Java Questions in Interview

[calvinho]

What are the thoughest java questions that u've encountered in an interview for a java-related jobs?
Pls raise some...

[mousami bhattacharya]

I was asked how the Garbage Collector algorithm of JVM works.
Any answers ??

[Mark Herschberg]

I ask this question of every candidate at every level. Well, I ask, "how does garbage collection work?" Here's generally what I get. The answers can be placed into handful of categories, although obviously answers can blur the line between the two, depending on how much detail is given.

• Automcatic
  "It's automatic."
  When I get this, usually form junior people, I prompt them for more details.
  
• Reference counting
  "It does reference counting, and when the references are 0, it collects it."
  This, with minimally more information, is baseline acceptable for a kid out of school. It's correct, but doesn't impress me. I may ask for more details.
  
• Mark and sweep/algorithm
  This answer goes into more detail about how it keeps reference counts. Talks a little about the algorithms used to determine if something is collectable. May comment on shortcomings of the algorithm. this is good for entry level people, and the minimum I expect of people with a few years experience.
  
• GC implementation details
  This builds upon the mark and sweep answer. The candidate will usually talk about advantages of automatic GC vs. languages without automatic memory management. May talk about how the HotSpot JVM uses a different algorithm. Advanced engineers will talk about reference classes (e.g. weak references). This is above average to above for mid-level engineers, dpeending on how much detail I get.
  
• Extensive GC knowledge
  Begins covering basic GC ideas. Talks about the pros and cons of different GC implementations, including handless vs. handle-based, single threaded vs. multithreaded, accurracy, concurrency, RTOS systems, etc. I like to see this in architects, although not all of them, even some good ones know it in this much detail. or engineers at other levels, this knowledge would deeply impress me.
  


--Mark

[mohit joshi]

Hello Mark,
Interesting post. GC is a complicated and interesting subject of study in itself. Do you mind sharing though, the percentage of candidates who have sound knowledge of GC, say who reach level 4 and 5 of your list of points..
Regards
Mohit Joshi

[Buzz Andersen]

The tough interview questions I have had have tended to be more language-neutral. Some of the most memorable:
- You have a gold bar, and you need to pay an employee 1/7 of this bar each day for a week. You can only cut it twice. How do you accomplish this? [Amazon.com]
- Sketch out a quick program to compute fibonacci numbers. [Amazon.com]
- You have an array 0-99 which contains a sequence of numbers 0-100. Obviously one number is missing out of the sequence. What is the most efficient way to determine which number is missing? [National Instruments]
Unfortunately I didn't get either of these jobs, despite answering the latter two questions correctly . The NI guy told a friend of mine who works there that I "didn't seem enthusiastic enough." Well sorreeey!
Oh well, live and learn...
Buzz
[This message has been edited by Buzz Andersen (edited August 30, 2001).]

[Mark Herschberg]

Originally posted by mohit joshi:
Hello Mark,
Interesting post. GC is a complicated and interesting subject of study in itself. Do you mind sharing though, the percentage of candidates who have sound knowledge of GC, say who reach level 4 and 5 of your list of points..
Regards
Mohit Joshi

Of the 60 some people I've interviewed, maybe 2-4 could reach about level 4. No one was at level 5 (of course, I met some smart architects who just only had 1-2 years of Java, so didn't know the nitty gritty details yet).
--Mark

[Mark Herschberg]

Originally posted by Buzz Andersen:

- You have a gold bar, and you need to pay an employee 1/7 of this bar each day for a week. You can only cut it twice. How do you accomplish this? [Amazon.com]

That's a good one. My only answera are outside the box problems, like melt it down.

Originally posted by Buzz Andersen:

[Amazon.com]
- You have an array 0-99 which contains a sequence of numbers 0-100. Obviously one number is missing out of the sequence. What is the most efficient way to determine which number is missing? [National Instruments]

Add 'em all up. If the sum is S and the missing number is X, then 5050 - S = X.
Alternatively, I would say write a trivial hash algorithm, and put all numbers into a hash table, such that number X, maps to bucket X. The mepty bucket is the missing number. Hence the solution is O(n). (But addition, while also O(n) is probably faster.)
I'll skip the fibonacci, which everyone probably knows. Here's one my friend likes to ask. You have two vials of pills of type A and type B. They look, smell, taste, etc alike (you can't tell them apart). You have 30 of each, and each day you must take exactly 1 of each. On the 15th day, you accidentally take out 2 from container A, and 1 from container B. But you weren't paying attention, and don't, of the 3 in your hand, know which is which. As these pills are very expensive, you don't want to throw them away. What do you do so you don't waste any pills.

--Mark

[Tighe Fagan]

Take one pill out of the more full jar,
cut the four in half. Make two piles
of halfs. Take one pile today, the
other tomorrow and you should be back on track.
Another one: You have a 3-gallon jar,
a 5-gallon jar, and unlimited water. How
do you get exactly 4 gallons in the 5 gallon jar?

[Florence Cheung]

step1: fill the 5 gallon jar.
step2: fill the 3 gallon jar with the water in the 5 gallon jar.
step3: empty the 3 gallon jar, at this point, there is 2 gallons water in the 5 gallon jar.
step4: pour the 2 gallons water in the 5 gallon jar into the 3 gallon jar. Now the 5 gallon jar is empty. There is 2 gallons water in the 3 gallon jar.
step5: fill the 5 gallon jar again.
step6: fill the 3 gallon jar with the water in the 5 gallon jar.There is 4 gallons water left in the 5 gallon jar.

[Karthik Guru]

Java does not use Reference counting algorithm i feel.
because then it means we w'd've a definitive garbage collection which is'nt the case i guess.
We just are'nt sure when finalize gets called. Had it been reference counting then we can be sure of finalize getting called ( i mean when).
I was asked the same question in a interview i attended last week!!
A language like say python i guess uses reference counting which i guess has lots of advantages. But then i guess it fails to resolve cyclic references?
So they use m&s in such cases.
Indeterministic garbage collection according to me is a problem especially when using scarce resources.
I knew it before but i badly felt the need for deterministic GC only when i started working :-)
We encountered this while usign our company's framework (esp the DatabaseManager part of it.)
After getting an access to a connection they expected us to close that as well.
(i write the business logic and obviously am the client as far as the DBManager is concerned and to place the onus of closing the connection after using it (on me) is'nt a good idea or is it?)
Problem: DBManager guys are'nt just sure when am done with the connection object ( yeah they did have a timeout though to do the clean up things) But to make things work perfectly they wanted all of us to close the DB Access instance once we were done.
class connection{
method(){
}
finalize(){
// clean up
}
}
class Client{
func(){
connection = new connection();
//do something with the connection
/* moment this method exits refernce to connection object is zero!
which means the DBManager guys can take some action in finalize?? in java they can't. But in say Python yes.
The destructor might have the code to release something.*/
}
anotherFunc(){
}
}
Even .NET'c GC is'nt reference counting based and heard that the COM guys out there were on a warpath against MS on hearing this!. It anyway did'nt work though :-).
and yeah be prepared to expect questions on design patterns.
I encountered many.
You s'd atleast be able to identify
factory,singleton, adapter/wrapper, enumeration, decorator, command patterns and give illustrations.
Infact there were'nt many on server side programming!.

karthik.

[Buzz Andersen]

Originally posted by karthik Guru:

Java does not use Reference counting algorithm i feel.
because then it means we w'd've a definitive garbage collection which is'nt the case i guess.
We just are'nt sure when finalize gets called. Had it been reference counting then we can be sure of finalize getting called ( i mean when).

It does, though--the only reason you can't be sure about when finalize() is called is that garbage collection runs in a low priority thread that the VM merely gets to whenever it can.
Incidentally, in response to Mark's original GC question--would I be too much of a smart ass interviewee if I responded that the Java spec makes no specific statements about how the garbage collection algorithm should work and that it is left entirely up to the VM implementer ? At least that's what I learned when I studied for my Java cert...
Buzz Andersen
- Sun Certified Java Programmer for the Java 2 Platform

[Buzz Andersen]

Originally posted by Mark Herschberg:
Add 'em all up. If the sum is S and the missing number is X, then 5050 - S = X.

You got it...

Alternatively, I would say write a trivial hash algorithm, and put all numbers into a hash table, such that number X, maps to bucket X.

That was one of my earlier answers, but then the guy prompted me for a better solution. After some thought I finally came up with the sum thing.

I'll skip the fibonacci, which everyone probably knows.

Yeah, I suppose it is sort of a textbook recursion illustration. At least he didn't pull the "Towers of Hanoi" on me--I've always hated that one .

Here's one my friend likes to ask. You have two vials of pills of type A and type B.

That is a good one! I got as far as pulling out the extra pill from b, but Tighe answered it before I was able to make the leap to cutting them in half .
Buzz Andersen
- Sun Certified Programmer for the Java 2 Platform

[Sridhar Garimella]

Hi all,
I was asked the following question in Verizon India interview.
which is the best algorithm to find whether the given number is multiple of 2 or not?
Sri.

[William Barnes]

From a presentation by Joshua Bloch: (I don't really understand it.)

if( (n & -n) == n)
{
  // n is power of 2.
}

[William Barnes]

You have the characters "ABCDEF" write the sudo-code to reverse them, "FEDCBA". You only have one variable to work with which can only hold one character at a time.

[shashank shah]

reverse(){
for (i=0; i<str.length/2; i++)
swap(i, str.length-i)
}
swap(i,j){
tmp = str[i];
str[i] = str[j];
str[j] = tmp;
}
[This message has been edited by shashank bapat (edited September 04, 2001).]

[Daniel Dunleavy]

I count way more that one variable in that.
Dan

[shashank shah]

R u sure about that requirment?
Because this is a typical question in which variable means do not use another array variable
ie inplace string reversal.
What is ur defintn of variables ?
If i write assembly and keep things in registers would u consider it usage of variable ?
I am pretty sure u havent understood the question correctly here. because if u have array u ll at least need index variable to do anything which becomes a second variable first being array itself.
GOT THE POINT ?
-Shashank
[This message has been edited by shashank bapat (edited September 04, 2001).]

[John Dale]

You can swap integers a and b without using any other variables:
<code>
b = a ^ b;
a = a ^ b;
b = a ^ b;
</code>

[shashank shah]

Let me make it simple for u
char[] str= "abcdef"
for (i=0; i<3; i++){
tmp = str[i];
str[i] = str[6-i];
str[6-i] = tmp;
}
Sorry thats the closest i can come still 3 variables including array itself.

[shashank shah]

Originally posted by John Dale:
You can swap integers a and b without using any other variables:
<code>
b = a ^ b;
a = a ^ b;
b = a ^ b;
</code>

Cool !!
Though from his description of problem it seems that he is ok with using one char type for swapping. but anyways it reduces one more variable. (Note that we are assuming that impl language is going to allow us treating chars as ints)

[Sameer Jamal]

another way to swap variables without using other variable is :-
a=a+b;
b=a-b;
a=a-b;

Originally posted by John Dale:
You can swap integers a and b without using any other variables:
<code>
b = a ^ b;
a = a ^ b;
b = a ^ b;
</code>

[Sameer Jamal]

One question which I faced in an interview and I find it tough to answer was why Java does not require pointers like c++ ??

[William Barnes]

Sorry about that. You would need an array to hold the string and one more variable to which could hold a single character.
The trick I missed was dividing the length by 2. If you loop through the entire string you end up with what you started with.
And the reason that Java doesn't allow pointers is that most programmers don't know how to clean up after they use them. (It would be too hard to have a garbage collector if you allowed pointers, as the programmer would have an easy way of not letting something get garbage collected. But of course you can still use resource pools to get around the garbage collector.)

[Mark Herschberg]

Originally posted by Buzz Andersen:

Incidentally, in response to Mark's original GC question--would I be too much of a smart ass interviewee if I responded that the Java spec makes no specific statements about how the garbage collection algorithm should work and that it is left entirely up to the VM implementer ;)? At least that's what I learned when I studied for my Java cert...
Buzz Andersen
- Sun Certified Java Programmer for the Java 2 Platform

I got answers like that a couple times. It raised a yellow flag, i.e. I'm about to pay attention to whether or not your trying to dodge the question. The fact that it isn't specified is the point of the question. The average programmer will say "gee, not specified, so what do I care, it's taken care of for me." The good programmer says, "I need to look under the hood to understand what's going on and get really good performence."
--Mark

[Steve Chernyak]

About GC. If JVM uses mark and sweep algorithm then how can refrence counting be the right answer? I have just read an article on GC and would appreciate response
Thanks, Steve

[shashank shah]

Originally posted by Mark Herschberg:

I got answers like that a couple times. It raised a yellow flag, i.e. I'm about to pay attention to whether or not your trying to dodge the question. The fact that it isn't specified is the point of the question. The average programmer will say "gee, not specified, so what do I care, it's taken care of for me." The good programmer says, "I need to look under the hood to understand what's going on and get really good performence."
--Mark

Sorry I am still in favour of smart ass guy.
My view is Ref counting and mark and sweep certainly make good interview questions but way one phrases it may make these technically wrong (ie "which GC algorithm is used by *java*?")
Anyways, I have following doubts though:
A.) Knowing the fact that we do not have control over GC so we should as far as possible avoid finalizers (or use them as a fall back) while providing explicit close() destroy() methods. Or making sure u null the references whenever needed and avoid memory leaks ... and other concepts like that have to my knowledge nothing to do with underlying impl of GC. But may be I do not know how knowing about specifics of gc impl helps in improving performance. So please throw some light. As a java developer how can i write different better performing code to take advantage of different GC impls? Please give url if space is not enough.
B.) Assuming that u write something assuming GC impl so it becomes faster for that particular GC algorithm for that particular VM. I think it *may* lose platform independence as its performance wont be fine tuned for other JVM's ... something similar to thread priorities on diff. OS's.
I think there are SO many good questions u can ask about things �over the hood� which u 'll find that many candidates wont know. I wonder if it really makes a good test to ask these questions unless u r hiring a JVM engineer.
You shouldnt ask things because you know about it and in position to ask anything (esp. in todays market). There should be good reason why "he" should know about it. I am not saying there cannot be in this particular case ... just telling you to throw some light if there is.
-Shashank
MS(CS),BE(CS),SCJP2
-----------
Another thing I want to add is my interpretation of his reply is: Smart-ass programmer would say �Not specified So UNRELIABLE�.
[This message has been edited by shashank bapat (edited September 05, 2001).]
Mark,
On the second thought, avoiding cycles if GC is ref counting can be one good reson to why know about GC ?? something else ?
[This message has been edited by shashank bapat (edited September 05, 2001).]

[John Coxey]

All:
In response to Shashank Bapat.
----
From personal experience, I find the technical interview to be a place of "sink or swim". But, I do it a little bit differently than most.
Rather than confront Mark's question head on - which personally, I don't know much more other than you cannot "force" GC to run - I try to turn a confrontational situation into one of collaboration.
I try to employ the technique of "give and take". I am certainly not going to know everything about garbage collection - but I can pick out bits and pieces here and there. If nothing else, I would have Mark go to the board and write out his solutions.
In this manner - I can judge Mark as much as he is judging me. Meaning, is this a person I want to work for?
In addition, I will be taking notes as Mark writes on the board. I will then refer to these notes when I write my thank-you letter. In addition, I will use these notes as a study guide for the next interview - if Mark doesn't think I am a good fit.
--------
Hopefully, during Mark's answer - he will stumble upon a subject that I know about. If he know's how to conduct an interview, Mark will give me the opportunity to run with topic. Perhaps, I will be able to teach him something.
What I hate is an interviewer who can't be flexible enough to adjust to the situation as the interview progresses.
-------
Given our "discussion" rather than "interrogation" - I hope to give Mark a better picture of what level I am comming in at and also convey a sense of "leadership" and a "take charge" attitude - along with a demonstration of my communication skills.
Now it's up to him to see where I can fit in the organization.
Just my two cents.
John Coxey
([e-mail])

[shashank shah]

Originally posted by John Coxey:
...
In this manner - I can judge Mark as much as he is judging me. Meaning, is this a person I want to work for? ...

John,
Did u try searching for job recently?
Forget about person, I have degraded myself to a level where I say that I am ready to work on �Microsoft's .NET�.
About ur style of interviewing. I used to employ that when I used to get 10 offers in 10 days 10 months back. Now � I get 1 interview in 20 days and I am afraid of losing that rare chance by creating smart-ass impression.
About interrogation, I had few really bad experiences lately:
1.) At VSP in Sacramento one guy asked to start an interview if Loyola University of Chicago, where I did my MS is accredited. Now I honestly think if he had any such doubt about my credentials he should have checked it BEFORE calling me for an interview, all the way down to Sacramento. Later they found someone with 6 years of java experience for the position. Do u believe that?? Well I do because in last one month many of my close friends gained additional 3 years experience. So it is possible.
2.) At one Indian consulting company called Global InfoTech:
Do u have Sun Certification as Java Programmer?
Yes (U r reading my resume)
Is that java 2?
Yes
When did u take that?
About 1 � years back.(E'thing is on the resume ... why ask again?)
He went: �There was no Java 2 Certification 1 � years back� (THE MOTIVE)
I was stunned. He wouldn't dare say that in good job market. And if he does he 'll see me walking out the next moment.
I said �I took it in Chicago � may be it wasn't available in India.�. I somehow managed to keep my cool.
May be you meet 10 candidates in a day with false experience degrees and certificates but 11th can be real one and deserves at least the respect if not a job.
-Shashank

[Buzz Andersen]

Originally posted by shashank bapat:
John,
Did u try searching for job recently?

Agreed, Shashank. With all due respect to the more senior members of our forum, I am getting a little weary of advice telling me that the point of an interview is to make sure the company and I are a good fit. Maybe this rule applies if you are a) a developer with 5+ years of experience or b) in a good economy, but people in my position can't afford to be so choosy.
I also can't agree enough with Shashank's comment that hiring managers shouldn't ask questions just because they know the answers and can. I am so sick of sitting across a desk from some smug bad-ass who thinks he is freaking Alan Turing just because he knows some arcane trivia. Sometimes I turn the tables by asking *them* a question from my archive, and you know what--they rarely can answer it!
And, lastly, I can certainly sympathize with the frustration against people who have inflated their experience on resumes. One thing that annoys me no end is that there are whole bunches of clueless Java programmers out there who have jobs, while I don't! A friend of mine who is a Senior Java Architect with a firm in Austin (and, thankfully, one of the few senior people in the industry who seems to give a damn about helping out a guy like me) told me a ridiculous story the other day. He is working on a project here in Denver, cleaning up a mess that was created by Java coders from a local consulting firm (which shall remain nameless). One day he suggested to said programmers that they should place some functionality that cut across several classes in the system in a superclass. Their response? "We don't believe in polymorphism."
Brilliant!
Buzz Andersen
- Sun Certified Java Programmer
[This message has been edited by Buzz Andersen (edited September 07, 2001).]

[shashank shah]

Originally posted by Buzz Andersen:
... local consulting firm (which shall remain nameless)....
Buzz Andersen
- Sun Certified Java Programmer

Buzz,
Even, I thought about keeping them nameless. But for few reasons:
1> They already think that I don't deserve a job at their �esteemed� organizations, which I wont even look at during good economy. So who cares what they think. And they did not seem technical enough that they ll ever notice javaranch. Not a single technical question from VSP guys.
2> They could challenge my real technical knowledge/aptitude for which I have no proof on paper. But they had guts to challenge my degree and certificates, which I have proof for.
3> I have seen both my startups folding during last one year. So there is twice the reason for feeing irritated.
4> I saw people vs. Larry Flynt last week
-Shashank
MS(CS),BE(CS),SCJP2

[Buzz Andersen]

Originally posted by shashank bapat:

At VSP in Sacramento one guy asked to start an interview if Loyola University of Chicago, where I did my MS is accredited.

Sorry, I was just reading back over your post and I couldn't get over this! Is Loyola University accredited? What a clown!
Congrats on getting your masters, BTW. I must admit that as this economy goes from bad to worse, that's what I'm seriously considering doing as well. It's funny--when I was an undergrad, I couldn't wait to get out. Now, I want back in !

[shashank shah]

Originally posted by Buzz Andersen:
Sorry, I was just reading back over your post and I couldn't get over this! Is Loyola University accredited? What a clown!
Congrats on getting your masters, BTW. I must admit that as this economy goes from bad to worse, that's what I'm seriously considering doing as well. It's funny--when I was an undergrad, I couldn't wait to get out. Now, I want back in !

Note that was the beginning of an interview.
I hear similar �polymorphic� stories from my ex-colleague at his present work place. (And when he tried forwarding my resume they said no H1s right now **Pl. dont change the topic Alex and Neha)
Another one of my ex-colleagues said that if someone is interviewing you today; 50% chances are that he knows much less than you because he is �boom time hired� who just ended up at right place and climbed ladder during boom time by jumping if needed. And then he won't even have enough technical knowledge to judge ur skills and will reject u. He has suddenly discovered this �right to choose� which he enjoys a lot and uses without thinking much .. or may be thinking is lost art for him.
Another funny reason for rejection can be "I am the GOD I am right" attitude interviewer. Its just bad luck that where u ll prefer comparable he ll insist on comparator ... where u ll say visitor he ll say composit. U like spending more time on design but he likes xp .. u use some IDE and he says nothing like emacs. Or u use JBuilder and he uses InteliJ.
This God has generally written 2 3 books and research papers hear and there. God is really knowledgable person. But in this case this God chooses one who 'll blindly follow him even if he is dumb rather than someone like u who has different views and ideas. So u lose the job because "u know" not because "u dont" or "he does not".
About studies, I am feeling the same right now and I am thinking about PhD ... So next time around I can design my own GC algorithm

[Junaid Bhatra]

Later they found someone with 6 years of java experience for the position. Do u believe that?? Well I do because in last one month many of my close friends gained additional 3 years experience. So it is possible.

LOL
Sorry state of affairs. Sad but true
It is so difficult to compete against such people, who behave as if forging/faking their experience/education is their birthright.
All you can do is hope that employers are smart enough not to take experience at face value, and instead thoroughly check the competence and ability of a person during the interview process. Also I know of some companies (mostly in the financial sector) which thoroughly cross-check the person's credentials, and verify each and every freaking job experience that the candidate has listed on his resume, instead of simply believing whatever the candidate claims. The sad part though is that most software companies do NOT do such a check

[This message has been edited by Junaid Bhatra (edited September 08, 2001).]

[Mark Herschberg]

Originally posted by John Coxey:

In this manner - I can judge Mark as much as he is judging me. Meaning, is this a person I want to work for?
...
What I hate is an interviewer who can't be flexible enough to adjust to the situation as the interview progresses.
-------
Given our "discussion" rather than "interrogation" - I hope to give Mark a better picture of what level I am comming in at and also convey a sense of "leadership" and a "take charge" attitude - along with a demonstration of my communication skills.

Well said, John. There's been a couple of companies where I interviewed and was totally turned off. I litterally said to myself, "I couldn't stand to work for a guy like this." True, for junior guys these days, maybe they're a little desperate and can't afford to get turned off, but more senior people and in better markets you can.
You also have an excellent point about the interviewer being able to adjust to the responses. If they are rigid in terms of "I have this checklist of questions I need to ask you," I wonder how well they can deal with all the curve balls that get thrown during the lifecycle of a project. Will they be able to recognize and compensate for environmental changes, or will they be just as rigid?

--Mark