Which Design Pattern?

Question

Hi all,
First of all, im pretty new to design patterns so apologies for any errors. I am creating a program that maintains a large index of string values (maybe a vector or hash table) but for the sake of performance, i would like more than one instance of the application to run at once, so that they can each add to the index.
As i said, i am new to patterns, but this seems like an ideal scenario for the Singleton. Would i be right in this assumption and could you give me any advice or example code as to how i might implement it?
Thanks very much
Wraith

Answer 1

georgemc wrote:
Imagist wrote:
final public class Singleton
private Singleton() { }
final private static class SingletonHolder
final private static Singleton instance = new Singleton();
final private static Singleton getInstance()
return SingletonHolder.instance;
}This is the paradigmatic way to implement a singleton in Java these days. There are other ways more commonly used, but this has the advantage of allowing lazy loading AND thread safety without incurring the performance penalty of synchronization.
Edited by: Imagist on Nov 19, 2008 9:53 AM
public class Singleton
private Singleton() { }
final public static Singleton instance = new Singleton();
}Oh look. Lazy loading, thread safety and simplicity, all in one lump. Why did you bother making the class final, anyway? The singleton pattern as prescribed by the GoF book explicitly talks about inheritanceAs far as I can tell, that's not lazy-loading. If you call Singleton.someStaticMethod(), the classloader will initialize the instance variable, causing the constructor to call. Admittedly, that doesn't happen often. I haven't looked at Bloch's book in a while, but if I remember correctly this is the method he recommended.
I made the class final because that allows more optimizations to occur at compile time. The JIT compiler usually figures stuff like that out, but in some older versions of Java it might not happen until it has been running for a while. I use older versions of Java often, so that's a good habit to be in (and it doesn't hurt anything).