Core Language: general syntax, primitive types and strings, classes and interfaces, etc. Collections APIs: core interfaces (Collection, List, Set, Map), implementations and their performance characteristics, etc. Concurrency: Thread and Runnable basics, the synchronized keyword and its semantics, the "java.util.concurrent" API, etc. Relevant APIs, Libraries, and Frameworks: Swing, J2EE (Servlets, JSP, JDBC), MVC, ORM, RMI, ImageIO, NIO, etc. Choose a couple that are important to your organization/project and see what they know.