Java Minutiae - Map.remove()

Suppose you'd like to remove an entry from a Map, and perform some action only if the remove actually removed. Since Map.remove() returns the value associated with the removed key, we can do this:

  if (map.remove(key) != null) {
   deleteAssociatedFiles(key);
  }`</pre>
Unfortunately, if the map tolerates `null` values, this approach doesn't differentiate between removing 'key&rarr;null' and not removing. The standard workaround is to do two map lookups (which fails for [ConcurrentMaps](http://java.sun.com/javase/6/docs/api/java/util/concurrent/ConcurrentMap.html)):
<pre class="prettyprint">`  if (map.containsKey(key)) {
    map.remove(key);
    deleteAssociatedFiles(key);
  }`</pre>
But there is an elegant solution. Although `Map.remove()` doesn't know 'null' from 'nothing', it turns out that `Set.remove()` makes the distinction. That method returns `true` only if the set changed. We simply operate on the map's keys, which will write-through:
<pre class="prettyprint">`  if (map.keySet().remove(key)) {
    deleteAssociatedFiles(key);
  }

I learned this trick from Jared of the Google Collections team.