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.
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.