

Dog Over a year ago

No, I set it by doing this: java A -Dlevel=1

Gray Over a year ago

The -D has to be before the A class @Dog. I'd debug your code or print out the system property to verify it.

Ankit Bansal Over a year ago

I agree. @Dog why don't you print i just before calling addToLevel to verify.

Dog Over a year ago

It sounds like you are going on from a speculation. Even if the getInteger was returning null, the cast should make it 0. I don't create other threads, but I don't know.. I thought perhaps the VM instantiates my singleton on another thread so that could by why I'm getting null for s?

Gray Over a year ago

Nope. That's not how auto-boxing works. You will and are getting a NPE because your system property is not set right @Dog.

|

sp00m · Accepted Answer · 2013-03-26 22:08:57Z

2

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).

answered Mar 26, 2013 at 22:08

sp00m

49k31 gold badges152 silver badges263 bronze badges

Comments

Dude Bro · Accepted Answer · 2013-03-27 17:34:46Z

1

static Singleton instance = new Singleton(); should be final to prevent race condition.

answered Mar 27, 2013 at 17:34

Dude Bro

1,1521 gold badge10 silver badges13 bronze badges

Collectives™ on Stack Overflow

Java Singleton.getInstance() returns null?

4 Answers 4

Comments

6 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

6 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related