What is a Singleton Class?

[nagaraj reddy]

please anybody explain about singleton class
[ April 11, 2005: Message edited by: Barry Gaunt ]

[levani dvalishvili]

as far as I know sigleton is the methodology which states the mechanizm where ONLY one instance of the certian class is maintained throught the program execution
[ April 10, 2005: Message edited by: levani dvalishvili ]

[nagaraj reddy]

what you told about singleton class is correct.Please explain through the code.


thanks in advance

[Jeroen Wenting]

not scjp

try design forums instead, do your own homework. There's tons of resources available explaining in fine detail what a singleton is.

[Barry Gaunt]

Transferring to ...

[peter wooster]

A Singleton is a class that can only be instantiated once, it's an OOP substitute for global variables. There are two basic idioms for this, eager instantiation and lazy instantiation. I'm writing this code from memory, the bugs are left as an exercise for the reader.

public class EagerSingleton {
     private static EagerSingleton SINGLETON = new EagerSingleton();
 
    private EagerSingleton() {
     // whatever your constructor needs
    }
    
    public static EagerSingleton getInstance() {
        return SINGLETON;
    }
    // the rest of your class
}
 
public class LazySingleton {
    private static LazySingleton SINGLETON = null;
 
    private LazySingleton() {
     // whatever your constructor needs
    }
    
    public static synchronized LazySingleton getInstance() {
        if (null == SINGLETON) {
             SINGLETON = new LazySingleton();
        }
        return SINGLETON;
    }
    // the rest of your class
}
 

[ April 11, 2005: Message edited by: peter wooster ]

[Ilja Preuss]

See http://faq.javaranch.com/view?SingletonPattern

[Pradeep bhatt]

Originally posted by Ilja Preuss:
See http://faq.javaranch.com/view?SingletonPattern

A singleton is a class of which there cannot be multiple instances.

Is the above correct ? I think GoF states that the pattern can control the number of instances and it could be more than 1.