Java SortedMap-TreeMap键超出范围

Question

I'm working on a project where I use a TreeMap as chronological index.

SortedMap<Long, String> expiryIndex = new TreeMap<>();
Long now = System.currentTimeMillis();
expiryIndex.put(now, null);

The aim is to later be able to create TailSet of "older" items without having to iterate. However, I'm not getting this far because already at put I get an exception:

SEVERE: Unhandled exception
java.lang.IllegalArgumentException: key out of range

I don't see what I could have done wrong here.

Answer 1

As a matter of fact this was indeed my fault.

I had a code block further down, where I sliced a tailset out of it. Since these slices are value by reference, the put in the corresponding tailslice was out of bounds, although it was made into the full set. Hence Davidxxx's comment was basically the hint.

Still.. weird. Thanks