@Ingvald wrote: Hello everybody,   I'm a beginner and i follow a tutorial on cloudera to do my first MapReduce but i have a problem when i run my Jar file :   Error: java.lang.NullPointerException at MonReducer.reduce(MonReducer.java:23) at MonReducer.reduce(MonReducer.java:1) at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:171) at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:627) at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:389) at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:164) 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:1917) at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:158)   Okay, so my problem is my reducer but i can't find the solution and i don't know how to debug it.     Did you see any errors ? How can i debug it ?   Thank you !!   I'm bumping up this question. I hope someone can help. ... View more