用eclipse运行 hadoop程序时，就出现这个错误，请问是什么问题？

java.lang.RuntimeException: java.lang.ClassNotFoundException: org.apache.hadoop.examples.Score$Map
at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:809)
at org.apache.hadoop.mapreduce.JobContext.getMapperClass(JobContext.java:157)
at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:569)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:305)
at org.apache.hadoop.mapred.Child.main(Child.java:170)
Caused by: java.lang.ClassNotFoundException: org.apache.hadoop.examples.Score$Map
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:247)
at org.apache.hadoop.conf.Configuration.getClassByName(Configuration.java:762)
at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:807)
... 4 more

路径没错，是不是包不对啊？我用的0.20的包
15/07/22 22:38:25 INFO mapred.JobClient: Task Id : attempt_201507222220_0009_m_000000_0, Status : FAILED
java.lang.RuntimeException: java.lang.ClassNotFoundException: