You will always see some interview questions from Exception and Error
handling in core Java Interviews. Exception handling is an important aspect of
Java application development and its key to writing robust, stable Java programs,
which makes it natural favorites on interviews. Questions from Error and
Exception in Java mostly based on concept of Exception and Error in Java, How
to handle Exception , best
practices to follow during Exception handling etc. Though multithreading,
garbage collection, JVM concepts and questions from object oriented design
rules these interviews, you should always expect and prepare some questions on
effective error handling. Some Interviewer also test debugging skill of programmers, as
resolving Exceptions quickly is another trait of solid Java programming
knowledge. If programmer is familiar with infamous and dodgy ClassNotFoundException
or OutOfMemoryError,
there is a good chance that he has some good practical experience under his
belt. In this article we will see some Java Error and Exception interview
questions asked to fresher, experienced and senior Java developers in Java J2EE
interviews.
Java Exception and Error Interview Questions
Here is my list of frequently asked questions from Java Error andException topics in various programming interviews to Java and J2EE
developers. I have also shared my
answers for these questions for quick revision, and provided source for more in depth understanding. I have tried to include questions of various difficulty
level, including simplest of simple for freshers and some tricky questions for
senior Java developers. If you think, there is a good question, which is not
included in this list, please feel free to share it via comment. You can also
share error handling questions asked to you on interviews or any question, for
which you don’t know the answer.
1) What
is Exception in Java?
This is always been first interview question on Exception and mostly
asked on fresher level interviews. I haven't seen anybody asking about what is
Exception in senior and experienced level interviews, but this is quite popular
at entry level. In simple word Exception is Java’s way to convey both system
and programming errors. In Java Exception feature is implemented by using class
like Throwable, Exception, RuntimeException and
keywords like throw, throws, try, catch and finally. All
Exception are derived form Throwable class.
Throwable further divides errors in too category one is java.lang.Exception and other
is java.lang.Error. java.lang.Error deals with
system errors like java.lang.StackOverFlowError or Java.lang.OutOfMemoryError
while Exception is mostly used to deal with programming mistakes, non
availability of requested resource etc.
2) What
is difference between Checked and Unchecked Exception in Java ?
This is another popular Java Exception interview question appears in
almost all level of Java interviews. Main difference between Checked and
Unchecked Exception lies in there handling. Checked Exception requires to be
handled at compile time using try, catch and finally keywords
or else compiler will flag error. This is not a requirement for Unchecked
Exceptions. Also all exceptions derived from java.lang.Exception classes
are checked exception, exception those which extends RuntimeException,
these are known as unchecked exception in Java. You can also check next
article for more
differences between Checked and Unchecked Exception.
3) What
is similarity between NullPointerException and ArrayIndexOutOfBoundException in
Java?
This is Java Exception interview question was not very popular, but
appears in various fresher level interviews, to see whether candidate is
familiar with concept of checked and unchecked exception or not. By the way
answer of this interview question is both of them are example of unchecked
exception and derived form RuntimeException. This
question also opens door for difference of array in Java and C programming
language, as arrays in C are unbounded and never throw ArrayIndexOutOfBoundException.
4) What
best practices you follow while doing Exception handling in Java ?
This Exception interview question in Java is very popular while hiring
senior java developer of Technical Lead. Since exception handling is crucial
part of project design and good knowledge of this is desirable. There are lot
of best practices, which can help to make your code robust and flexible at same
time, here are few of them:
1) Returning boolean instead of returning null to avoid
NullPointerException at callers end. Since NPE is most infamous of all Java exceptions,
there are lot of techniques and coding
best practices to minimize NullPointerException. You can check that link
for some specific examples.
2) Non empty catch blocks. Empty catch blocks are considered as one of the bad practices in
Exception handling because they just ate Exception without any clue, at bare
minimum print stack trace but you should do alternative operation which make
sense or defined by requirements.
3) Prefer Unchecked exception over checked until you have a very good
reason of not to do so. it improves readability of
code by removing boiler plate exception handling code
.
4) Never let your database Exception flowing till client error. since
most of application deal with database and SQLException
is a checked Exception in Java you should consider handling any database
related errors in DAO layer of your application and only returning alternative
value or something meaningful RuntimeException which
client can understand and take action.
5) calling close() methods for connections,
statements, and streams on finally block in Java.
I have already shared lot of these in my post Top
10 Java exception handling best practices, you can also refer that for more
knowledge on this topic.
5) Why do
you think Checked Exception exists in Java, since we can also convey error
using RuntimeException ?
This is a controversial question and you need to be careful while
answering this interview question. Though they will definitely like to hear
your opinion, what they are mostly interested in convincing reason. One of the
reason I see is that its a design decision, which is influenced by experience
in programming language prior to Java e.g. C++. Most of checked exceptions are
in java.io package, which make sense because if you request any system resource and
its not available, than a robust program must be able to handle that situation
gracefully. By declaring IOException as checked Exception, Java ensures
that your provide that gracefully exception handling. Another possible reason
could be to ensuring that system resources like file descriptors, which are
limited in numbers, should be released as soon as you are done with that using
catch or finally block. Effective Java book from Joshua Bloch has couple of items in this topic, which is again
worth reading.
6) What
is difference between throw and throws keyword in Java?
One more Java Exception interview questions from beginners kitty. throw and throws keyword
may look quite similar, especially if you are new to Java programming and
haven't seen much of it. Though they are similar in terms that both are used in
Exception handling, they are different on how and where they are used in code. throws keyword is
used in method signature to declare which checked exception method can throw,
you can also declare unchecked exception, but that is not mandatory by compiler.
This signifies lot of things like method is not going to handle Exception
instead its throwing it, if method throws checked Exception then caller should
provide compile time exception handling etc. On the other hand throw keyword is
actually used to throw any Exception. Syntactically you can throw any Throwable (i.e. Throwable or any
class derived from Throwable) , throw keyword transfers control of execution to
caller so it can be used in place of return keyword. Most common example of
using throw in place of return is throwing UnSupportedOperationException from an
empty method as shown below :
private static void show() {
throw new
UnsupportedOperationException("Not yet implemented");
}
See this
article for more differences between these two keywords in Java.
7) What
is Exception chaining in Java?
Exception chaining is a popular exception handling concept in Java, where
another exception is thrown in response of an exception and creating a chain of
Exceptions. This technique mostly used to wrap a checked exception into an
unchecked or RuntimeException. By the way if you are
throwing new exception due to another exception then always include original
exception so that handler code can access root cause by using methods like getCause() and initCause().
8) Have
you written your own custom Exception in Java? How do you do that?
Ofcourse most of us has written custom or business Exceptions like AccountNotFoundExcepiton. Main
purpose of asking this Java Exception interview question is to find out how you
use this feature. This can be used for sophisticated and precise exception
handling with tweak involved in whether you would choose a checked or unchecked
exception. By creating a specific exception for specific case, you also gives
lot of options to caller to deal with them elegantly. I always prefer to have a
precise exception than a general exception. Though creating lots of specific
exceptions quickly increase number of classes in your project, maintaining a
practical balance between specific and general exceptions are key to success.
9) What
changes has been introduced in JDK7 related to Exception handling in Java ?
A relatively new and recent Exception interview question in Java. JDK7
has introduced two major feature which is related to Error and Exception
handling, one is ability to handle multiple
exception in one catch block, popularly known as multi cache block and
other is ARM
blocks in Java 7 for automatic resource management, also known as try with
resource. Both of these feature can certainly help to reduce boiler plate code
required for handling checked exceptions in Java and significantly improves
readability of code. Knowledge of this feature, not only helps to write better error and exception code in Java, but also helps to do well during interviews. I
also recommend reading Java 7 Recipes
book to get more insight on useful features introduced in Java 7, including
these two.
10) Have
you faced OutOfMemoryError in Java? How did you solved that?
This Java Error interview questions is mostly asked on senior level Java
interviews and here interviewer is interested on your approach to tackle
dangerous OutOfMemoryError. Admit it we always face this
error no matter which kind of project you are working so if you say no it
doesn't go very well with interviewer. I suggest even if you are not familiar
or not faced it in reality but have 3 to 4 years of experience in Java, be
prepare for it. At the same time, this is also a chance to impress interviewer
by showing your advanced technical knowledge related to finding memory leaks,
profiling and debugging. I have noticed that these skills almost always creates
a positive impression. You can also see my post on how
to fix java.lang.OutOfMemoryError for more detail on this topic.
11) Does
code form finally executes if method returns before finally block or JVM exits
?
This Java exception interview question can also be asked in code format,
where given a code with System.exit() in try block and something in
finally block. It’s worth knowing that, finally block in Java executes even
when return keyword is used in try block. Only time they don’t execute is when you
call JVM to exit by executing System.exit(0)from try
block in Java.
12) What
is difference in final, finalize and finally keyword in Java?
Another classic interview question in core Java, this was asked to one of
my friend on his telephonic interview for core Java developer with Morgan
Stanley. final and finally are
keyword, while finalize is method. final keyword is very
useful for creating ad Immutable
class in Java By making a class final, we prevent it from being extended,
similarly by making a method final, we prevent it from being overridden,. On
the other hand, finalize() method is called by garbage collector, before that object is
collected, but this is not guaranteed by Java specification. finally keyword is
the only one which is related to error and exception handling and you should
always have finally block in production code for closing connection and
resources. See here
for more detailed answer of this question.
13) What
is wrong with following code :
public static void start() throws IOException, RuntimeException{
throw new RuntimeException("Not able to Start");
}
public static void main(String args[]) {
try {
start();
} catch (Exception ex) {
ex.printStackTrace();
} catch (RuntimeException re) {
re.printStackTrace();
}
}
This code will throw compiler error on line where RuntimeException variable “re” is written
on catch block. since Exception is super class of RuntimeException, all RuntimeException thrown by start() method
will be captured by first catch block and code will never reach second catch
block and that's the reason compiler will flag error as “exception
java.lang.RuntimeException has already been caught".
14) What
is wrong with following code in Java:
public class SuperClass
{
public void start() throws IOException{
throw new IOException("Not able
to open file");
}
}
public class SubClass
extends SuperClass{
public void start() throws Exception{
throw new Exception("Not able
to start");
}
}
In this code compiler will complain on sub class where start() method
gets overridden. As per rules
of method overriding in Java, an overridden method can not throw Checked
Exception which is higher in hierarchy than original method. Since here start() is
throwing IOException in super class, start() in sub
class can only throw either IOException or any sub
class of IOException but not super class of IOException e.g.
Exception.
15) What
is wrong with following Java Exception code:
public static void start(){
System.out.println("Java Exception interivew question Answers for
Programmers");
}
public static void main(String args[]) {
try{
start();
}catch(IOException ioe){
ioe.printStackTrace();
}
}
In this Java Exception example code, compiler will complain on line where
we are handling IOException, since IOException is a
checked Exception and start() method doesn't throw IOException, so
compiler will flag error as "exception
java.io.IOException is never thrown in body of corresponding try statement", but
if you change IOException to Exception compiler error will
disappear because Exception can be used to catch all RuntimeException which
doesn't require declaration in throws clause. I like this little tricky Java
Exception interview question because its not easy to figure out result by chaining
IOException to Exception. You can
also check Java Puzzlers by Joshua Bloch and Neil Gafter for some tricky questions based
on Java Errors and Exceptions.
These are some of Java Error and Exception interview questions, I have
mostly seen in both fresher and experienced level of Java interviews. There are
a lot more questions on Exception which I haven't included and if you think you
have a good question missed out than let me know and I will make effort to
include it on this list of java exceptions question and answers. One last
question of Java Exception I am leaving for you guys is "Why
Java Exception considered to be better alternative of returning error codes" ,
let me know what is your thought on this list of Java Exception interview
questions and answers.
Recommended Books in this article
Effective Java
By Joshua Bloch
Java 7Recipes: A Problem-Solution Approach By Josh Juneau, Carl Dea, Freddy Guime
and John O'Conner
Java Puzzlers
by Joshua a Bloch and Neil Gafter
Source:http://javarevisited.blogspot.com/2013/06/10-java-exception-and-error-interview-questions-answers-programming.html
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