title: java.lang.IllegalArgumentException: Can not create a Path from a null string
body: <pre><code>**“Path inputPath = new Path(job.get(KnnDriver.QUERY_DIR_PROPERTY));”**//it  gives  out   error。
      if (inputPath == null)
          throw new UnsupportedOperationException("ERROR: query directory not set");
      job.setInputFormat(NonSplitableTextInputFormat.class);
      NonSplitableTextInputFormat.addInputPath(job, inputPath);
      Path outputPath = new Path(job.get(KnnDriver.RESULT_DIR_PROPERTY));
      FileSystem.get(job).delete(outputPath, true);
      // Change to FileOutputFormat to see output
  
      job.setOutputFormat(TextOutputFormat.class);
      TextOutputFormat.setOutputPath(job, outputPath);
  
      KnnDriver.run(job);
  }
  </code></pre>
  
  <p>I  have  taged  the   error  in  the  text。
  The  following   will   be the   static  constant  I have set  in  knnDriver.</p>
  
  <pre><code>public static final String QUERY_DIR_PROPERTY = "knn.query.dir";
  public static final String RESULT_DIR_PROPERTY = "knn.result.dir";
  public static final String THRESHOLD_PROPERTY = "knn.sim.threshold";
  </code></pre>