Today while Getting Started with the Closure Compiler Application I ran into the following trouble:

jj5@mercy:~/Desktop/compiler-latest$ java -jar compiler.jar --help

Exception in thread "main" java.lang.UnsupportedClassVersionError: com/google/javascript/jscomp/CommandLineRunner : Unsupported major.minor version 51.0
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
        at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: com.google.javascript.jscomp.CommandLineRunner. Program will exit.

To fix the problem I installed jdk-7:

root@mercy:/home/jj5# apt-get install openjdk-7-dbg openjdk-7-demo openjdk-7-doc openjdk-7-jdk openjdk-7-jre