One-time object

[cthulhufhtagn]

How to make an object, that can only be used in java-prog one time, e.g. when I try to make an second line like that:

FooObj bar = new FooObj (baz, quz, quuz)

the compiler will start to complain. How to do that?
P.S.: Sorry for my poor english.

[PhHein]

Read about Singletons.

[cthulhufhtagn]

Read about Singletons.

Mmkay, I've done it and tried to implement some stuff, but I'M getting error msgs and I don't understand them. The Prog:
hxxp://pastebin.ca/1708189
and here are the error msgs:

javac "MySingletonTest.java" (in directory: /media/DATA AND SH/uni/semester_3/inf_iii/uebeungsblaetter/a_7)
MySingletonTest.java:3: cannot find symbol
symbol  : constructor MySingleton()
location: class MySingleton
	private static MySingleton _instance = new MySingleton();
	                                       ^
MySingletonTest.java:34: MySingleton(java.lang.String) has private access in MySingleton
		MySingleton IAmSingleWhatsWithYou = new MySingleton ("uk");
		                                    ^
2 errors
Compilation failed.

I've used the official tutorial from sun:
hxxp://java.sun.com/developer/technicalArticles/Programming/singletons
what to do

[wtd_nielsen]

public class Elvis
{
     private static Elvis instance;
      
     private Elvis()
     {

     }

     public static Elvis getInstance()
     {
             if(instance==null)
                instance = new Elvis();

             return instance;
     }
}

this is one way of implementing the Singleton pattern...

[Tolls]

this is one way of implementing the Singleton pattern...

That way is not threadsafe.
The more common technique is:

public class Elvis
{
     private static Elvis instance = new Elvis();
      
     private Elvis()
     {

     }

     public static Elvis getInstance()
     {
             return instance;
     }
}

This ensures no threading issues.
Obviously in a stand alone, single thread, application it doesn't matter, but it's good to get used to using the above anyway. Makes sure you don't accidentally do the non-threadsafe one in a webapp, for example.

[wtd_nielsen]

thats true, it depends on what you want. there is also a solution that combinds lazy initialization and thread safety..
I forgot that this was in the Threads and Synchronization forum.. :)

[Tolls]

-Tolls looks up-
Oh, so it is!

Have to say I've never used the lazy initialisation one since I've never written a Singleton that had any other static methods, or public static attributes, than getInstance().