parquet-tools/src/main/java/org/apache/parquet/tools/command/MergeCommand.java
private void checkParquetFiles(List<Path> inputFiles) throws IOException {
    if (inputFiles == null || inputFiles.size() <= 1) {
      throw new IllegalArgumentException("Not enough files to merge");
    }

    for (Path inputFile: inputFiles) {
      FileSystem fs = inputFile.getFileSystem(conf);
      FileStatus status = fs.getFileStatus(inputFile);

      if (status.isDir()) {
        throw new IllegalArgumentException("Illegal parquet file: " + inputFile.toUri());
      }
    }
  }
private void checkParquetFiles(List<Path> inputFiles) throws IOException {
    if (inputFiles == null || inputFiles.size() <= 1) {
      throw new IllegalArgumentException("Not enough files to merge");
    }

    for (Path inputFile: inputFiles) {
      FileSystem fs = inputFile.getFileSystem(conf);
      FileStatus status = fs.getFileStatus(inputFile);

      if (status.isDir()) {
        throw new IllegalArgumentException("Illegal parquet file: " + inputFile.toUri());
      }
    }
  }
Check input files basically.
ParquetFileReader will throw exception when reading an illegal parquet file.

@param inputFiles files to be merged.
@throws IOException