Monday, September 10, 2012

Top 10 Tricky Java interview questions and Answers

What is a tricky question? Well, tricky Java interview questions are those questions which has some surprise element on it. If you try to answer a tricky question with common sense, you will most likely fail because they require some specific knowledge. Most of the tricky Java questions comes from confusing concepts like function overloading and overriding,  Multi-threading which is really tricky to master, character encoding, checked vs unchecked exceptions and subtle Java programming details like Integer overflow. Most important thing to answer a tricky Java question is attitude and analytical thinking , which helps even if you don't know the answer. Anyway in this Java article we will see 10 Java questions which is real tricky and requires more than average knowledge of Java programming language to answer them correctly. As per my experience there are always one or two tricky or tough Java interview question on any core Java or J2EE interviews, so its good to prepare tricky questions from Java in advance. If I take interview, I purposefully put these kind of question to gauge depth of candidate's understanding in Java. Another advantage of asking such question is surprise element, which is key factor to put candidate on some pressure during interviews. Since these questions are less common, there is good chance that many Java developer doesn't know about it.


10 Tricky Java interview question - Answered

Tricky Java interview question and answer for experienced programmerHere is my list of 10 tricky Java interview questions, Though I have prepared and shared lot of difficult core Java interview question and answers, But I have chosen them as Top 10 tricky questions because you can not guess answers of this tricky Java questions easily, you need some subtle details of Java programming language to answer these questions.

Question: What does the following Java program print?
public class Test {
    public static void main(String[] args) {
        System.out.println(Math.min(Double.MIN_VALUE, 0.0d));
    }
}
Answer: This questions is tricky because unlike the Integer, where MIN_VALUE is negative, both the MAX_VALUE and MIN_VALUE of the Double class are positive numbers. The Double.MIN_VALUE is 2^(-1074), a double constant whose magnitude is the least among all double values. So unlike the obvious answer, this program will print 0.0 because Double.MIN_VALUE is greater than 0. I have asked this question to Java developer having experience up to 3 to 5 years and surprisingly almost 70% candidate got it wrong.


Question : What will happen if you put return statement or System.exit () on try or catch block ? Will finally block execute?
This is a very popular tricky Java question and its tricky because many programmer think that no matter what, but finally block will always execute. This question challenge that misconcept by putting return statement in try or catch block or calling System.exit from try or catch block. Answer of this tricky question in Java is that finally block will execute even if you put return statement in try block or catch block but finally block won't run if you call System.exit form try or catch.


Can you override private or static method in Java ?
Another popular Java tricky question, As I said method overriding is a good topic to ask trick questions in Java.  Anyway, you can not override private or static method in Java, if you create similar method with same return type and same method arguments in child class then it will hide the super class method, this is known as method hiding. Similarly you cannot override private method in sub class because it's not accessible there, what you do is create another private method with same name in child class. See Can you override private method in Java or more details.


Question: What does the the expression 1.0 / 0.0 will return? will it throw Exception? any compile time error?
Answer : This is another tricky question from Double class. Though Java developer knows about double primitive type and Double class, while doing floating point arithmetic they don't pay enough attention to Double.INFINITY, NaN, and -0.0 and other rules that govern the arithmetic calculations involving them. Simple answer to this question is that it will not throw ArithmeticExcpetion and return Double.INFINITY. Also note that the comparison x == Double.NaN always evaluates to false, even if x itself is a NaN. To test if x is a NaN, one should use the method call Double.isNaN(x) to check if given number is NaN or not. If you know SQL, this is very close to NULL there.


Does Java support multiple inheritance ?
This is the trickiest question in Java, if C++ can support direct multiple inheritance than why not Java is the argument Interviewer often give. Answer of this question is much more subtle then it looks like, because Java does support multiple inheritance of Type by allowing interface to extend other interfaces, what Java doesn't support is multiple inheritance of implementation. This distinction also get blur because of default method of Java 8, which now provides Java, multiple inheritance of behaviour as well. See Why multiple inheritance is not supported in Java to answer this tricky Java question.


What will happen if we put a key object in a HashMap which is already there ?
This tricky Java questions is part of another frequently asked question, How HashMap works in Java. HashMap is also a popular topic to create confusing and tricky question in Java. Answer of this question is, if you put the same key again than it will replace the old mapping because HashMap doesn't allow duplicate keys. Same key will result in same hashcode and will end up at same position in bucket. Each bucket contains a linked list of Map.Entry object, which contains both Key and Value. Now Java will take Key object form each entry and compare with this new key using equals() method, if that return true then value object in that entry will be replaced by new value. See How HashMap works in Java for more tricky Java questions from HashMap.

Question : What does the following Java program print?
public class Test {
    public static void main(String[] args) throws Exception {
        char[] chars = new char[] {'\u0097'};
        String str = new String(chars);
        byte[] bytes = str.getBytes();
        System.out.println(Arrays.toString(bytes));
    }
}

Answer: The trikyness of this question lies on character encoding and how String to byte array conversion works. In this program, we are first creating a String from a character array, which just has one character '\u0097', after than we are getting byte array from that String and printing that byte. Since \u0097 is within the 8-bit range of byte primitive type, it is reasonable to guess that the str.getBytes() call will return a byte array that contains one element with a value of -105 ((byte) 0x97). However, that's not what the program prints and that's why this question is tricky. As a matter of fact, the output of the program is operating system and locale dependent. On a Windows XP with the US locale, the above program prints [63], if you run this program on Linux or Solaris, you will get different values.

To answer this question correctly, you need to know about how Unicode characters are represented in Java char values and in Java strings, and what role character encoding plays in String.getBytes(). In simple word, to convert a string to a byte array, Java iterate through all the characters that the string represents and turn each one into a number of bytes and finally put the bytes together. The rule that maps each Unicode character into a byte array is called a character encoding. So It's possible that if same character encoding is not used during both encoding and decoding then retrieved value may not be correct. When we call str.getBytes() without specifying a character encoding scheme, the JVM uses the default character encoding of platform to do the job. The default encoding scheme is operating system and locale dependent. On Linux, it is UTF-8 and on Windows with a US locale, the default encoding is Cp1252. This explains the output we get from runing this program on Windows machines with a US locale. No matter which character encoding scheme is used, Java will always translate Unicode characters not recognized by the encoding to 63, which represents the character U+003F (the question mark, ?) in all encodings.


If a method throws NullPointerException in super class, can we override it with a method which throws RuntimeException?
One more tricky Java questions from overloading and overriding concept. Answer is you can very well throw super class of RuntimeException in overridden method but you can not do same if its checked Exception. See Rules of method overriding in Java for more details.


What is the issue with following implementation of compareTo() method in Java
public int compareTo(Object o){
   Employee emp = (Employee) emp;
   return this.id - o.id;
}
where id is an integer number ?
Well three is nothing wrong in this Java question until you guarantee that id is always positive. This Java question becomes tricky when you can't guaranteed that id is positive or negative. tricky part is, If id becomes negative than subtraction may overflow and produce incorrect result. See How to override compareTo method in Java for complete answer of this Java tricky question for experienced programmer.


How do you ensure that N thread can access N resources without deadlock
If you are not well versed in writing multi-threading code then this is real tricky question for you. This Java question can be tricky even for experienced and senior programmer, who are not really exposed to deadlock and race conditions. Key point here is order, if you acquire resources in a particular order and release resources in reverse order you can prevent deadlock. See how to avoid deadlock in Java for a sample code example.


Question : Consider the following Java code snippet, which is initializing two variables and both are not volatile, and two threads T1 and T2 are modifying these values as following, both are not synchronized
int x = 0;
boolean bExit = false;

Thread 1 (not synchronized)
x = 1; 
bExit = true;

Thread 2 (not synchronized)
if (bExit == true) 
System.out.println("x=" + x);
Now tell us, is it possible for Thread 2 to print “x=0”?

Answer: It's impossible for a list of tricky Java questions to not contain anythign from multi-threading. This is the simplest one I can get. Answer of this question is Yes, It's possible that thread T2 may print x=0.Why? because without any instruction to compiler e.g. synchronized or volatile, bExit=true might come before x=1 in compiler reordering. Also x=1 might not become visible in Thread 2, so Thread 2 will load x=0. Now, how do you fix it? When I asked this question to couple of programmers they answer differently, one suggest to make both thread synchronized on a common mutex, another one said make both variable volatile. Both are correct, as it will prevent reordering and guarantee visibility. But best answer is you just need to make bExit as volatile, then Thread 2 can only print “x=1”. x does not need to be volatile because x cannot be reordered to come after bExit=true when bExit is volatile.


What is difference between CyclicBarrier and CountDownLatch in Java
Relatively newer Java tricky question, only been introduced form Java 5. Main difference between both of them is that you can reuse CyclicBarrier even if Barrier is broken but you can not reuse CountDownLatch in Java. See CyclicBarrier vs CountDownLatch in Java for more differences.


What is difference between StringBuffer and StringBuilder in Java ?
Classic Java questions which some people think tricky and some consider very easy. StringBuilder in Java was introduced in JDK 1.5 and only difference between both of them is that StringBuffer methods e.g. length(), capacity() or append() are synchronized while corresponding methods in StringBuilder are not-synchronized. Because of this fundamental difference, concatenation of String using StringBuilder is faster than StringBuffer. Actually its considered bad practice to use StringBuffer any more, because in almost 99% scenario, you perform string concatenation on same thread. See StringBuilder vs StringBuffer for more differences.


Can you access non static variable in static context?
Another tricky Java question from Java fundamentals. No you can not access non-static variable from static context in Java. If you try, it will give compile time error. This is actually a common problem beginners in Java face, when they try to access instance variable inside main method. Because main is static in Java, and instance variables are non-static, you can not access instance variable inside main. Read why you can not access non-static variable from static method to learn more about this tricky Java questions.

Now, its practice time, here are some questions for you guys to answer, these are contributed by readers of this blog, big thanks to them.
  1. When Singleton doesn't remain Singleton in Java?
  2. is it possible to load a class by two ClassLoader?
  3. is it possible for equals() to return false, even if contents of two Objects are same?
  4. Why compareTo() should be consistent to equals() method in Java?
  5. When do Double and BigDecimal give different answers for equals() and compareTo() == 0. 
  6. How does "has before" apply to volatile work?
  7. Why is 0.1 * 3 != 0.3,
  8. Why is (Integer) 1 == (Integer) 1 but (Integer) 222 != (Integer) 222 and which command arguments change this.
  9. What happens when exception is thrown by a Thread?
  10. Difference between notify() and notifyAll() call?
  11. Difference between System.exit() and System.halt() method?
  12. Does following code legal in Java? is it example of method overloading or overriding?
  13. public String getDescription(Object obj){
       return obj.toString;
    }
    public String getDescription(String obj){
       return obj;
    }
    and
    public void getDescription(String obj){
       return obj;
    }


This was my list of Some of the most common tricky question in Java . It's not a bad idea to prepare tricky Java question before appearing for any core Java or J2EE interview. One or two open ended or tricky question is quite common in Java interviews.

Hungry for more Java Interview Question and Answer post, check out these articles
18 Java design pattern question asked in interviews
10 Java coding interview question answer for 2  to 4 years experience
Top 21 Most Frequently Asked Java Questions and Answers

35 comments:

  1. Great questions, How about adding tricky questions related to programming exercise ?

    ReplyDelete
    Replies
    1. One of the most tricky questions, I have face in a Java interview was, Does two object will always be equal, when there compareTo() method returns zero? I said, Yes, but that was not true. Though most of the classes will be equal if there compareTo() return true e.g. java.lang.String, but it's not mandatory. compareTo() may be inconsistent to equals(), which means compareTo() may return zero, but object will not be equal by equals() method. One of the prime example of this is java.math.BigDecimal class, whose equals() method return true if two BigDecimal object is equal in both value and scale e.g. 6.0 and 6.00 will not be equal, but compareTo() will return zero if both objects are compared. This was really tricky, until you had faced similar question previously. In another interview, my friend was asked this question little differently, Can we store BigDecimal class in TreeSet? obviously No, because of above reason, BigDecimal class can produce unexpected behavior when stored in SortedSet or SortedMap.

      Delete
  2. Good work done! It will definitely help me in my interviews... :)
    thanks .. :)

    ReplyDelete
  3. Hi ,

    I am able to override public static method declared in base class in its subclass.
    It didn't throw any compilation or runtime exception

    ReplyDelete
    Replies
    1. It won't because compiler will treat it as different method. This is called method hiding and its indeed one of the tricky Java question. Remember that this method is not overriding super class method as static method bonded using type information. That's the reason this Java question is tricky.

      Delete
  4. One of the tricky Java question I faced is "What is difference between Collection and Generics in Java", its not tricky because I don't know either collection or Generics but How can you compare Generics with Collection ?

    ReplyDelete
  5. Hi
    Is the line in codes correct.
    It's one of ur question.

    Can you access non static variable in static context?
    Another tricky Java question from Java fundamentals. "No you can not access static variable in non static context in Java". Read why you can not access non-static variable from static method to learn more about this tricky Java questions.

    ReplyDelete
    Replies
    1. Hi Ankur, Thanks for pointing out, you should read opposite i.e non static variable can not accessible from static context. This always confuse if you don't remember it and that's why its one of the tricky question.

      Delete
    2. Hi
      This line is not correct.
      "No you can not access static variable in non static context in Java".

      Delete
  6. all are tricky questions?

    ReplyDelete
  7. its really nice tricky question to read to face interview.

    ReplyDelete
  8. these are not tricky rather easy questions ...

    ReplyDelete
  9. i agree nice question to prepare before appearing for interview

    ReplyDelete
  10. These are definitely a good set of tricky questions that a candidate may face during an interview. I certainly enjoyed going through them but I have a different opinion regarding the last question: Can you access non static variable in static context?

    I think the answer to that question is a YES albeit one should note that you need an object reference of the associated class to access the variable (I assume this variable is an instance variable). And likewise you can directly access a static variable in non static context too. Please refer to this link for more information http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html

    ReplyDelete
  11. Is the compareTo code correct ? the references look to be wrong.

    ReplyDelete
  12. The explanation of the last question is little bit confusing. Actually static variables are always accessible in non static context however the reverse is not true.

    ReplyDelete
    Replies
    1. In static context, you can access non-static variables - by creating a new instance of the object. Hence the reverse is partly true.

      Delete
  13. public int compareTo(Object o){
    Employee emp = (Employee) emp;
    return this.id - o.id;
    }

    I think this is what it should be...

    public int compareTo(Object o){
    Employee emp = (Employee) o;
    return this.id - o.id;
    }

    ReplyDelete
    Replies
    1. Wrong, it should be:

      public int compareTo(Object o){
      Employee emp = (Employee) o;
      return this.id - emp.id;
      }

      Delete
    2. I should also mention that you shouldn't cast to Employee without knowing it's an Employee via instanceof, or just using Comparable generic interface.

      Delete
  14. Q 1. about try and catch block
    class Main
    {
    public static void main(String args[])
    {

    try
    {
    int a=2/0;
    System.exit(0);
    }


    catch(Exception e)
    {

    System.out.println("i am in catch block");

    }

    finally
    {

    System.out.println("finally");
    }


    }
    }
    /*OUTPUT
    i am in catch block
    finally*/

    finally block get executed if we place System.exit(0) in try block

    ReplyDelete
    Replies
    1. Before calling System.exit(0) you raised excetion "int a=2/0".
      Onces system.exit(0) is called finally will never called.
      plz Don't confused others :)

      Delete
    2. 2/0 is arithmetic exception so before going to system.exit it will jump to catch block and then finally but once system.exit(0) executes ...finally will not execute then .. try it with
      class Main
      {
      public static void main(String args[])
      {

      try
      {
      int a=2/1;
      System.exit(0);
      }


      catch(Exception e)
      {

      System.out.println("i am in catch block");

      }

      finally
      {

      System.out.println("finally");
      }


      }
      }

      Delete
    3. as per my java knowledge,try,finally blocks will execute parallely,because if single statement(inside try block first line or empty block) is executed corresponding catch may or may not execute but finally block will execute automatically,otherwise it wont execute,so without try&catch blocks we cant write finally block in java.

      Delete
  15. I was asked during an interview the difference between "arraylist" and "linkedlist" and when I should use them
    Also, found a site where you can give online Java mock interviews. Its www.preparestreet.com

    ReplyDelete
  16. What is so tricky about these question, to me they look most simplest question, which doesn't even qualify for Interviews. Tricky questions are those, who challenge your notion e.g.

    When Singleton doesn't remain Singleton in Java?
    is it possible to load a class by two ClassLoader?
    is it possible for equals() to return false, even if contents of two Objects are same?
    Why compareTo() should be consistent to equals() method in Java?
    is Following code legal in Java? is it example of method overloading or overriding?

    public String getDescription(Object obj){
    return obj.toString;
    }

    public String getDescription(String obj){
    return obj;
    }

    and

    public void getDescription(String obj){
    return obj;
    }

    Anyone disagree with my questions not being tricky?

    ReplyDelete
    Replies
    1. Actually your questions are better than Javin's Questions...

      Delete
    2. Trickier) When do Double and BigDecimal give different answers for equals() and compareTo() == 0.

      Delete
  17. Hi,
    Good collection of tricky questions, However I feel that the questions in the interview becomes tricky because we need to answer the same as what interviewer in thinking is correct, this is the most tricky part. It is always better to clarify the question correctly by rephrasing it.

    ReplyDelete
  18. Some trickier questions.

    1a) how do you prevent a return statement from calling finally.
    1b) how do you make a System.exit() call the finally block.

    3) Not sure multiple inheritance is the trickest question in Java. How about;
    3a) how does "has before" apply to volatile work?
    3b) why is 0.1 * 3 != 0.3,
    3c) why is (Integer) 1 == (Integer) 1 but (Integer) 222 != (Integer) 222 and which command arguments change this.

    7) how can you release synchronized locks in an order which is not the reverse order? (You can do this BTW)

    Good answers to some common questions, I look forward to some trickier questions. ;)

    ReplyDelete
    Replies
    1. I would add :
      What happens when exception is thrown in a Thread?
      Difference between notify and notifyAll call?
      Difference between System.exit() and System.halt() method?

      do you agree these are much trickier question for an average Java programmers?

      Delete
  19. Thanks for sharing these. I need some good Java interview questions and a few of these might help me out.

    ReplyDelete
    Replies
    1. No problem Tom, thanks for dropping by, If you have not read already, you may find my article about 10 Questions to make Programming Interview Less Costly useful as well.

      Delete
  20. Answer to the question "is it possible to load a class by two ClassLoader?" is Yes, it quite possible if you are using a custom class loader or working on managed environment which uses classloader e.g. web and application server. This is one more reason why you should use instanceof instead of getClass() while overriding equals() mehtod, otherwise equals() will return false even if object are same but classloader is different.

    ReplyDelete

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