It’s called double curly brace initialization. (EDIT: Link removed, archived here)
It means you’re creating an anonymous subclass and the code within the double braces is basically a constructor. It’s often used to add contents to collections because Java’s syntax for creating what are essentially collection constants is somewhat awkward.
So you might do:
List<String> list = new ArrayList<String>() {{
add("one");
add("two");
add("three");
}};
instead of:
List<String> list = new ArrayList<String>();
list.add("one");
list.add("two");
list.add("three");
I actually don’t like that and prefer to do this:
List<String> list = Arrays.asList("one", "two", "three");
So it doesn’t make much sense in that case whereas it does for, say, Maps, which don’t have a convenient helper.