core java

What is core java? Well, basically all the javadoc for JDK 1.6, 1.7, and now 1.8. Need to learn a large majority of the classes documented. As well as the difference between versions 1.6 to 1.8. Right now most companies are bound to one of those 3 versions. And so if you want to work for a company that is using one of these JVMs, you need to have a lot of the fundamentals mesmerized.

For Example, java.lang.Object, which has the basic methods used for creating your own collections. These need to be understood and how to overwrite them.

Also, need to know Collections class and all of its implementations. When you would want to use one over the other, etc.

Also sorting algorithms, you should know how to write binarysort, bubblesort, and mergesort on a whiteboard really fast. I suggest practicing it in the evening, a couple of days before an interview.