ee http://java.sun.com/javase/reference for more details.
psutton@E-machine:~/minecraft$ java -version
java version "1.6.0_22"
OpenJDK Runtime Environment (IcedTea6 1.10.4) (6b22-1.10.4-0ubuntu1~11.04.1)
OpenJDK Client VM (build 20.0-b11, mixed mode, sharing)
psutton@E-machine:~/minecraft$ java version
Exception in thread "main" java.lang.NoClassDefFoundError: version
Caused by: java.lang.ClassNotFoundException: version
	at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: version. Program will exit.
psutton@E-machine:~/minecraft$ java -version
java version "1.6.0_22"
OpenJDK Runtime Environment (IcedTea6 1.10.4) (6b22-1.10.4-0ubuntu1~11.04.1)
OpenJDK Client VM (build 20.0-b11, mixed mode, sharing)
psutton@E-machine:~/minecraft$ java -Xmx1024M -Xms512M -cp Minecraft.jar net.minecraft.LauncherFrame