JavaScript and Java are two complete different languages. Altough they share most of the syntax they still do not share how they work. I am not an expert in any of the two, but what Ican say is that they way you do thing in Java are different then what you do in java script, since they are aimed for two complete different things.
Also one is a scripting language, while the other one is a compiled language. I saw some classes in javascript, but they are completly different from java classes.
However having said that, if you are willing to learn Java then you should do it, but I sugets to start from the very basic, and then keep going up little by little.