Re: ensuring only one instance of an object exists?

jcsnippets.atspace.com wrote:


Hi,

I read your article, and there's this line that jumps out at me:

"Both examples given above do NOT have an explicit constructor - which
means they both get the default private constructor."

This is wrong, if you do not define an explicit constructor, you
inherit the default PUBLIC constructor.

This test code (using the example singleton classes in your article):

public class Test {
  public static void main(String args[]){
    StaticSingleton s = new StaticSingleton();
    SynchronizedSingleton ss = new SynchronizedSingleton();
    System.out.println(StaticSingleton.getInstance().equals(s));
    System.out.println(SynchronizedSingleton.getInstance().equals(ss));
  }
}

Outputs:

% java Test
false
false

Which means I have two distinct instances of the singleton class. When
defining a singleton, you MUST define the private constuctor(s) -
unless you don't mind people being able to create new instances outside
your static one ;)