double brace initialization and "kind of" static anonymous class

Question

Sometimes for testing I use quick "double-brace" initialization which creates anonymous nested class in Outer class, for example:

static final Set<String> sSet1 = new HashSet<String>() {
    {
        add("string1");
        add("string2");
        // ...
    }
};

Edit I am correcting my previously faulty statement that this example keeps reference to Outer instance. It does not and it is effectively equivalent to the following :

static final Set<String> sSet2;
static {
    sSet2 = new HashSet<String>() {
        {
            add("string1");
            add("string2");
            // ...
        }
    };
}

both sSet1 and sSet2 are initialized with anonymous nested classes that keep no reference to Outer class.

Does it mean that these anonymous classes are essentially static nested classes ?

Your first example doesn't have a reference to Outer or am I missing something? — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Dec 15, 2013 at 18:22
@SotiriosDelimanolis Well, I believe anonymous inner class from first example does contain implicit reference to Outer.this — kiruwka
– kiruwka, Commented Dec 15, 2013 at 19:00
No, it doesn't. You're in a static context in both. There is no outer instance. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Dec 15, 2013 at 19:02
Lol, that's a little harsh. Usually, I'll try this in my IDE Outer.this.someMethod(). If it doesn't compile, then gotta rethink my logic. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Dec 15, 2013 at 19:09

Dmitry Tikhonov · Accepted Answer · 2013-12-15 18:38:19Z

3

As in the related question you are referencing to is discussed, an anonymous class cannot be static technically, but it can be so called effectively static if it is declared in a static context, that is it has no reference to the outer instance.

In your case, however, there is definitely no difference between two approaches, the initialization of static fields is a static context as well.

answered Dec 15, 2013 at 18:38

Dmitry Tikhonov

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2 Comments

kiruwka Over a year ago

+1 Yes, you are right about no difference, I completely overlooked that. Could you please give some practical difference between effectively static and normal static nested class. Tom referenced JLS 3rd Ed for this in his answer(to which I refer), but may be you could put quick summary for me ? thanks.

kiruwka Over a year ago

After reading all answers to this question, I realize that there is no technical difference, other then doing it is not really recommended as this Java feature could be pulled out.

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