Questions every good Java/J2EE Developer should be able to answer?

Question

I was going through Questions every good .Net developer should be able to answer and was highly impressed with the content and approach of this question, and so in the same spirit, I am asking this question for Java/J2EE Developer.

What questions do you think should a good Java/J2EE programmer be able to answer?

I am marking this question as community wiki as it is not user specific and it aims to serve programming community at large.

Looking forward for some amazing responses.

EDIT: Please answer questions too, as suggested in the comments, so that people could learn something new regarding the language, too.

Answer 1

What is the relationship between hashCode() and equals()? What is the significance of these methods? What are the requirements for implementing them?

Answer 2

What is the difference between Set, Map and List?

I'm still amazed how many people don't know this one in a telephone interview.

Answer 3

Describe the differences between the "four" (not three ;)) types of inner class..

Answer 4

Trick question: What kinds of parameters are passed by reference in Java?

It's amazing how many people still parrot the "primitives are passed by value, objects are passed by reference" mantra.

Answer 5

Advantages and disadvantages of thread-safe classes and explicitly synchronized code and examples of good applications of both. It is often not correct to trust on thread-safe classes as guarantees for data consistency in multi-threaded applications.

Answer 6

Many questions and interviews are available at http://www.techinterviews.com/interview-questions/java and I don't really see value in copy / pasting a selection of them.

No, it's up to you to create your own compilation of things you think are important. Personally, I proceed always in two steps: first a few questions to get a basic idea of the experience and skills, then a problem solving situation. I'm indeed not convinced that being able to answer any known questions makes you a good or bad unknown problems solver. So, I prefer to ask people to solve a given problem, to give them some requirements, and ask them to write code (but not on paper). I give them some time to come back to me and check how they did it, their coding style, how they used the suggested APIs, etc.

That all being said, my favorite question is "what don't you like about Java?" (in the spirit of this one). It is really a excellent question, it gives you an immediate feedback on how much a candidate has used Java and explored its API and if he just religious about it or not (as the OP wrote).

Update: As suggested by CPerkins, a better wording for the question suggested above might be "What would you most like to see changed in Java?". And indeed, I prefer this way.

Answer 7

A simple questions such as, What is JRE and JDK.? Why does java claim interoperability? though these are very basic still many developers do no know What is I suggest these are more important to know before heading for the code related queries.

Answer 8

What is 'System', 'out', 'println' in System.out.println ? What happens when you call 'put' on HashMap ?

Answer 9

How about what is a session bean and describe some differences between stateless and stateful session beans.

Answer 10

If you are hiring graduates with Java "experience" a simple question like Write some code that will cause a NullPointerException to be thrown can distinguish which candidates have used Java recently, and didn't just stop when they finished their unit/course.

Answer 11

One sure is comparison of string. Difference between

String helloWorld = "Hello World";
helloWorld == "Hello World";
"Hello World".equals(helloWorld);

Answer 12

1. Explain the various access modifiers used in Java. I have had lots of people struggle with this, especially default access.
2. If you could change one thing about the Java language or platform what would it be? Good developers will have an answer here while those who aren't really interested in development probably don't care.
3. If their CV says something like they use EJB2.1 then ask about EJB3 to see what they know about it. The best developers will keep up with the latest developments even if they don't use the newer versions.

Answer 13

"What's a deployment descriptor?"

If the candidate shudders involountarily, he has experience working with pre-3.0 EJBs.

Answer 14

Can an interface extend multiple interfaces?

Most people answer "no", because they know java doesn't have multiple inheritance. But an interface can still extend multiple interfaces (but a class can't extend multiple classes). This doesn't lead to the diamond problem.

If the answer is "no", the interviewer should ask "why would it be forbidden?". Then you start thinking about it and you should realize that there is not problem with it.

So you learned something (by yourself) in the interview and you showed the interviewer that you are able to reason about classes, objects, inheritance, polymorphism, etc. It's actually much better than a candidate who knows the answer by heart but doesn't understand why

Answer 15

• What is the general contract when overriding equals?
• Is better option prefer lists or arrays?
• What are the generally accepted naming conventions?
• How serialization works?
• How to implement Comparable?
• What are the advantages of using JDBC's Prepared Statements?
• What is Java EE?
• What is a container and what services does it provide?

Answer 16

Difference between and web server and a web container

Answer 17

You said "Good","Developer". Here are my 2 cents too.. :)

• What does a "checked exception" mean?
• Which one is better to use and when: Assertions or Exceptions to handle unexpected conditions?
• Why String class is final? (or is it not? ;) )
• are the wait, notify and notifyAll methods in Object class?
• Why isn't Thread class final? Why would I extend Thread, ever?
• Why there are two Date classes; one in java.util package and another in java.sql?
• What happens if an exception is thrown in finally block? Is the remaining finally executed or not?
• There is a garbage collector alright, but then is memory leak totally absent in a Java applications? If not, how so?

For J2EE:

• Is it good to have instance/static variables in a servlet? Why not? Then where do you store "state"?
• continuing on above question: what & where is a "state" for a (web) application?
• What happens if I started creating/closing DB connections in "JSP"?
• What are the ways to handle JSP exceptions? try-catch? Hmmm.. is there anything else?

I can think many, many, many more of 'em but this'll do for now :)

Answer 18

How do threads work? What is synchronized? If there are two synchronized methods in a class can they be simultaneously executed by two threads. You will be surprised to hear many people answer yes. Then all thread related question, e.g. deadlock, starvation etc.

Answer 19

What do you like most / least about Java and why?

Answer 20

What is the difference between an abstract class and an interface? When would you use each of them?

Lots of Java developers don't know this, I asked most people on my computer science course at university and the vast majority could not answer it.

Answer 21

why would you override the toString() method?

Answer 22

What is the difference between J2SE and J2EE (or JSE and JEE)?

A developer should be able to point out that the enterprise edition is basically an interface definition (i.e. a specification) which can be implemented by vendors. Whereas the standard edition is an implementation in its own right

Answer 23

How does volatile affect code optimization by compiler?

Answer 24

What is difference between String, StringBuffer and StringBuilder?

Answer 25

When would you use Session Beans or Message Driven Beans ?

How are transactions handled in session beans ?

What is the difference between local and remote session beans ? This question is more about knowing that RPCs are in the picture or not, since a method that is exposed as both Local or Remote might still work differently (side-effects on parameters are possible with Locals, while not possible with Remotes).

A good test is: never ask the questions in the same order. We had the experience with offshoring that sometimes they were actually giving the response in the wrong order :-). As a result you should make sure that you can actually see the person you are interrogating.

Answer 26

A more pure Java question:

What is the difference between sleep and wait ? Not many people actually understand how wait is working.

How do you need to handle InterruptedExceptions ?

Answer 27

Q. Give a real world example scenario where you will use GenericServlet not HttpServlet ?

Answer 28

Write a program to accept two integers and output the largest of two numbers to a file at a location of your choice. Now describe what each statement does.

It's possible to drill down pretty deep starting from the significance of the import statement, right down to abnormal termination

Answer 29

Java/J2EE Interview Questions and Answers have some very useful ones.

Answer 30

What will be printed?

public void testFinally(){
    System.out.println(setOne().toString());

}

protected StringBuilder setOne(){
    StringBuilder builder=new StringBuilder();
    try{
        builder.append("Cool");
        return builder.append("Return");
    }finally{
        builder.append("+1");
    }
}

Answer: CoolReturn+1

A bit more difficult:

public void testFinally(){
    System.out.println(setOne().toString());

}

protected StringBuilder setOne(){
    StringBuilder builder=new StringBuilder();
    try{
        builder.append("Cool");
        return builder.append("Return");
    }finally{
        builder=null;  /* ;) */
    }
}

Answer: CoolReturn

Answer 31

Core: 1. What are checked and unchecked exceptions ? 2. While adding new exception in code what type (Checked/Unchecked) to use when ?

Servlet: 1. What is the difference between response.sendRedirect() and request.forward() ?

Answer 32

One thing many Java programmers don't know is that Strings are immutable, so use StringBuilder or StringBuffer!

String s = "";
for(int i = 0; i < 100; i++) {
  s += "Strings " + "are " + "immutable, " + " so use StringBuilder/StringBuffer to reduce memory footprint";
}