Eclipse appears exception in thread "main" Java.lang.NoClassDefFoundError

Last night, while running the Hadoop program, a new project was created and somehow it always appeared exception in thread "main" Java.lang.NoClassDefFoundError

The error is as follows:

Exception in thread "main" Java.lang.noclassdeffounderror:wordcount/class
caused By:java.lang.ClassNotFoundException:WordCount.class
At Java.net.urlclassloader$1.run (urlclassloader.java:217)
At Java.security.AccessController.doPrivileged (Native method)
At Java.net.URLClassLoader.findClass (urlclassloader.java:205)
At Java.lang.ClassLoader.loadClass (classloader.java:321)
At Sun.misc.launcher$appclassloader.loadclass (launcher.java:294)
At Java.lang.ClassLoader.loadClass (classloader.java:266)
Could not find the main class:WordCount.class. Program would exit.

I've been trying a lot of ways, including:

1. Modify Classpath

2. Modify Path

3, in Eclipse's property-> Java build Path-> Libraries-> Add External Jars, add Tools.jat, Dt.jar all

All right, try reloading eclipse, suddenly brainwave, think of the run as-> run configurations-> Main, as shown in the following figure:

Get rid of the circle's ". Class" and it's OK.

The following is a successful operation:

13/07/21 10:28:58 INFO JVM. Jvmmetrics:initializing JVM Metrics with Processname=jobtracker, sessionid=
13/07/21 10:28:58 WARN mapred. Jobclient:use Genericoptionsparser for parsing the arguments. Applications should implement Tool for the same.
13/07/21 10:28:58 WARN mapred.  Jobclient:no job jar file set. User classes May is found. Jobconf (Class) or Jobconf#setjar (String).
13/07/21 10:28:58 INFO input. Fileinputformat:total input paths to process:1
13/07/21 10:29:03 INFO mapred. Jobclient:running job:job_local_0001
13/07/21 10:29:03 INFO input. Fileinputformat:total input paths to process:1
13/07/21 10:29:03 INFO mapred. MAPTASK:IO.SORT.MB = 100
13/07/21 10:29:05 INFO mapred. Jobclient:map 0% Reduce 0%
13/07/21 10:29:05 INFO mapred. Maptask:data buffer = 79691776/99614720
13/07/21 10:29:05 INFO mapred. Maptask:record buffer = 262144/327680
13/07/21 10:29:05 INFO mapred. maptask:starting Flush of map output
13/07/21 10:29:05 INFO mapred. Maptask:finished spill 0
13/07/21 10:29:05 INFO mapred. TaskRunner:Task:attempt_local_0001_m_000000_0 is done. and is in the process of commiting
13/07/21 10:29:05 INFO mapred. Localjobrunner:
13/07/21 10:29:05 INFO mapred. Taskrunner:task ' Attempt_local_0001_m_000000_0 ' done.
13/07/21 10:29:05 INFO mapred. Localjobrunner:
13/07/21 10:29:05 INFO mapred. Merger:merging 1 sorted Segments
13/07/21 10:29:05 INFO mapred. Merger:down to the last Merge-pass, with 1 segments left of total size:34 bytes
13/07/21 10:29:05 INFO mapred. Localjobrunner:
13/07/21 10:29:05 INFO mapred. TaskRunner:Task:attempt_local_0001_r_000000_0 is done. and is in the process of commiting
13/07/21 10:29:05 INFO mapred. Localjobrunner:
13/07/21 10:29:05 INFO mapred. Taskrunner:task Attempt_local_0001_r_000000_0 is allowed to commit now
13/07/21 10:29:05 INFO output. fileoutputcommitter:saved output of Task ' attempt_local_0001_r_000000_0 ' to output
13/07/21 10:29:05 INFO mapred. Localjobrunner:reduce > Reduce
13/07/21 10:29:05 INFO mapred. Taskrunner:task ' Attempt_local_0001_r_000000_0 ' done.
13/07/21 10:29:06 INFO mapred. Jobclient:map 100% Reduce 100%
13/07/21 10:29:06 INFO mapred. Jobclient:job complete:job_local_0001
13/07/21 10:29:06 INFO mapred. Jobclient:counters:12
13/07/21 10:29:06 INFO mapred. Jobclient:filesystemcounters
13/07/21 10:29:06 INFO mapred. jobclient:file_bytes_read=26254
13/07/21 10:29:06 INFO mapred. jobclient:file_bytes_written=53072
13/07/21 10:29:06 INFO mapred. Jobclient:map-reduce Framework
13/07/21 10:29:06 INFO mapred. Jobclient:reduce input groups=4
13/07/21 10:29:06 INFO mapred. Jobclient:combine Output records=4
13/07/21 10:29:06 INFO mapred. Jobclient:map input Records=1
13/07/21 10:29:06 INFO mapred. Jobclient:reduce Shuffle bytes=0
13/07/21 10:29:06 INFO mapred. Jobclient:reduce Output records=4
13/07/21 10:29:06 INFO mapred. Jobclient:spilled records=8
13/07/21 10:29:06 INFO mapred. Jobclient:map Output bytes=54
13/07/21 10:29:06 INFO mapred. Jobclient:combine input Records=9
13/07/21 10:29:06 INFO mapred. Jobclient:map Output records=9
13/07/21 10:29:06 INFO mapred. Jobclient:reduce input records=4

A bug really can make people crazy ... You don't know how to die before you solve a bug ...

Hope to give a little help to the students who meet this bug.