samskivert: “Fun with edge cases” or “Why your spec can never possibly be complete”

12 August 2011

(Apologies for the duplication with Google+, still sorting out my syndication process.)

What do you think should happen if you do the following in Java?

Map<String,String> map = new HashMap<String,String>();
map.put("foo", "bar");
Iterator<Map.Entry<String,String>> iter = map.entrySet().iterator();
Map.Entry<String,String> entry =;

Should the entry freak out because you tried to update the mapping after it has already been removed? Should it reintroduce a mapping from “foo” to “baz”? Should the call succeed but have no effect on the map?

Java actually specifies the behavior as undefined. So perhaps your spec can be complete by pushing the burden onto the developer of ensuring that they never rely on any undefined behavior. The HashMap implementation’s interpretation of “undefined” is “allow the call to succeed and don’t modify the underlying map.” That makes it more difficult to ensure you’re not relying on undefined behavior than if it, say, threw an exception.

Perhaps the question on your mind is “Gee Mike, how do you find yourself in the position of caring about something like this?” The answer to that lies in this code, which chooses a different interpretation of undefined.

©1999–2015 Michael Bayne