Cool Java code: anonymous class plus instance initializer

Ryan Sonnek posted a neat example of how to use anonymous classes and instance initializers to initalize a Map easily, right were you declare it. He's using this for his JavaScript builder.

Check out this code example:

Map options = new HashMap() {{
  put("key", "value");
  put("option", "foo");
  put("flag", "bar");

Ryan calls it a "little-known Java feature called an initializing block":

Huh! I didn't know you could do that. So I tried to figure out how it works. It's interesting. I've used both components separately, but never put them together like this.

The first set of { } creates an anonymous class as a subclass of HashMap. You see this type of code a lot, especially for event listeners. They're also nice for FileFilters.

The second { } I've also used before, but it is rare. It's an instance initializer. (See also this discussion of them.)

So basically you're creating a subclass of HashMap and then setting the values you want in its instance initializer. Clever.

I ran this code in BeanShell and sure enough it reported the class of the instance as "global$1" instead of HashMap.

I ran this past my friend Nick and he replied with another neat trick:

I've used the new <someclass>() {} trick so that I can distinguish between classes in the debugger.
class X {
  Map m = new HashMap() {};
class Y {
  Map m = new HashMap() {};
The debugger will say X$1, instead of HashMap -- useful when debugging deadlocks and it isn't clear which object is in contention.