For io
you should know all the input and ouput streams, all the readers and writers, all the buffered streams etc. Primarily you should know how they function, and how you can chain them together. From what I've read, you don't need to know them inside out, just know all the different constructors, and how each of them function. Note that you don't need to know about socket programming for the exam, but it's a good idea since it helps clarify some of this.