Questions every good Java Java EE Developer should be able to answer

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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 Java EE Developer   What questions do you think should a good Java Java EE 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

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why would you override the toString   method

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Top 5 J2EE JEE questions  The list of J2EE JEE middleware questions I have faced is exceptionally long  but here are the top 5 that I have been asked  and have lead to good discussions      What happens when an MDB encounters an exception that it cannot handle  This question usually leads to various interesting discussions about poison messages  error queues  etc     Given a JMS Topic  and a few consumers on different JVMs  questions on various scenarios with and without durable consumers  This question usually allows me to discuss in terms of when to use durable subscribers  when to use queues  etc   If stuck in a situation where accessing a Hibernate JPA POJO contents leads to exceptions  how would one resolve it  This leads to wonderful discussions about lazy loading  rehydrating  etc   It has even lead to comparing and contrasting JPA with Entity beans  I have found it useful to be prepared  and to be clear in concepts    How could a simple web service be provided  Any solution from simple web server based to sophisticated SOAP REST solutions  and any in between  should be good enough  Also  based on the interviewer  sometimes it leads to very interesting discussions on topics such as some design ideas - WSDL first  doc-style  SOAP intermediaries  etc    It might lead to questions such as listing improvements in JAX-WS over JAX-RPC  or SOAP1 2 over SOAP1 1  answers to which are usually based on how much I remember    JEE 5 resource injection This question is posed in many ways  starting from Service Locator pattern to javax naming Context questions    Another tricky question I find troubling  but have faced many times is  How are dependent libraries packaged into an archive  or Visibility of various classes in a bundled archive  If the discussion does not lead to class loader hierarchy of different application servers  resource archives  etc   it is best to resign and move on      Top 5 core Java questions    Questions on java util collections This is the mother of all questions  Once you can effectively land the interviewer in this area  and if you are prepared  the rest of the interview usually stays here  Be sure of knowing Set  List  Map  and the importance of Object equals   and Object hashCode   in every implementation of these interfaces  Refactoring questions These are good if the interviewer has an open mind  If the interviewer already has a specific solution in mind  and yours does not match his hers  you are pretty much doomed  It is best to agree with the answer  I understand other solutions are possible    Questions on multi-threading in JDK5  comparing with earlier JDK versions I have found it is best to be prepared with java util concurrent package classes  Doug Lea s book has most of the answers  What s new in JDK1 6 JDK1 7     This is a sure shot question with many interviewers  As much as I hate this  it is best to be prepared  At least remembering a few that I have worked with  and leading the discussion in some other direction  largely and effectively dodges and solves the problem    Patterns in Java API Time and again I have been asked to point out a GoF pattern in the Java API  Better be prepared for this one

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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

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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

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What do you like most   least about Java and why

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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

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Great questions and great answers by all It is nice to see such type of questions  I would like to add some more which can make this answer repository more rich   For Core Java Is main is keyword in java   More info  Java is plat form independent what about JVM   Can we have private constructor in class  If yes what the usage of it   Why we need to have constructors in abstract class though we cant instantiate it  More Info  Instead of above all there are many more question related to core java  More Info

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What s a deployment descriptor    If the candidate shudders involountarily  he has experience working with pre-3 0 EJBs

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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

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How does volatile affect code optimization by compiler

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Difference between and web server and a web container

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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

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Describe the differences between the  four   not three     types of inner class

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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  lt  100  i        s     Strings      are      immutable        so use StringBuilder StringBuffer to reduce memory footprint

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Usage of final keyword in method calls   For example why does the method test in below code does not give any compile error despite using final qualifier for the method parameter    class Name       private String name       public Name  String s            this name   s             public void setName String s            this name   s           private void test  final Name n        n setName  test

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

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This is the question I faced in my interview   Why is main method in Java called as public static void main String   args     Answer    1 main   must be declared public because it is invoked by JVM whenever the program execution starts JVM doesnot belong to our program package   Inorder to access main outside the package we have to declare it as public If we declare it as anything other than public it shows a Runtime Error but not Compilation time error  2 main   must be declared as static because if a method is declared as static then we can call that method outside the class using ClassName methodName      class Sample        static void fun                     System out println  Hello                     class Test         public static void main String   args                          Sample fun                The JVM will first Load the Test class and will check for the Commandline arguments and calls the main method as Test main     3 main   must be declared as void main   because JVM is not expecting any value from main   So it must be declared as void   If other return type is provided the it is a RunTimeError i e NoSuchMethodFoundError   4 main   must have String arguements as arrays because JVM calls main method by passing command line arguement As they are stored in string array object it is passed as an argument to main

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Explain the various access modifiers used in Java  I have had lots of people struggle with this  especially default access  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  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

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One sure is comparison of string  Difference between      String helloWorld    Hello World      helloWorld     Hello World       Hello World  equals helloWorld

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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

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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

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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

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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

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

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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

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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  amp  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

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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

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What is difference between String  StringBuffer and StringBuilder

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Simple questions such as    What is JRE and JDK   Why does java claim interoperability    Though these are very basic  many developers do not know the answers  I suggest these be asked before the code-related queries

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What is  System    out    println  in System out println   What happens when you call  put  on HashMap

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

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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

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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

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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

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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

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