Remove expired channel numbers when writing a channel

I'm concerned that this is potentially pretty slow, but writing is fairly infrequent, so hopefully isn't a massive issue. Would welcome suggestions on how to improve efficiency though.
I'm not totally sure but looks like you're computing a difference between two sets?
Can this be useful in that case? http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Sets.html#difference(java.util.Set, java.util.Set)