Why and What changes should be done to Driver class in mapreduce program when using stringtokenizer instead of split()

Question

I am new to java and hadoop. I was practicing mapreduce wordcount example where I came across 2 way of splitting the line in mapper class.

1st one

public class WordCountMapper extends
                Mapper<LongWritable, Text, Text, IntWritable> {
@Override
public void map(LongWritable key, Text value, Context output)
          throws IOException, InterruptedException {
String line = value.toString();
String[] splits = line.split(" ");

 for (String split : splits) {
 output.write(new Text(split), new IntWritable(1));
}
}
}

2nd one

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

private final static IntWritable one = new IntWritable(1);
private Text word = new Text();

     public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException
     {
         String line = value.toString();
         StringTokenizer tokenizer = new StringTokenizer(line);

         while (tokenizer.hasMoreTokens())
         {
            word.set(tokenizer.nextToken());
            context.write(word, one);
         }
     }
 }

Driver class

public class WordCountDriver 
{
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException 
    {
        Configuration conf = new Configuration();
        Job job = new Job(conf, "WordCount");
        job.setJarByClass(WordCountDriver.class);

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

        job.setMapperClass(WordCountMapper.class);
        job.setReducerClass(WordCountReducer.class);

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

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

        job.waitForCompletion(true);
    }
}

when I am using 1st one as mapper class I am getting proper output and if I change only mapper class to 2nd code and Keeping the rest same I get an error as follow

Exception in thread "main" java.lang.NoClassDefFoundError:

I understood that some changes should done in Driver class but my concern is to know what is causing this error!! This may help me understand concepts of java and Hadoop.