public HasNext hasNext() {
    if (currentEntry == null) {
      return tryAdvanceEntry();
    } else {
      return HasNext.YES;
    }
  }
Try advance the stream if there is no entry yet. See the javadoc for {@link HasNext} for more
details about the meanings of the return values.
<p/>
You can call {@link #peek()} or {@link #next()} to get the actual {@link Entry} if this method
returns {@link HasNext#YES}.
hbase-server/src/main/java/org/apache/hadoop/hbase/replication/regionserver/WALEntryStream.java