The important areas one must know for Job Interviews are:

• Core Java Concepts (you can expect these questions even after 4 years)
• OO Concepts (try to develop practical experience all these concepts and impress by writing code in front of an interviewer)
• JSP and Servlets (life cycle methods, difference between GenericServlet and HttpServlet, Session handling, Cookies, URL-rewriting, annotations, Servlet filters, web.xml vs. annotations, JSP rags.. etc)
• Multithreading, concurrency and thread basics
• Usage of abstract class and interface
• User defined custom exceptions and its advantages
• String vs. StringBuffer vs. StringBuilder
• Java Collections (util package)
• Exception handling

All the best 👍

This is an extension to Ipseeta Priyadarshini 's answer:

Java4-6: Generics, @interface which allows you to write custom annotations, some of the Collection helpers, NIO

Java7: Try-with-resource, Strings in switch, catching multiple exceptions in one catch statement, NIO2

Java8: Lambdas, Streams, Default implementations in interfaces, new DateTime api.

Java9: Know about jshell and microbenchmarking (currently that's being done via JMH). Java9 coming soon: java9countdown.xyz

General things that might help you in the process is if you have a high-level understanding of how the garbage collector works and if you can name the different garbage collectors available.

Also understand access modifiers and their impact on code as well as the difference between private static and public static

Some knowledge on different frameworks might benefit your interview as well, but this is usually an added bonus, focus on the basics first if you only have limited time.

Otherwise, a google search for Java interview questions might help you as well, there are dozens of sites with collections of things that are typically asked. You'll learn something in the proces of going through those questions, even if it's for interview preparation.