Getting a weird null pointer exception

Question

I get a null pointer exception at this line :

private ArrayList<DrawableEntity> entitiesToDraw = Loader.instance().getDrawableEntities();

There is apparently no problem with the constructor of Loader :

public static Loader instance() {
    if (instance == null) {
        new Loader();
        System.out.println("Loader ready");
    }
    return instance;
}

Because I get the message "Loader ready". I don't understand, the problem seem to be before the call to getDrawableEntities() but I see nothing and it is not inside getDrawableEntities().

If you step through this code in a debugger, the source of your "weird" problem would be obvious. — Rob, Nov 15 '14 at 15:47

score 5 · Answer 1 · edited May 23 '17 at 11:59

5

You forgot to assign it to instance

   public static Loader instance() {
      if (instance == null) {
        instance = new Loader();
        System.out.println("Loader ready");
       }
     return instance;
   }

And by the way, if thats a singleton then it's wrong (not thread-safe), Here's a way to implement the singleton pattern.

edited May 23 '17 at 11:59

Community

1
1

answered Nov 15 '14 at 15:44

Sleiman Jneidi

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@Spooky Then it's fine, but can you guarantee that?You shouldn't take such assumptions that might not be true in the future. – Sleiman Jneidi Nov 15 '14 at 15:58
Yeah, I know, re-usability, but I don't really understand the way it is done in your link, do I have to change the loader class to an enum ? – Spooky Nov 15 '14 at 16:07
@Spooky Exactly, you change to an enum , Watch this if you found it hard to understand https://www.youtube.com/watch?v=pi_I7oD_uGI#t=28m50s – Sleiman Jneidi Nov 15 '14 at 16:10
I understood, I have made some change of course, everything work except that Loader need a string (gameName) to be initialized, but how to pass a parameter to the constructor if I can't access it ? I can't put the gameName field in the enum, there is an error : Syntax error on token "String", strictfp expected – Spooky Nov 15 '14 at 16:28
@Spooky Singletons cannot take parameters, I think it's not the right pattern for you. – Sleiman Jneidi Nov 15 '14 at 16:31
Yes I know but without the enum I can initialize this field before the instance is created : public static String gameName; – Spooky Nov 15 '14 at 16:33
Adding synchronized to the constructor or the instance() method can avoid problem too, am I right ? – Spooky Nov 15 '14 at 17:20

score 4 · Accepted Answer · answered Nov 15 '14 at 15:45

4

public static Loader instance() {
    if (instance == null) {
        instance = new Loader();
        System.out.println("Loader ready");
    }
    return instance;
}

You forgot to asign your instance variable

answered Nov 15 '14 at 15:45

Anthony Raymond

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score 2 · Answer 3 · answered Nov 15 '14 at 15:45

2

You are not setting instance value when its null. It should be:

if (instance == null) {
    instance = new Loader();
    System.out.println("Loader ready");
}

answered Nov 15 '14 at 15:45

SMA

36,381
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Getting a weird null pointer exception

3 Answers3