The Principle of Least Astonishment, and Naming

The Principle of Least Astonishment states that code should behave in expected ways without hidden surprises. The following is an example of how improperly named things will introduce bugs into systems.

Given a list of players, each player has multiple games, we want to build a map from game IDs to game names. We could do so using a Map and loops Java:

Map<Integer, String> games = ...;
for (var player : players) {
  for (var game : {

We could also do it using the stream API with the toMap collector:

Map<Integer, String> games =

Both seems reasonable and clear at first.

A Map in Java doesn't contain duplicates, when putting a key value into a map it will override existing mapping that has the same key, this is standard behavior and everybody expects it. So when multiple players share the same game, putting the same game ID to name mapping into a map has no effect.

However the toMap collector breaks this expectation, it will throw an exception if it sees duplicates, this is rather surprising given the name toMap implies it would behave like a normal map would. As more and more developers are refactoring loops into streams, hopefully this difference is caught during testing instead of things breaking in production.

The other problem is you tend to forget about things after a while, so you started to make the same mistake again by using toMap thinking it behaves like a normal map, only to find yourself relearning the same lesson again, and again, when it surprises you by breaking things. Same goes for your friends.

It should have been given an abnormal name that fits with the abnormal behavior.