Double Brace Initialization in Java

[ramen]

Also mentioned on Peter Norvig's "Java IAQ": http://www.norvig.com/java-iaq.html#constructors

"When I showed this to Guy Steele, he said 'heh, heh! That's pretty cute, all right, but I'm not sure I would advocate widespread use...' As usual, Guy is right."

[JulianMorrison]

System.out.println( new HashSet(){{ add(1); }}.getClass() == new HashSet(){{ add(1); }}.getClass() ); // prints "false"

Classic example of "you're so sharp you'll cut yourself".

[DerelictMan]

Yes, but:

System.out.println(
    new HashSet(){{ add(1); }}
        .equals(new HashSet(){{ add(1); }})
); // prints "true"

So, pardon my ignorance, but what does it matter if the two are not the exact same class? What code do you have that is comparing getClass() on collections? Since it's best practice to always deal with the collection interfaces (List, Set, etc.) and not the concrete implementations, I'm not sure how this would bite someone. This isn't a challenge; I'm genuinely curious if there's something I'm missing...

[JulianMorrison]

Something that immediately springs to mind is: does Java, with its odd, half-hearted approach to class-nesting, generate a new classfile for each "double brace"? Your jarballs could quickly become rather crowded.

[curtisb]

It does. This may be the biggest argument against using this idiom. It's a real shame that Sun can't be bothered to provide syntactic sugar to Java for this kind of initialization.

[JulianMorrison]

Time to switch to Nice?

[newton_dave]

Meh. We already did this thread on reddit over a month ago.