Added comments to explain offset field better

extensions/parquet/base/src/main/java/io/deephaven/parquet/base/ColumnPageReaderImpl.java

@@ -54,11 +54,19 @@ public class ColumnPageReaderImpl implements ColumnPageReader {
  private final Path filePath;
  private final List<Type> fieldTypes;
 
+ /**
+ * Stores the offset from where the next byte should be read. Can be the offset of page header if
+ * {@link #pageHeader} is {@code null}, else will be the offset of data.
+ */
  private long offset;
  private PageHeader pageHeader;
  private int numValues;
  private int rowCount = -1;
 
+ /**
+ * @param offset The offset for page header if supplied {@code pageHeader} is {@code null}. Else, the offset of data
+ * in the page.
+ */
  ColumnPageReaderImpl(SeekableChannelsProvider channelsProvider,
  CompressorAdapter compressorAdapter,
  Supplier<Dictionary> dictionarySupplier,
@@ -108,6 +116,10 @@ public IntBuffer readKeyValues(IntBuffer keyDest, int nullPlaceholder) throws IO
  }
  }
 
+ /**
+ * If {@link #pageHeader} is {@code null}, read it from the {@link #offset} and increment the {@link #offset} by the
+ * length of page header. Also, read the number of values in the page from the header.
+ */
  private synchronized void ensurePageHeader(SeekableByteChannel file) throws IOException {
  if (pageHeader == null) {
  offset = file.position();