There is a HashMap<String,Thread> requests in class sun.net.www.protocol.http.AuthenticationInfo.

            Thread t, c;
            c = Thread.currentThread();
            if ((t = requests.get(key)) == null) {
                requests.put (key, c);
                assert cached == null;
                return cached;
            }
            if (t == c) {
                assert cached == null;
                return cached;
            }
Instead of separate HashMap.get/put calls we can improve code by HashMap.putIfAbsent.
It results in cleaner and a bit faster code.