Question

Why can you not do the following?

class foo {
  private static foo instance  = new foo();
  List <string> messages = new List<string>();


  private foo(){}
  public static void doSomething(){..}

}

Edit:

I mean is there a difference between doing this:

class foo {
  private static foo instance  = new foo();
  List <string> messages = new List<string>();


  private foo(){}
  public static void doSomething(){..}

}

or

class foo {
  private static foo instance;
  List <string> messages = new List<string>();


  private foo(){}
  public static void doSomething(){..}

  public foo getInstance(){
    if(instance!=null){
       return instance;
    }
 }
}

Answer 1

This is the non-lazy method of implementing the Singleton pattern for languages that support it. It’s perfectly acceptable to do this, especially since it’s one of the ways of implementing a thread-safe Singleton. However, if your Singleton object is expensive (see lazy initialization) to create, then it might not be appropriate to create it in this manner.

If your code doesn’t compile, as the other answers point out, your syntax for the doSomething method is incorrect.