Games : Recent update to Minecraft fails to launch (Mac OSX)

My user output tab continually reads as follows:

Exception in thread "main" java.lang.NoClassDefFoundError: net/minecraft/client/main/Main Caused by: java.lang.ClassNotFoundException: net.minecraft.client.main.Main at java.net.URLClassLoader$1.run(URLClassLoader.java:202) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:306) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:247)

Are there any suggestions for what I ought to do? Thank you.