TomWij, it depends on the potential employer. If the employer understand the interchangeability of knowledge among engineers, it should not be a problem. Unfortunately, most aren't like that. In fact, I only know of one employer in the commercial sector that does that. Government agencies and defense contractors are usually much better at this. That is, they give you a chance to apply for a job on X technology provided that you can demonstrate proficiency in a similar technology (or that you have very strong engineering skills.)
My suggestion to you is not to worry about interview questions. It is a hit and miss. Either you get an interview with silly language questions that you can pass just by looking at the Javadocs (if you are good enough.) Or you get hammered with a real interview that you cannot possibly answer without having actual work experience.
For the later, the reason being is that in Java development, it is a lot more than just knowing the language. You have to be proficient in the JEE architecture stack (sometimes down to the database/Java interoperability... typically Oracle or MySQL); you need to know either the standards (EJB 3.x, JSF, JPA) or alternate stacks (Spring, etc.)
I have no doubt that if you are good in C# you would have no problem doing well in Java. The problem is convincing employers that you are good without having experience. It is the stupid chicken-n-egg problem that plagues everybody in the software industry, not just us Java and C# developers.
If you feel that you really need to switch to Java, but have a hard time getting a break w/o work experience because employers are simply too stupid to give you one, I'd suggest you work on becoming a Sun certified Professional.
I'd suggest you work your way through the following, at a minimum:
Sun Certified Java Programmer (SCJP)
Sun Certified Web Component Developer (SCWCD)
Sun Certified Developer for Java Web Services (SCDJWS)
These three should start helping you a bit. However, I would strongly suggest you go for the following as well:
Sun Certified Business Component Developer (SCBCD)
Sun Certified Enterprise Architect (SCEA)
It is not a guarantee, but it will improve your odds. Taking the first 3 should take about a year. All five should take about 2. So you have to look at it as a mid-term investment. Even if you land a Java job without completing these, I'd suggest you do.
You can always try with defense contractors looking for Java developers (Lockheed Martin, General Dynamics, Northon Grumman, Honeywell, Harris) or with government agencies (DOD, DOE, DOT, which you canfind at usajobs.gov... if you are in the USA, though.). They'll be far more willing to take a good C# programmer for a Java position than a commercial entity.
Then, you earn your bones on the Java field and you are in.
I do wish you luck you get a chance to work on Java. Although the JVM and .NET have a lot in c