Help! Since I installed Robocode, my original Java isn't working anymore

Hello, please forgive me if this isn't the right place to post this. I'm a noob to Java; two months ago I installed the JDK from Oracle and played around with the examples from the book "Java - A Beginner's Guide". It worked nicely until I installed Robocode. Now, Robocode is working fine, but regardless which of the example programs I try to run, I get the same error:
 cd the_directory_that_contains_Example.class 
 java Example.class 
 Error: Could not find or load main class Example.class
I tried recompiling Example.class from Example.java, but that didn't help. 
I googled for this error; I found a nice reply at stackoverflow.com/questions/3005433/java-problem-could-not-find-main-class-helloworld, so I tried to explicitly point it to the class directory:
 java -classpath . Example.class
But this didn't help either. BTW, I had added the follwing two environment variables for Robocode, but from what I read, they shouldn't matter:
 JAVA_HOME="C:\Program Files\Java\jdk1.7.0_07"
 Path=C:\Program Files\Java\jdk1.7.0_07\bin;C:\WINDOWS\system32;...
Please help me with this. Thanks!