Double Brace Initialisation

"Double Brace Initialisation" is one of the lesser known features in Java. It can be useful for initialising collections that have to be passed to other methods.

For example, without double-brace initialisation:

Map<String,String> map = new HashMap <String,String>();
map.put("key","value");
map.put("key2","value2");
printMap(map);

With double-brace initialisation:

printMap(new HashMap <String,String>(){{
            put("key","value");
            put("key2","value2");
        }});

The first brace creates a new AnonymousInnerClass, the second declares an instance initializer block that is run when the anonymous inner class is instantiated. This only works only for non-final classes because it creates an anonymous subclass.

Don't go too overboard with this because it creates a new anonymous inner class just for making a single object which might not be very efficient! It will also create an additional class file on disk.

Another example:

//double-brace initialisation
List<String> list = new ArrayList<String>(){{
   add("apple");
   add("banana");
}};

//better alternative
List<String> list2 = Arrays.asList("apple","banana");