I was reading the Singleton article on Wikipedia and I came across this example:
public class Singleton {
// Private constructor prevents instantiation from other classes
private Singleton() {}
/**
* SingletonHolder is loaded on the first execution of Singleton.getInstance()
* or the first access to SingletonHolder.INSTANCE, not before.
*/
private static class SingletonHolder {
private static final Singleton INSTANCE = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}
While I really like the way this Singleton behaves, I can't see how to adapt it to incorporate arguments to the constructor. What is the preferred way to do this in Java? Would I have to do something like this?
public class Singleton
{
private static Singleton singleton = null;
private final int x;
private Singleton(int x) {
this.x = x;
}
public synchronized static Singleton getInstance(int x) {
if(singleton == null) singleton = new Singleton(x);
return singleton;
}
}
Thanks!
Edit: I think I have started a storm of controversy with my desire to use Singleton. Let me explain my motivation and hopefully someone can suggest a better idea. I am using a grid computing framework to execute tasks in parallel. In general, I have something like this:
// AbstractTask implements Serializable
public class Task extends AbstractTask
{
private final ReferenceToReallyBigObject object;
public Task(ReferenceToReallyBigObject object)
{
this.object = object;
}
public void run()
{
// Do some stuff with the object (which is immutable).
}
}
What happens is that even though I merely pass a reference to my data to all of the tasks, when the tasks are serialized, the data gets copied over and over. What I want to do is share the object among all of the tasks. Naturally, I might modify the class like so:
// AbstractTask implements Serializable
public class Task extends AbstractTask
{
private static ReferenceToReallyBigObject object = null;
private final String filePath;
public Task(String filePath)
{
this.filePath = filePath;
}
public void run()
{
synchronized(this)
{
if(object == null)
{
ObjectReader reader = new ObjectReader(filePath);
object = reader.read();
}
}
// Do some stuff with the object (which is immutable).
}
}
As you can see, even here I have the issue that passing a different file path means nothing after the first one is passed. This is why I like the idea for a store which was posted in the answers. Anyhow, rather than including the logic for loading the file in the run method, I wanted to abstract this logic into a Singleton class. I will not provide yet another example, but I hope you get the idea. Please let me hear your ideas for a more elegant way to accomplish what I am trying to do. Thank you again!
