SimpleSingleton

SimpleSingleton.java

public class Singleton
{
  private Singleton singleton;
  
  //make the constructor protected- to defeat instantiation.
  protected Singleton(){}
  
  public static Singleton getInstance() {
      if(singleton == null) {
         singleton = new ClassicSingleton();
      }
      return singleton;
}

This is not thread-safe though. Once you have public static method/property, there's always danger of thread-safety. You need to guard your null check/construction on the singleton member instance. you can do something like this:

public static Singleton getInstance() {
      if(singleton == null) {
         try {
               singletonLock.lock(); //define singletonLock as a member field in Singleton class

               //need to check for null again (double-checked locking //ref:https://en.wikipedia.org/wiki/Double-checked_locking)
               if(singleton == null) {
                    singleton = new ClassicSingleton();
               }
          }
          finally {
               singletonLock.unlock();
          }
      }
      return singleton;
}