@Override
  public void readFields(final DataInput in) throws IOException {
    this.length = in.readInt();
    this.bytes = new byte[this.length];
    in.readFully(this.bytes, 0, this.length);
    this.offset = 0;
  }
Return the offset into the buffer.
hbase-common/src/main/java/org/apache/hadoop/hbase/io/ImmutableBytesWritable.java