Kafka SimpleConsumer not working

Hi,
I am using kafka for the first time, and was running the sample from
https://cwiki.apache.org/confluence/display/KAFKA/0.8.0+SimpleConsumer+Example

However, I cannot read any data from kafka. The kafka has 10 partitions,and
I tried to read from any of them. The fetch can succeed, however, the
message size returned is always 0( System.out
.println("the message size is" + messageSet
.kafka$javaapi$message$ByteBufferMessageSet$$underlying()
.size());). Is there something apparent missing for my case?

while (a_maxReads > 0) {
if (consumer == null) {
consumer = new SimpleConsumer(leadBroker, a_port, 100000,
10240 * 1024, clientName);

System.out.println("start fetching");
System.out.println("the readoffset is" + readOffset);

FetchRequest req = new FetchRequestBuilder().clientId(clientName)
.addFetch(a_topic, a_partition, readOffset, 1000000)
.build();
FetchResponse fetchResponse = consumer.fetch(req);
System.out.println("finish fetching");
if (fetchResponse.hasError()) {
numErrors++;
// Something went wrong!
short code = fetchResponse.errorCode(a_topic, a_partition);
System.out.println("Error fetching data from the Broker:" +
leadBroker + " Reason: " + code);
if (numErrors > 5)
break;
if (code == ErrorMapping.OffsetOutOfRangeCode()) {
// We asked for an invalid offset. For simple case ask for
// the last element to reset
readOffset = getLastOffset(consumer, a_topic, a_partition,
kafka.api.OffsetRequest.LatestTime(), clientName);
continue;

consumer.close();
consumer = null;
leadBroker = findNewLeader(leadBroker, a_topic, a_partition,
a_port);
continue;

numErrors = 0;

long numRead = 0;
System.out.println("The topic is:" + a_topic + " partition is : " +
a_partition);
ByteBufferMessageSet messageSet = fetchResponse.messageSet(a_topic,
a_partition);
System.out
.println("the message size is" + messageSet
.kafka$javaapi$message$ByteBufferMessageSet$$underlying()
.size());
for (MessageAndOffset messageAndOffset: messageSet) {
long currentOffset = messageAndOffset.offset();
if (currentOffset < readOffset) {
System.out.println("Found an old offset: " + currentOffset + "
Expecting: " + readOffset);
continue;

readOffset = messageAndOffset.nextOffset();
ByteBuffer payload = messageAndOffset.message().payload();

byte[] bytes = new byte[payload.limit()];
payload.get(bytes);
System.out.println(String.valueOf(messageAndOffset.offset()) + ": "
+ new String(bytes, "UTF-8"));
numRead++;
a_maxReads--;

if (numRead == 0) {
try {
Thread.sleep(1000);
} catch (InterruptedException ie) {}

Thanks much!
Chen