How to initialize my singleton object

Question

public class FooClient {

    private Foo foo;
    private final static String key = "<api-key>";

    private static FooClient client = new FooClient();

    private FooClient() {
        foo = new Foo(key);
    }

    public static FooClient getFooClient() {
        return client;
    }
}

Is it ok to initialize client in the above fashion.
Should I declare private Foo foo; as static, I am guessing its not the case.
If I have to support different singletons for different keys, should I modify getFooClient(String key) to take in a key and cache it, so that I can return singleton FooClients which are key specific.

Please stop using singletons. It makes horrible classes to test. http://stackoverflow.com/questions/137975/what-is-so-bad-about-singletons You don't really need them. — zengr, Sep 09 '11 at 07:09
If you can, stay away from singletons. They will most likely cause problems in the long run. ´new FooClient(key)´ is IMHO the way to go. — Matt, Sep 09 '11 at 07:18

score 4 · Accepted Answer · answered Sep 09 '11 at 06:59

4

Yes. In the constructor you can check if client != null and if it is - throw an error. (this will counter reflection instantiations)
No, it is an instance field of the singleton
Yes. And you should have a Map<String, Foo>. But note that that is not "different singletons" - your singleton is the "client". The other classes can be instantiated multiple times.

answered Sep 09 '11 at 06:59

Bozho

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[1] is not clear, can you provide the code snippet of how it needs to look and why you think of it as problematic. – Jason Sep 09 '11 at 07:38
if `(client != null) throw new Error("attempted second instantiation)`. It can be problematic because one can instantiate objects even if their constructor is private - through reflection – Bozho Sep 09 '11 at 07:51

score 2 · Answer 2 · answered Sep 09 '11 at 07:02

2

Usually you declare

private static final FooClient client = new FooClient();

This is the traditional Singleton implementation. See wikipedia page for other implementation options.

I would not declare Foo foo as static.

If your singleton can return different instances based on the key, then it's a good idea to pass the key value in the getFooClient() method.

answered Sep 09 '11 at 07:02

MarcoS

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+1 That singleton works 95% of the times. For the rest 5% you need double-check locking with volatile. – zengr Sep 09 '11 at 07:07

score 1 · Answer 3 · answered Sep 09 '11 at 07:03

If you have more than one of something, its not a singleton.

I would use enum in both cases.

For the case where this is just one.

enum FooClient {
    INSTANCE;

    private final Foo foo = new Foo("<api-key>");
}

for the case where there is more than one.

enum FooClient {
    INSTANCE1("<api-key>"), INSTANCE2("<api-key2>");

    private final Foo foo;
    FooClient(String apiKey) {
        foo = new Foo(apiKey);
    }
}

How to initialize my singleton object

3 Answers3