public class MyManager{

  private static class MyManagerContainer{
     private static MyManager instance = new MyManager();
  }

  public static MyManager getInstance(){
     return MyManagerContainer.instance;
  }

  private MyManager(){

  }

}

public class MyManager{
  private static MyManager instance;

  public static getInstance(){
     return instance;
  }

  public MyManager(){
      if(instanct!=null){
         throw new IllegalStateException("Already exist");
      }
      instance = this;
  }
}