Exception in thread "main" java.lang.UnsupportedClassVersionError: a (Unsupported major.minor version 51.0) [duplicate]

Try sudo update-alternatives --config java from the command line to set the version of the JRE you want to use. This should fix it.

Copy the contents of the PATH settings to a notepad and check if the location for the 1.4.2 comes before that of the 7. If so, remove the path to 1.4.2 in the PATH setting and save it.

After saving and applying "Environment Variables" close and reopen the cmd line. In XP the path does no get reflected in already running programs.

Assuming you are using Eclipse, on a MAC you can:

1. Launch Eclipse.app
2. Choose Eclipse -> Preferences
3. Choose Java -> Installed JREs
4. Click the Add... button
5. Choose MacOS X VM as the JRE type. Press Next.
6. In the "JRE Home:" field, type /Library/Java/JavaVirtualMachines/1.7.0.jdk/Contents/Home
7. You should see the system libraries in the list titled "JRE system libraries:"
8. Give the JRE a name. The recommended name is JDK 1.7. Click Finish.
9. Check the checkbox next to the JRE entry you just created. This will cause Eclipse to use it as the default JRE for all new Java projects. Click OK.
10. Now, create a new project. For this verification, from the menu, select File -> New -> Java Project.
11. In the dialog that appears, enter a new name for your project. For this verification, type Test17Project
12. In the JRE section of the dialog, select Use default JRE (currently JDK 1.7)
13. Click Finish.

Hope this helps