java MainServer gives java.lang.UnsupportedClassVersionError: Unsupported major.minor version 51.0

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http://askubuntu.com – Compiling is easy but when using java to run the programs, it gives Exception found. I am using Ubuntu 12.04 without internet connections. Also, installed Oracle JDK7 and JRE7. Also did the update-alternatives command on java, javac and javaws. Changed the machine java in /etc/profiles. PATH is pointing to oracle java folder. I did many searches on Google about this topic but each time jdk and jre version are different. Also check version of jdk and jre using java -version and javac - version. Both of them are the same. The system also contains OpenJdk6 and OpenJdk7. So, how do I remove these (HowTos)