is the Java HashMap keySet() iteration order consistent?

You can use a LinkedHashMap if you want a HashMap whose iteration order does not change.

Moreover you should always use it if you iterate through the collection. Iterating over HashMap's entrySet or keySet is much slower than over LinkedHashMap's.

The API for Map does not guarantee any ordering whatsoever, even between multiple invocations of the method on the same object.

In practice I would be very surprised if the iteration order changed for multiple subsequent invocations (assuming the map itself did not change in between) - but you should not (and according to the API cannot) rely on this.

EDIT - if you want to rely on the iteration order being consistent, then you want a SortedMap which provides exactly these guarantees.

If it is not stated to be guaranteed in the API documentation, then you shouldn't depend on it. The behavior might even change from one release of the JDK to the next, even from the same vendor's JDK.

You could easily get the set and then just sort it yourself, right?

Map is only an interface (rather than a class), which means that the underlying class that implements it (and there are many) could behave differently, and the contract for keySet() in the API does not indicate that consistent iteration is required.

If you are looking at a specific class that implements Map (HashMap, LinkedHashMap, TreeMap, etc) then you could see how it implements the keySet() function to determine what the behaviour would be by checking out the source, you'd have to really take a close look at the algorithm to see if the property you are looking for is preserved (that is, consistent iteration order when the map has not had any insertions/removals between iterations). The source for HashMap, for example, is here (open JDK 6): http://www.docjar.com/html/api/java/util/HashMap.java.html

It could vary widely from one JDK to the next, so i definitely wouldn't rely on it.

That being said, if consistent iteration order is something you really need, you might want to try a LinkedHashMap.