我是hadoop和java的初学者，我正在编写map，reduce函数，将一组纬度和经度基于接近度聚集在一起，并设置一个震级（一个簇中的lat，long pair的数量）和一个代表lat，long pair（到目前为止，这是遇到的第一个lat，long pair）
这是我的密码：

package org.myorg;

import java.io.IOException;
import java.util.*;

import org.apache.hadoop.fs.Path;
import org.apache.hadoop.conf.*;
import org.apache.hadoop.io.*;
import org.apache.hadoop.mapreduce.*;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;

import util.hashing.*;

public class LatLong {

 public static class Map extends Mapper<Object, Text, Text, Text> {
    //private final static IntWritable one = new IntWritable(1);

    public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
        String line = value.toString();
        String[] longLatArray = line.split(",");
        double longi = Double.parseDouble(longLatArray[0]);
        double lat = Double.parseDouble(longLatArray[1]);
        //List<Double> origLatLong = new ArrayList<Double>(2);
        //origLatLong.add(lat);
        //origLatLong.add(longi);
        Geohash inst = Geohash.getInstance();
        //encode is the library's encoding function
        String hash = inst.encode(lat,longi);
        //Using the first 5 characters just for testing purposes
        //Need to find the right one later
        int accuracy = 4;
        //hash of the thing is shortened to whatever I figure out
        //to be the right size of each tile
        Text shortenedHash = new Text(hash.substring(0,accuracy));
        Text origHash = new Text(hash);
        context.write(shortenedHash, origHash);
    }
 } 

 public static class Reduce extends Reducer<Text, Text, Text, Text> {

     private IntWritable totalTileElementCount = new IntWritable();
     private Text latlongimag = new Text();
     private Text dataSeparator = new Text();

     @Override
     public void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
      int elementCount = 0;
      boolean first = true;
      Iterator<Text> it= values.iterator();
      String lat = new String();
      String longi = new String();
      Geohash inst = Geohash.getInstance();

      while (it.hasNext()) {
       elementCount = elementCount+1;
       if(first)
       {
           lat = Double.toString((inst.decode(it.toString()))[0]);
           longi = Double.toString((inst.decode(it.toString()))[1]);
           first = false;

       }
       @SuppressWarnings("unused")
       String blah = it.next().toString();

      }
      totalTileElementCount.set(elementCount);
      //Geohash inst = Geohash.getInstance();

      String mag = totalTileElementCount.toString();

      latlongimag.set(lat+","+ longi +","+mag+",");
      dataSeparator.set("");
      context.write(latlongimag, dataSeparator );
     }
 }

 public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    Job job = new Job(conf, "wordcount");
    job.setJarByClass(LatLong.class);

    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(Text.class);

    job.setMapperClass(Map.class);
    job.setReducerClass(Reduce.class);

    job.setInputFormatClass(TextInputFormat.class);
    job.setOutputFormatClass(TextOutputFormat.class);

    FileInputFormat.addInputPath(job, new Path(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));

    job.waitForCompletion(true);
 }

}

我要参加npe。我不知道如何测试这个，我也无法在我的代码中找到错误。
hadoop错误：

java.lang.NullPointerException
    at util.hashing.Geohash.decode(Geohash.java:41)
    at org.myorg.LatLong$Reduce.reduce(LatLong.java:67)
    at org.myorg.LatLong$Reduce.reduce(LatLong.java:1)
    at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:176)
    at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:663)
    at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:426)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:255)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1132)
    at org.apache.hadoop.mapred.Child.main(Child.java:249)

geohash库中的decode函数返回一个双精度数组。任何指点都将不胜感激！谢谢你的时间！
edit1（测试后）：
我意识到问题在于reduce函数中需要有it.next（）.tostring（），而不仅仅是it.tostring（），但是当我做了这个更改并进行测试时，我发现了这个错误，我不知道为什么在while循环条件中检查hasnext（）时会出现这个错误。

java.util.NoSuchElementException: iterate past last value
    at    org.apache.hadoop.mapreduce.ReduceContext$ValueIterator.next(ReduceContext.java:159)
    at org.myorg.LatLong$Reduce.reduce(LatLong.java:69)
    at org.myorg.LatLong$Reduce.reduce(LatLong.java:1)
    at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:176)
    at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:663)
    at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:426)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:255)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1132)
    at org.apache.hadoop.mapred.Child.main(Child.java:249)

edit2（进一步测试）：解决方案
我不止一次地调用它.next（），作为一个迭代器，这只会导致它继续，两次，在最后一次迭代中，它检查条件并输入，但我随后调用它.next（）两次，这会导致问题，因为只有一个下一个元素（最后一个）