I have installed JDK 6, but I can't execute a Java program.

For example, I have made test.java. I compile it with javac tes.java and there's no error when I compile it, but when I want to execute that program it always displays an error. I execute the Java program with java tes.

Exception in thread "main" java.lang.UnsupportedClassVersionError: tes : 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: tes. Program will exit.

My javac version is 1.7.0, my java version is 1.6.0.

Here is my tes.java code:

class tes{

    public static void main(String[]args){
        System.out.println("hello");    
    }   

}