Could not find main class : Program will exit

Question

I was messing with the JRE setup on Eclipse. Originally the project was set to 1.6 and when set to 1.7, my program started showing a lot of errors. So when i switched back to 1.6, all the compile time errors vanished but i have a new problem now. This was shown on the console when i tried to run the started class containing the main function. The .class files all exist. Please advice.

java.lang.UnsupportedClassVersionError: Demo : Unsupported major.minor version 51.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(Unknown Source)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.security.SecureClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.access$000(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
Exception in thread "main" 

Answer 1

Version 51.0 means the are still class files compiled with Java 7. This files wont run with a JRE 6. You have to recompile them first (Rebuild project or clean compile using ant/maven).