Very simple problem with my first "Hello World" Java code

Question

I saved this code in FirstApp.java:

class FirstApp {
    public static void main (String[] args) {
        System.out.println("Hello World");
    }
}

Then this is what I get when trying to compile and run:

$ javac FirstApp.java
$ java FirstApp
Exception in thread "main" java.lang.UnsupportedClassVersionError: FirstApp : 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: FirstApp. Program will exit.

What I am doing wrong?

Answer 1

You may have multiple versions of the JDK installed. See: http://www.java.net/node/664117

It means that you compiled your classes under a specific JDK, but then try to run them under older version of JDK.