About singleton

Question

Hello everyone:
I'm not sure if my class is Thread safe. Can you state me if it is right?
This class is to share outputstream used by socket tcp/ip.
The HashMap object: is itself thread safe?
Regards, Ulises.
class SingletonOutStream{
     private static SingletonOutStream instance;
     HashMap outsToModems;
     public synchronized static SingletonOutStream getInstance(){
          if ( instance == null) {
               instance = new SingletonOutStream();
          return instance;
     private SingletonOutStream(){
     void addOutToModem(String key, OutputStream value){
       if(outsToModems==null){
          outsToModems = new HashMap();
       if ((OutputStream) outsToModems.get(key)==null) {
                        outsToModems.put(key, value);
     OutputStream getOutToModem(String key){
       return (OutputStream) outsToModems.get(key);
     void removeOutToModem(String key){
       outsToModems.remove(key);
}

Answer 1

Add the synchronized keyword to all methods using the
HashMap. Also note that if the get and remove methods
are called before the add method you'll get a
null-pointer exception.Great advice idiot.
Just get rid of the lazy initialization and call
synchronizedMap on the HashMap.Even an "idiot" can see that synchronizing the HashMap won't make addOutToModem threadsafe. For that to happen the second if-statement must be "atomic". The easiest way is to synchronize the whole method.
An advice. If you want to show attitude make sure you're correct. Right now you're just a lousy programmer with your pants at the ankles.