Singleton vs. class with only static methods

[erikd]

Hi there,

Having read about the Singleton, I'm wondering: Why a Singleton with the private constructor stuff and all? Isn't it easier to create a class with static methods and a static initializer, something for example like:

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public class Test {   private static String value;   static {     value = "test";   }     public static String getValue() {     return value;   } }

Isn't that easier than the java singleton stuff?
What great advantage about the singleton am I missing?

Greetings,
Erik

[Markus_Persson]

singletons are just "prettier", as it's basically just a class with no public constructor and a public factory method.
You can even hide the fact it is a singleton from the user of the class.. only the class itself needs to know how it works. That means you can refactor it into a non-singleton class in the future without having to break other code.


/ Markus "Wishing all constructors were private" Persson

[erikd]

I don't think a private constructor is that pretty  :-/
Creating a static class instead of forced single instance  seems more natural to me.

When you have to refactor the singleton to a non-singleton, you probably made a design error, but hey everybody makes them (I should know  )
However, I suspect when you do refactor your singleton to a non-singleton, chances are than you break code that relies on the singleton being in fact a singleton. And why creating a singleton in the first place when you can't rely on the singleton being in fact a singleton?

Erik

Games published by our own members! Check 'em out!

[cfmdobbie]

One useful point about them is that they are still normal objects.  You can obtain a reference to a singleton and pass it to someone else, and they can use it as any other object.  If you changed it to an all-static interface this just wouldn't be possible.

[erikd]

With an all static class, you would never have to pass a reference to it to another object. I can't think of a reason why you would have to be able to in a singleton. I mean there's always just one and every object can get its reference using the method in the singleton class that is meant to provide that reference.
I suppose I'm still missing the clue

[cfmdobbie]

But how about if the specific object type isn't something that the target code needs to know?  You can still manipulate the object-type singleton via an interface, but with the static-type singleton the target code would have to refer directly to the particular implementation.

EDIT: This really needs an example to be perfectly clear!  How about storing a singleton object in a JNDI repository?  Or implementing a JDBC DataSource as a singleton (for whatever reason)?

[erikd]

Ah yes, I suppose you'll have a problem when you want the all static class to implement an interface which would be no problem for the singleton.

[tortoise]

Many things expect objects, not classes. If you just want one of those objects, a singleton's a good way to do it.

In my current program I have a toolbox window. I only want one. But it still has to be an instance of JInternalFrame. The static approach wouldn't work.

[Mojomonkey]

I try to follow this rule (whether or not it's a good rule is debatable).

If I have something that I'll only one a single instance of in a JVM:

1. If it maintains state, singleton. Otherwise, static.

[Markus_Persson]

Good rule, imo. *wishing there was a thumbs-up emoticon*

Games published by our own members! Check 'em out!

[cfmdobbie]

Good rule, imo. *wishing there was a thumbs-up emoticon*

/me hands Markus a

[Markus_Persson]

Ooohh. How/where did you find that? =D

[cfmdobbie]

It's one of the options for a message logo (see above) so it's already on the JGO server.  I just copied the url and chucked it between a [ img ] [ / img ] pair.  This is also the only way of getting the animated laugh smiley:

.

I think I've been here too long...

[Preston]

If I have something that I'll only one a single instance of in a JVM:

1. If it maintains state, singleton. Otherwise, static.

This is a good rule. I used singletons for a quite large C++ project some time ago and we used practically the same rule and it went well. :-)

Whenever a (single instanced) class has got some kind of states, so that the order of when it is beeing created could matter, a singleton is much safer than a static only class.

For a "function"-like class (like Math.sin() etc) pure static classes makes perfect sense. :-)

[singleton]

Almost a decade has passed since the previous response and the use of dependency injection is common now. Often times the injected instance is a stateless Singleton. However, a class with static methods would not lend itself to injection. Note that this predicament is essentially the same as the one referred to by tortoise in an earlier response.

[HeroesGraveDev]

Just WHY did you necro an almost 10 year old thread?!

WHY????!!!!!!!

[princec]

He DANCES ON THE GRAVE!

Cas

[Roquen]

Since "singleton" is the username I was assuming this was a pro-wrestling match.  I feel so let down.

[matheus23]

Just WHY did you necro an almost 10 year old thread?!

WHY?!!!!!!!

He only cam here to do EXACTLY THAT...

[tyeeeee1]

Since this thread had already been necro'd and I read the OP...

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public class Test {   private static String value;   static {     value = "test";   }     public static String getValue() {     return value;   } }

Wont this code give a compile error? I've never seen code that just says "static{}" and does stuff within that.

[princec]

No, it's fine.

Cas

[Cero]

Since this thread had already been necro'd and I read the OP...

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public class Test {   private static String value;   static {     value = "test";   }     public static String getValue() {     return value;   } }

Wont this code give a compile error? I've never seen code that just says "static{}" and does stuff within that.

So what it does is, like variables, it just executes the code right away when the class is first loaded somewhere somehow. It's actually pretty cool. Its like an initialize without having to call it

[tyeeeee1]

O.o That's probably one of the most useful things I've learned yet. Necro posts are awesome.

[matheus23]

O.o That's probably one of the most useful things I've learned yet. Necro posts are awesome.

'aaahahhhhhH!!!!' STATIC *aaaahhh*

It's probably useful to do stuff, which is near the machine, but I really hate it actually... In my opinion nothing really needs to be static, unless it's close to the machine...

[tyeeeee1]

'aaahahhhhhH!!!!' STATIC *aaaahhh*

It's probably useful to do stuff, which is near the machine, but I really hate it actually... In my opinion nothing really needs to be static, unless it's close to the machine...

I was actually just about to edit my post to ask if there was another way to do it without using static{}.

[sproingie]

That particular static initializer is also completely pointless since it's exactly the same as saying:

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public class Test {   private static String value = "test";   public static String getValue() {     return value;   } }

However, they do come in handy later: http://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom

Namely, you can have the holder execute more or less arbitrary initialization code in its static initializer. I use that trick here: https://bitbucket.org/chuck/jcod/src/c2e8ae6fd81bafc50bce94e2b2c9a0a26f3137d6/src/main/java/net/fishbulb/jcod/display/TileDisplay.java?at=default#cl-150

[Damocles]

Wont this code give a compile error? I've never seen code that just says "static{}" and does stuff within that.

This also works, can be run without a main method.

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public class StartStatic {    static    {       System.out.println("Hello, main is overrated...");    } }

or even

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public class StartStatic {    static    {       System.out.println("Dont like main");       new Thread()       {          public void run()          {          for(int i=0;i<10;i++)          {             System.out.println("Im running! "+i);             try             {                Thread.sleep(500);             }             catch (InterruptedException e)             {                e.printStackTrace();             }          }                    }             }.start();    } }

Would be a good way to make your teacher mad in a programming class.

[Oskuro]

A good example of when to use the static initialization is Riven's implementation of sin/cos using lookup tables: here

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private static final float   RAD,DEG; private static final int     SIN_BITS,SIN_MASK,SIN_COUNT; private static final float   radFull,radToIndex; private static final float   degFull,degToIndex; private static final float[] sin, cos; static {     RAD = (float) Math.PI / 180.0f;     DEG = 180.0f / (float) Math.PI;         SIN_BITS  = 12;     SIN_MASK  = ~(-1 << SIN_BITS);     SIN_COUNT = SIN_MASK + 1;         radFull    = (float) (Math.PI * 2.0);     degFull    = (float) (360.0);     radToIndex = SIN_COUNT / radFull;     degToIndex = SIN_COUNT / degFull;     sin = new float[SIN_COUNT];     cos = new float[SIN_COUNT];     for (int i = 0; i < SIN_COUNT; i++)     {         sin[i] = (float) Math.sin((i + 0.5f) / SIN_COUNT * radFull);         cos[i] = (float) Math.cos((i + 0.5f) / SIN_COUNT * radFull);     } }

In other words, when you need complex static data structures that can be generated procedurally.

Just WHY did you necro an almost 10 year old thread?!

WHY?!!!!!!!

Have you noticed who made the first reply to the thread? 

[relminator]

No one is going to win this argument.  Some like to use singletons while others like to use statics.  I kind of like to mix it up depending on mostly my mood and not from a coding standpoint.

I have a friend at EA UK who told me that they don't care whether you use singletons or static globals (namespaced as this was in c++) as long as your sibgletons are not "lazy" (for memory efficiency obviously).