I need to create a singleton class without keeping a static method.
How can i do that?
8
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Only call the constructor once.M Platvoet– M Platvoet2011-12-22 10:43:05 +00:00Commented Dec 22, 2011 at 10:43
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2@M Platvoet - That's not a singleton, because you can call the constructor a second time.Petar Minchev– Petar Minchev2011-12-22 10:43:34 +00:00Commented Dec 22, 2011 at 10:43
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1@Peter, wrong. That's not a singleton pattern, it is however a singleton instance.M Platvoet– M Platvoet2011-12-22 10:46:13 +00:00Commented Dec 22, 2011 at 10:46
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@M Platvoet - Wrong, the OP asks for a singleton class and I was talking about singleton pattern, not instance.Petar Minchev– Petar Minchev2011-12-22 10:55:29 +00:00Commented Dec 22, 2011 at 10:55
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@Peter Did you really want to get down Sunny on his interview? ;)andrey– andrey2011-12-22 10:56:21 +00:00Commented Dec 22, 2011 at 10:56
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7 Answers
Create an enum with one instance
enum Singleton {
INSTANCE;
private Field field = VALUE;
public Value method(Arg arg) { /* some code */ }
}
// You can use
Value v = Singleton.INSTANCE.method(arg);
EDIT: The Java Enum Tutorial shows you how to add fields and methods to an enum.
BTW: Often when you can use a Singleton, you don't really need one as utility class will do the same thing. The even shorter version is just
enum Utility {;
private static Field field = VALUE;
public static Value method(Arg arg) { /* some code */ }
}
// You can use
Value v = Utility.method(arg);
Where Singletons are useful is when they implement an interface. This is especially useful for testing when you using Dependency injection. (One of the weakness of using a Singleton or utility class substitution in unit tests)
e.g.
interface TimeService {
public long currentTimeMS();
}
// used when running the program in production.
enum VanillaTimeService implements TimeService {
INSTANCE;
public long currentTimeMS() { return System.currentTimeMS(); }
}
// used in testing.
class FixedTimeService implements TimeService {
private long currentTimeMS = 0;
public void currentTimeMS(long currentTimeMS) { this.currentTimeMS = currentTimeMS; }
public long currentTimeMS() { return currentTimeMS; }
}
As you can see, if your code uses TimeService everywhere, you can inject either the VanillaTimeService.INSTANCE or a new FixedTimeService() where you can control the time externally i.e. your time stamps will be the same every time you run the test.
In short, if you don't need your singleton to implement an interface, all you might need is a utility class.
3 Comments
remi bourgarel
I don't get it ... you can't call any method on your singleton
josefx
@remi bourgarel a java enum is just a class. Just add the variables, methods and a constructor to the Singleton enum (after the declaration of INSTANCE) as you would with any other class and INSTANCE has them.
Peter Lawrey
@remibourgarel I have added a link, I hope that helps. ;)
public class Singleton {
public static final Singleton instance = new Singleton();
private Singleton() {}
public void foo() {}
}
then use
Singleton.instance.foo();
Another approach is the singleton holder idiom which offers initialization on demand:
public class Something {
private Something() {
}
private static class LazyHolder {
public static final Something INSTANCE = new Something();
}
public static Something getInstance() {
return LazyHolder.INSTANCE;
}
}
Note that standalone singletons like this should be avoided where possible because it promotes global state, leads to hard-to-unittest code and depends on a single classloader context to name a few possible drawbacks.
3 Comments
Irfan Latif
How non-static
getInstance method would be accessible outside without creating new instance of Something?
Adriaan Koster
@IrfanLatif I have no idea who edited this. It's wrong, so changing getInstance back to static
Adriaan Koster
I am guessing that someone saw the OP asked for a solution without a static method, and then just made getInstance non-static in this answer, which is wrong. I don't know if I overlooked the 'without using a static method' part of the question at the time (9 years ago) or that this was changed later on.
Follow the Joshua Bloch enum recipe in "Effective Java" 2nd edition. That's the best way to create a singleton.
I don't understand why this comes up so much. Isn't singleton a discredited design pattern? The GoF would vote it off the island today.
Comments
You can use one of IoC containers (e.g. Google Guice) and use your class as singleton(eager or lazy - it depends on your needs). It's easy and flexible as instantiating is controlled by IoC framework - you don't need any code changes if, for example, you will decide to make your class not singleton later.
3 Comments
A.H.
In the context of an interview question it is not a good idea to solve little problems like that by introducing huge frameworks like an IoC container.
Artem
Why? IoC is quite an ordinary thing now. And it's a plus if interviewee knows about it and propose using IoC framework as one of possible solutions.
A.H.
Yes, IoC is common knowlegde nowaday. By far it is not used in all and every project. So unless the the interviewer showed an interest in any IoC framework before, I would assume that the question wants to test the knowledge about the language itself. If you don't show that you know how simple things can be done in a simple way but only in a complicated way involving several heavy frameworks then it can be assumed, that your normal problem solving skill works the same way. <br/>The best thing to do: Show both solutions.
Use an object factory, fetch your singleton object from this factory.
ObjectFactory factory;
....
MySingletonObject obj = factory.getInstance(MySingletonObject.class);
of course, there are many frameworks to help you to achieve this. the spring framework is a popular choice.
1 Comment
Eduard
Isnt factory using a static method? That is something he doesn want.
From within constructor you need to chceck if there is already instance of the class somewhere. So you need to store reference to your singleton instance in static variable or class. Then in contructor of singleton I would always check if there is existing instance of singleton class. If yes I wouldnt do anything if not I would create it and set reference.
