Java Hack: Double Brace Initialization

Watch out - double-brace initialization is cool looking, but it's orders of magnitude slower than regular initialization as it must generate an anonymous class.<p>Also, the generated class is put into the permgen space, which is not garbage collected. The permgen is pretty small by default - and if you fill it up, your system is hosed.<p>I use them all the time in unit tests, but never in production code.

For initializing lists there is a less verbose expression in Java:<p><pre><code> List&#60;Integer&#62; list = Arrays.asList(1,2,3,4,5);</code></pre>

Nice. Could have used that thought about 9 years ago :)

This is where Google Collections comes in handy:<p><pre><code> Lists.newArrayList(1, 2, 3) </code></pre> Or, if you don't need to modify it:<p><pre><code> ImmutableList.of(1, 2, 3) </code></pre> Such an awesome library.

I've been using this for years. It is a fantastic way to build up hierarchies. XML, maps, JSON, UI code etc. It is very concise, fast and produces clean code which also just happens to correctly indent when you apply code formatting.<p><pre><code> School s = new School() {{ add(new Student() {{ name = "Bobby"; age = 15; add(new Pet() {{ name = "Fluffy"; }}); }}); }}; vs. School sc = new School(); Student st = new Student(); st.name = "Bobby"; st.age = 15; Pet p = new Pet(); p.name = "Fluffy"; st.add(p); sc.add(st);</code></pre>

One thing though. I love this technique but the home built Eclipse compiler is not fully Java language compliant and it will reject certain forms of this as invalid. Specifically, the sun and eclipse compiler treat "this" differently in these code blocks. My project will not compile under eclipse because of certain usages of this pattern when combined with inner classes. The bug crops up when you need to pass the parent to the child object being initialized in the static block.