Java collections: TreeMap.size() and TreeSet.size(): O(1) or O(n)?

Question

Do TreeMap and TreeSet keep track of how many items they contain, or do they have to count them every time you call size() ? The javadocs remain mute on the subject.

Answer 1

Take a look:

http://www.docjar.com/html/api/java/util/TreeMap.java.html

http://www.docjar.com/html/api/java/util/TreeSet.java.html

For future reference, the google search was "java source code treemap". (I'm not saying that to be snarky -- it's not entirely obvious that the source code would be out there for the googlin').

tl;dr version is that they keep track, so it's O(1).

Answer 2

是的，它们会跟踪它们包含多少个对象，因此调用size()会产生O（1）运行时。