25 Core Java Questions and Answers from Job Interviews Software developer
Core Java Interview Question Answer
This is a new series of sharing core Java interview question and answer on Finance domain and mostly on big Investment bank.Many of these Java interview questions are asked on JP Morgan, Morgan Stanley,Barclays or Goldman Sachs. Banks mostly asked core Java interview questions from multi-threading, collection, serialization, coding and OOPS design principles. Anybody who is preparing for any Java developer Interview on any Investment bank can be benefited from these set of core Java Interview questions and answers. I have collected these Java questions from my friends and I thought to share with you all. I hope this will be helpful for both of us. It's also beneficial to practice some programming interview questions because in almost all Java interview, there is at-least 1 or 2 coding questions appear. Please share answers for unanswered Java interview questions and let us know how good these Java interview questions are?
These Java interview questions are mix of easy, tough and tricky Java questions e.g. Why multiple inheritance is not supported in Java is one of the tricky question in java. Most questions are asked on Senior and experienced level i.e. 3, 4, 5 or 6 years of Java experience e.g. How HashMap works in Java, which is most popular on experienced Java interviews. By the way recently I was looking at answers and comments made on Java interview questions given in this post and I found some of them quite useful to include into main post to benefit all. By the way apart from blogs and articles, you can also take advantage of some books, which are especially written for clearing any programming interviews and some focused on Java programming, two books which comes in minds are programming interview exposed and Java/J2EE interview companion from fellow blogger Arulkumaran. Former is focused on programming in general and lot of other related topics e.g. data structures, algorithms, database, sql, networking and behavioral questions, while later is completely dedicated to Java J2EE concepts.
Core Java Interview Questions Answers in Finance domain
1. What is immutable object? Can you write immutable object?
Immutable classes are Java classes whose objects can not be modified once created. Any modification in Immutable object result in new object. For example is String is immutable in Java. Mostly Immutable are also final in Java, in order to prevent sub class from overriding methods in Java which can compromise Immutability. You can achieve same functionality by making member as non final but private and not modifying them except in constructor.
2. Does all property of immutable object needs to be final?
Not necessary as stated above you can achieve same functionality by making member as non final but private and not modifying them except in constructor.
3. What is the difference between creating String as new() and literal?
When we create string with new() Operator, it’s created in heap and not added into string pool while Stringcreated using literal are created in String pool itself which exists in PermGen area of heap.
does not put the object in String pool , we need to call String.intern() method which is used to put them into String pool explicitly. its only when you create String object as String literal e.g. String s = "Test" Java automatically put that into String pool.
4. How does substring () inside String works?
Another good Java interview question, I think answer is not sufficient but here it is “Substring creates new object out of source string by taking a portion of original string”. see my post How SubString works in Java for detailed answer of this Java question.
5. Which two method you need to implement for key Object in HashMap ?
In order to use any object as Key in HashMap, it must implements equals and hashcode method in Java. Read How HashMap works in Java for detailed explanation on how equals and hashcode method is used to put and get object from HashMap. You can also see my post 5 tips to correctly override equals in Java to learn more about equals.
6. Where does these two method comes in picture during get operation?
This core Java interview question is follow-up of previous Java question and candidate should know that once you mention hashCode, people are most likely ask How its used in HashMap. See How HashMap works in Java for detailed explanation.
7. How do you handle error condition while writing stored procedure or accessing stored procedure from java?
This is one of the tough Java interview question and its open for all, my friend didn't know the answer so he didn't mind telling me. my take is that stored procedure should return error code if some operation fails but if stored procedure itself fail than catching SQLException is only choice.
8. What is difference between Executor.submit() and Executer.execute() method ?
This Java interview question is from my list of Top 15 Java multi-threading question answers, Its getting popular day by day because of huge demand of Java developer with good concurrency skill. Answer of this Java interview question is that former returns an object of Future which can be used to find result from worker thread)
By the way @vinit Saini suggested a very good point related to this core Java interview question
There is a difference when looking at exception handling. If your tasks throws an exception and if it was submitted with execute this exception will go to the uncaught exception handler (when you don't have provided one explicitly, the default one will just print the stack trace to System.err). If you submitted the task with submit any thrown exception, checked exception or not, is then part of the task's return status. For a task that was submitted with submit and that terminates with an exception, the Future.get will re-throw this exception, wrapped in an ExecutionException.
9. What is the difference between factory and abstract factory pattern?
This Java interview question is from my list of 20 Java design pattern interview question and its open for all of you to answer.
@Raj suggestedAbstract Factory provides one more level of abstraction. Consider different factories each extended from an Abstract Factory and responsible for creation of different hierarchies of objects based on the type of factory. E.g. AbstractFactory extended by AutomobileFactory, UserFactory, RoleFactoryetc. Each individual factory would be responsible for creation of objects in that genre.
You can also refer What is Factory method design pattern in Java to know more details.
10. What is Singleton? is it better to make whole method synchronized or only critical section synchronized ?
Singleton in Java is a class with just one instance in whole Java application, for example java.lang.Runtimeis a Singleton class. Creating Singleton was tricky prior Java 4 but once Java 5 introduced Enum its very easy. see my article How to create thread-safe Singleton in Java for more details on writing Singleton using enum and double checked locking which is purpose of this Java interview question.
11. Can you write critical section code for singleton?
This core Java question is followup of previous question and expecting candidate to write Java singleton using double checked locking. Remember to use volatile variable to make Singleton thread-safe. check 10 Interview questions on Singleton Pattern in Java for more details and questions answers
12. Can you write code for iterating over hashmap in Java 4 and Java 5 ?
13. When do you override hashcode and equals() ?
Whenever necessary especially if you want to do equality check or want to use your object as key in HashMap. check this for writing equals method correctly 5 tips on equals in Java
14. What will be the problem if you don't override hashcode() method ?
You will not be able to recover your object from hash Map if that is used as key in HashMap.
See here How HashMap works in Java for detailed explanation.
15. Is it better to synchronize critical section of getInstance() method or whole getInstance() method ?
Answer is critical section because if we lock whole method than every time some one call this method will have to wait even though we are not creating any object)
16. What is the difference when String is gets created using literal or new() operator ?
When we create string with new() its created in heap and not added into string pool while String created using literal are created in String pool itself which exists in Perm area of heap.
17. Does not overriding hashcode() method has any performance implication ?
This is a good question and open to all , as per my knowledge a poor hashcode function will result in frequent collision in HashMap which eventually increase time for adding an object into Hash Map.
18. What’s wrong using HashMap in multithreaded environment? When get() method go to infinite loop ?
Another good question. His answer was during concurrent access and re-sizing.
19. Give a simplest way to find out the time a method takes for execution without using any profiling tool?
this questions is suggested by @Mohit
Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.
To put it in code…
long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();
System.out.println (“Time taken for execution is ” + (end – start));
Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing
Recommended Books to prepare Java Interviews
Apart from blogs and articles, you can also take help of some books, which are especially written to help with programming interviews, covering wide range of questions starting from object oriented design, coding, Java basic concepts, networking, database, SQL, XML and problem solving skills. Following books are from my personal collection, which I often used to revise before going for any interview.
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