Here's my only-somewhat-contrived example of a case where the null key can be useful:
public class Timer {
private static final Logger LOG = Logger.getLogger(Timer.class);
private static final Map<String, Long> START_TIMES = new HashMap<String, Long>();
public static synchronized void start() {
long now = System.currentTimeMillis();
if (START_TIMES.containsKey(null)) {
LOG.warn("Anonymous timer was started twice without being stopped; previous timer has run for " + (now - START_TIMES.get(null).longValue()) +"ms");
}
START_TIMES.put(null, now);
}
public static synchronized long stop() {
if (! START_TIMES.containsKey(null)) {
return 0;
}
return printTimer("Anonymous", START_TIMES.remove(null), System.currentTimeMillis());
}
public static synchronized void start(String name) {
long now = System.currentTimeMillis();
if (START_TIMES.containsKey(name)) {
LOG.warn(name + " timer was started twice without being stopped; previous timer has run for " + (now - START_TIMES.get(name).longValue()) +"ms");
}
START_TIMES.put(name, now);
}
public static synchronized long stop(String name) {
if (! START_TIMES.containsKey(name)) {
return 0;
}
return printTimer(name, START_TIMES.remove(name), System.currentTimeMillis());
}
private static long printTimer(String name, long start, long end) {
LOG.info(name + " timer ran for " + (end - start) + "ms");
return end - start;
}
}