count

(count coll)

Answers the number of elements of coll. Table-aware: maps and sets answer their hash-table-count directly (no seq built), everything else the length of the seq view (a lazy seq realizes whole first). A Java Collection or Map answers its size, a CharSequence its length; any other Java Iterable signals, like the oracle. Anything that is no collection (a keyword, false, a number, an atom) signals UnsupportedOperationException, like the oracle.