Address comment and add javadoc

Author: yuzhu

parquet-hadoop/src/main/java/org/apache/parquet/hadoop/ParquetFileReader.java

@@ -729,7 +729,22 @@ public static ParquetFileReader open(InputFile file, ParquetReadOptions options,
     return new ParquetFileReader(file, options, f);
   }
 
-  protected SeekableInputStream f;
+  /**
+   * Open a {@link InputFile file} with {@link ParquetMetadata footer} and {@link ParquetReadOptions options}.
+   *
+   * @param file    an input file
+   * @param footer  a {@link ParquetMetadata} footer already read from the file
+   * @param options parquet read options
+   * @param f       the input stream for the file
+   * @return an open ParquetFileReader
+   * @throws IOException if there is an error while opening the file
+   */
+  public static ParquetFileReader open(InputFile file, ParquetMetadata footer, ParquetReadOptions options, SeekableInputStream f)
+      throws IOException {
+    return new ParquetFileReader(file, footer, options, f);
+  }
+
+  protected final SeekableInputStream f;
   private final InputFile file;
   private final ParquetReadOptions options;
   private final Map<ColumnPath, ColumnDescriptor> paths = new HashMap<>();
@@ -930,6 +945,12 @@ public ParquetFileReader(InputFile file, ParquetReadOptions options) throws IOEx
     this(file, options, file.newStream());
   }
 
+  /**
+   * @param file    Path to a parquet file
+   * @param options {@link ParquetReadOptions}
+   * @param f       a {@link SeekableInputStream} for the parquet file
+   * @throws IOException if the file can not be opened
+   */
   public ParquetFileReader(InputFile file, ParquetReadOptions options, SeekableInputStream f) throws IOException {
     this.converter = new ParquetMetadataConverter(options);
     this.file = file;
@@ -938,6 +959,8 @@ public ParquetFileReader(InputFile file, ParquetReadOptions options, SeekableInp
     try {
       this.footer = readFooter(file, options, f, converter);
     } catch (IOException e) {
+      // In case that reading footer throws an exception in the constructor, the new stream
+	    // should be closed. Otherwise, there's no way to close this outside.
       f.close();
       throw e;
     }
@@ -971,6 +994,13 @@ public ParquetFileReader(InputFile file, ParquetReadOptions options, SeekableInp
     }
   }
 
+  /**
+   * @param file    Path to a parquet file
+   * @param footer  a {@link ParquetMetadata} footer already read from the file
+   * @param options {@link ParquetReadOptions}
+   * @param f       a {@link SeekableInputStream} for the parquet file
+   * @throws IOException if the file can not be opened
+   */
   public ParquetFileReader(InputFile file, ParquetMetadata footer, ParquetReadOptions options, SeekableInputStream f)
       throws IOException {
     this.converter = new ParquetMetadataConverter(options);