Bug in ThreadLocal documentation?

Question

This example shows up in the docs here: http://download.oracle.com/javase/6/docs/api/java/lang/ThreadLocal.html
<code>
import java.util.concurrent.atomic.AtomicInteger;
public class UniqueThreadIdGenerator {
private static final AtomicInteger uniqueId = new AtomicInteger(0);
private static final ThreadLocal < Integer > uniqueNum =
new ThreadLocal < Integer > () {
@Override protected Integer initialValue() {
return uniqueId.getAndIncrement();
public static int getCurrentThreadId() {
return uniqueId.get();
} // UniqueThreadIdGenerator
</code>
I believe that getCurrentThreadId() should be changed to:
<code>
return uniqueNum.get();
</code>
Otherwise initialValue is never executed. Also, uniqueId is not ThreadLocal so all threads calling getCurrentThreadId() will return the same value all the time.
Am I missing something?
Sorry for the code formatting, it's not clear to me how to properly format it for this forum.

Answer 1

843904 wrote:
I believe that getCurrentThreadId() should be changed to:
return uniqueNum.get();Otherwise initialValue is never executed. Correct. That's a bug in the doc.
Also, uniqueId is not ThreadLocal so all threads calling getCurrentThreadId() will return the same value all the time.Wrong. At least, wrong once you make the above fix.
Both of which you could have found out by simply running, fixing, and re-reunning the code. At which point your real question becomes, "Why do the different threads not share the same value?"
Remember, once you fix the typo, uniqueId is only ever used by initialValue(), which is only called once per thread. So thread T1 calls uniqueNum.get() the first time, which calls initialValue(), which, which calls getAndIncrement(). From then on, any time T1 calls uniqueNum.get(), it's just getting the value that was set the first time by initialValue(). T1 never looks at uniqueId again. T2 comes in, does the same thing on its first call to get(), and gets the next ID, and then never looks at uniqueId again. And so on...
Am I missing something?Apparently either the fact that each thread only calls initialValue() once, or the fact that initialValue() is the only place that looks at uniqueId. Maybe both. :-)
Sorry for the code formatting, it's not clear to me how to properly format it for this forum.[code] and [/code] or just {code} before and after.