@Override
  public Object evaluate(DeferredObject[] arguments) throws HiveException {
    if (arguments[0].get() == null || arguments[1].get() == null) {
      return null;
    }

    String str = ObjectInspectorUtils.copyToStandardJavaObject(
        arguments[0].get(), strObjectInspector).toString();

    if (set == null) {
      String filePath = (String)ObjectInspectorUtils.copyToStandardJavaObject(
        arguments[1].get(), fileObjectInspector);
      loadFromFile(filePath);
    }

    return set.contains(str);
  }
IN_FILE(str, filename) returns true if 'str' appears in the file specified
by 'filename'.  A string is considered to be in the file if it that string
appears as a line in the file.

If either argument is NULL then NULL is returned.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFInFile.java