ThreadPoolExecutor.setCorePoolSize goes to a lot of trouble to 
kill off excess idling threads, but the code is not correct.

--------------
import java.util.*;
import java.util.concurrent.*;

public class Bug4 {
    static void report(String label, ThreadPoolExecutor tpe) {
	System.out.printf("%10s: active=%d, submitted=%d, completed=%d%n",
			  label,
			  Thread.activeCount() - 1,
			  tpe.getTaskCount(),
			  tpe.getCompletedTaskCount());
    }

    public static void main(String[] args) throws Throwable {
	final int count = 8;
	final CyclicBarrier barrier = new CyclicBarrier(count + 1);
	ThreadPoolExecutor tpe =
	    new ThreadPoolExecutor(10, 30, 1L, TimeUnit.HOURS,
				   new LinkedBlockingQueue<Runnable>());

	report("newborn", tpe);

	for (int i = 0; i < count; i++)
	    tpe.execute(new Runnable() {
		    public void run() {
			try { barrier.await(); barrier.await(); }
			catch (Throwable t) { t.printStackTrace(); }
		    }});

	barrier.await();
	report("started", tpe);

	barrier.await();
	Thread.sleep(1000);
	report("idling", tpe);

	tpe.setCorePoolSize(count/2);
	Thread.sleep(1000);

	report("shrunk", tpe);

	System.out.printf("Shutting down...%n");
	tpe.shutdown();
	tpe.awaitTermination(1L, TimeUnit.HOURS);

	report("terminated", tpe);
    }
}
--------------
 $ jver 6 jr Bug4
==> javac -source 1.6 -Xlint:all Bug4.java
==> java -esa -ea Bug4
   newborn: active=0, submitted=0, completed=0
   started: active=8, submitted=8, completed=0
    idling: active=8, submitted=8, completed=8
    shrunk: active=8, submitted=8, completed=8
Shutting down...
terminated: active=0, submitted=8, completed=8