2016-07-18 11:51:36,921 WARN [main] org.apache.hadoop.mapred.YarnChild: Exception running child : java.lang.RuntimeException: java.lang.ClassNotFoundException: Class MyNLineInputFormat$MyNLineInputFormatMapper not found
	at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:2195)
	at org.apache.hadoop.mapreduce.task.JobContextImpl.getMapperClass(JobContextImpl.java:186)
	at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:745)
	at org.apache.hadoop.mapred.MapTask.run(MapTask.java:341)
	at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:168)
	at java.security.AccessController.doPrivileged(Native Method)
	at javax.security.auth.Subject.doAs(Subject.java:415)
	at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1657)
	at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:162)
Caused by: java.lang.ClassNotFoundException: Class MyNLineInputFormat$MyNLineInputFormatMapper not found
	at org.apache.hadoop.conf.Configuration.getClassByName(Configuration.java:2101)
	at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:2193)
	... 8 more

Try adding this

Job job = new Job(conf, "wordcount");
job.setJarByClass(WordCount.class);