Refer to the Exam check section in Kathy Serra. You will definitely get lot questions from those concepts. Some of the questions I remember -
peek and poll in priority queue
subset in Trees, adding few values and then answering the content of two trees
Valid method declarations keeping in mind the legal overloading and overridding
compareTo and compare methods belong to which interfaces.
equals and HashCode
public static and final can be valid modifiers for interface variables.
Valid class path setting upon compiling and executing java files
Inheritance and Composition questions which says "Man has a best friend who is a Dog"
Use of assert.