IllegalAcessException when creating a Block object via WritableFactories

When I ran the dfs namenode, I received an error message listed below. Changing Block class to be public will be able to fix the problem.

java.lang.RuntimeException: java.lang.IllegalAccessException: Class org.apache.hadoop.io.WritableFactories can not access a member of class org.apache.hadoop.dfs.Block with modifiers "public"
java.lang.RuntimeException: java.lang.IllegalAccessException: Class org.apache.hadoop.io.WritableFactories can not access a member of class org.apache.hadoop.dfs.Block with modifiers "public"
at org.apache.hadoop.io.WritableFactories.newInstance(WritableFactories.java:49)
at org.apache.hadoop.io.ObjectWritable.readObject(ObjectWritable.java:226)
at org.apache.hadoop.io.ObjectWritable.readObject(ObjectWritable.java:163)
at org.apache.hadoop.io.ObjectWritable.readObject(ObjectWritable.java:211)
at org.apache.hadoop.ipc.RPC$Invocation.readFields(RPC.java:88)
at org.apache.hadoop.ipc.Server$Connection.run(Server.java:154)
Caused by: java.lang.IllegalAccessException: Class org.apache.hadoop.io.WritableFactories can not access a member of class org.apache.hadoop.dfs.Block with modifiers "public"
at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65)
at java.lang.Class.newInstance0(Class.java:344)
at java.lang.Class.newInstance(Class.java:303)
at org.apache.hadoop.io.WritableFactories.newInstance(WritableFactories.java:45)
... 5 more