hi ..hope this isnt silly question..what are essential package you should know..what an employer aspects you to know...i know lang is essential..i am talking on the whole..
Hm... Actually I know 40% of the API. I guess that non of it is obrigatory for you to know - a simple search at Google and you can find the documentation.
Now, other packages (for Graphics, 3D stuff) it'll depend of your project. I guess that for Graphics one of the best is JFreeChart. For 3D, JMonkey Engine and for image processing, JAI.
I couldn't answer your question very well because I did not understand it.
You mean just the names of the packages? Did any one ask you this in an interview? Generally, in my point of view, knowledge lies in abstraction than in detials.
I'd say the two packages you can't live without are java.lang and java.util. For the rest it is up to the code you're developing: - java.io for file / stream code - java.net for network code - java.awt and javax.swing and their subpackages for user interfaces