Points to note when getting started in mapreduce

In mapreduce, for example, there are the following words:

I love beijing
i love cina
beijing is the captial of china
   When the statistics are as follows:




Note that in the above figure, the leftmost offset is the first 1, and then I LOVE CHINA In, I is the 4th word; so the offset is 4;
then the word segmentation is performed,
and then K3 is the KEY that classifies each word, and then V3 (1, 1), that is, the word I, the frequency of statistics ;


Related mapper writing:

public class WordCountMapper extends Mapper<LongWritable, Text, Text, LongWritable> {

	@Override
	protected void map(LongWritable key, Text value, Context context)
			throws IOException, InterruptedException {
		/*
		 * key: the entered key
		 * value: data I love Beijing
		 * context: Map context
		 */
		String data= value.toString();
		//Participle
		String[] words = data.split(" ");
		
		// output each word
		for(String w:words){
			context.write(new Text(w), new LongWritable(1));
		}
	}

}

reduce:

public class WordCountReducer extends Reducer<Text, LongWritable, Text, LongWritable>{

	@Override
	protected void reduce(Text k3, Iterable<LongWritable> v3,Context context) throws IOException, InterruptedException {
		//v3: is a set, each element is v2
		long total = 0;
		for(LongWritable l:v3){
			total = total + l.get();
		}
		
		// output
		context.write(k3, new LongWritable(total));
	}

}

Main program:

public class WordCountMain {

	public static void main(String[] args) throws Exception {
		//Create a job = map + reduce
		Configuration conf = new Configuration();
		
		//create a job
		Job job = Job.getInstance(conf);
		//Specify the entry of the task
		job.setJarByClass(WordCountMain.class);
		
		//Specify the mapper of the job
		job.setMapperClass(WordCountMapper.class);
		job.setMapOutputKeyClass(Text.class);
		job.setMapOutputValueClass(LongWritable.class);
		
		//Specify the reducer of the job
		job.setReducerClass(WordCountReducer.class);
		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(LongWritable.class);
		
		// Specify the input and output of the task
		FileInputFormat.setInputPaths(job, new Path(args[0]));
		FileOutputFormat.setOutputPath(job, new Path(args[1]));		
		
		//Submit the task
		job.waitForCompletion(true);
	}