获取一个奇怪的空指针异常

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().

Answer 1

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 .

Answer 2

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

You forgot to asign your instance variable

Answer 3

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

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