A Nice Java HashMap Antipattern

Don't you just love how simple it is to instantiate maps in Java? Like, could it be any uglier than
Map<String, String> map = new HashMap<String, String>();
map.put("fruit", "banana");
map.put("vegetable", "carrot");
I saw someone using a pattern that made it look a bit nicer, like
Map<String, String> map = new HashMap<String, String>() {
  put("fruit", "banana");
  put("vegetable", "carrot");
It kinda puts a bit more structure to this mess. However, you'll get some unwanted runtime side-effects.
When you use this "syntax", you'll actually create a new anonymous inner class. Each anonymous inner class instace will have a runtime reference to thethis object for the context where you define the anonymous inner class in. Thus, if your this happens to have references to 100 megabytes of porn, that stuff will not be GC'd as long as this Map or yours is alive. Also, should you try to serialize your map for sending or storing it somewhere, you'll also be sending your 100 megabytes of porn.
Enjoy your cup of Java!


  1. It is called "Double Brace Initialization" (you are missing one pair) and there is a long write-up of it in this Stack Overflow question: http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization