Problem with a pair of singletons

[willemien]

I have a problem with a pair of singletons

program (simplified) :

public static void main(String[] args){
    SingletonOne other1 = SingletonOne.getInstance();
    SingletonTwo other2 = SingletonTwo.getInstance();
    other1.doSomething();
    other2.doSomething();
}



public class SingletonOne{
  
  
  // singleton
  private static final SingletonOne unique = new SingletonOne();
  
  private final SingletonTwo other = SingletonTwo.getInstance();
  
  private SingletonOne(){
    // private constructor
  }
  
  public static SingletonOne getInstance(){
    return unique;  
  }

  public void doSomething()  {    
    System.out.println("singleton ONE do something 1");
    other.doSomething2();
  }
  
  public void doSomething2() {
    System.out.println(" singleton ONE do something 2");
  }
  // other methods
}


public class SingletonTwo{
  // singleton
  private static final SingletonTwo unique = new SingletonTwo();
  
  private final SingletonOne other = SingletonOne.getInstance();
  
  private SingletonTwo(){
    // private constructor
  }
  
  public static SingletonTwo getInstance(){
    return unique;  
  }
  
  public void doSomething()  {    
    System.out.println("singleton TWO do something 1");
    other.doSomething2();
  }

  public void doSomething2() {
    System.out.println("singleton TWO do something 2");    
  }
  
  // other methods
}

If I run it I get:

singleton ONE do something 1
singleton TWO do something 2
singleton TWO do something 1
Exception in thread "main" java.lang.NullPointerException
at test.SingletonTwo.doSomething(SingletonTwo.java:21 ) (what is line 54 above)

What is happening here and what is the best way to prevent it?

[JosAH]

Your singleton(s) try to get a reference to the other singleton just before their constructor code is run. The constructor is run at class initialization time of the other constructor and just before the 'unique' reference is initialized it is asked for by the static getInstance() method. The remedy is simple: don't do other.doSomething() but take care of lazy (singleton) instantiation and do 'SingtonOne.getInstance().doSomething()' (and similar in the other singleton class).

kind regards,

Jos

[Tolls]

Add a System.out.println() into each constructor so you can identify which constructor is currently being called.
That'll show you that the 'unique' variables are null until the relevant constructor has finished...but you are accessing them in order to populate the 'other' variable.

The solution?
Don't store a local version.
It's not needed.

ETA: And once again I'd left the window open too long!