Question

I am trying to implement in memory sorting,i.e. soring based on the no. of counts. I am facing the below problem, null pinter exception in output.collect in the close method of the Reduce class. Pls help!

Is my coding logic correct? I am keeping in memory the tokens from different instances of reduce method. Please help me! I wanted a sorted output of based on the TCounts.

package com.a;

import java.io.IOException;
import java.util.ArrayList;
import java.util.StringTokenizer;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.MapReduceBase;
import org.apache.hadoop.mapred.Mapper;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reporter;


public class Map1 extends MapReduceBase implements Mapper<LongWritable, Text, Text, 
Text >{
    public void map(LongWritable key, Text value, OutputCollector<Text, Text> output,
            Reporter reporter) throws IOException {

        StringTokenizer tokenizer = new StringTokenizer(value.toString());

        String tk = tokenizer.nextToken();
        String id = tokenizer.nextToken();
        String name = tokenizer.nextToken();

        StringTokenizer tkz = new StringTokenizer(name, ",");


        ArrayList<String> al = new ArrayList<String>();

        while(tkz.hasMoreTokens())
        {
            name = tkz.nextToken();
            al.add(name);
        }

        for(int i = 0; i<al.size(); i++)
        {
            output.collect(new Text(t+" "+al.get(i)), new Text("1"));
            System.out.println("out key:----->"+t+" "+al.get(i));
        }

    }

}



public class Reduce1 extends MapReduceBase implements Reducer<Text, Text, Text, Text>{
    //  @SuppressWarnings("unchecked")

    ArrayList<TCount> al = new ArrayList<TCount>();
    String key_str = null;
    private OutputCollector<Text, Text> output;



    public void reduce (Text key, Iterator<Text> values, OutputCollector<Text,
            Text> output, Reporter reporter) throws IOException {

        int sum = 0;

        while(values.hasNext())
        {
            String val = values.next().toString();
            sum = sum+Integer.parseInt(val);;

        }

        String str_val = String.valueOf(sum);
        key_str = key.toString();
        //output.collect(key, new Text(str_val));
        TCount tc = new TCount(key.toString(), sum);
        al.add(tc);


    }

    private Text t = new Text();
    private Text txt_key = new Text();


    public void close() throws IOException {
        Collections.sort(al);

        for(int i = 0; i<al.size(); i++)
        {

            String tkn = al.get(i).getT();
            System.out.println("token:-------------------> "+tkn);

            System.out.println("output: "+output);
            txt_key = new Text(t);
            txt = new Text(String.valueOf(al.get(i).getCount()));
            output.collect(txt_key, t);

        }
    }
}

Answer 1

In class Reduce1, you’re declaring the output object:

private OutputCollector<Text, Text> output; 

without initializing it -> so it’s null at this time.

Also in the method reduce() you pass a parameter of the same type (OutputCollector<Text, Text> output) -> so in this method, I guess you wanna say:

this.output=output;
// if the object is null, initialize it if you wanna use it

The thing is you’ll get a null pointer exception as long as your object is not initialized (instantiated).