Java Interview Answers: core java interview question

Showing posts with label core java interview question. Show all posts

Saturday 31 August 2013

Top Most Best Practices while dealing with Password in Java

While working in core Java application or enterprise web application there is always a need of working with passwords in order to authenticate user. Passwords are very sensitive information like Social Security Number(SSN) and if you are working with real human data like in online banking portal or online health portal its important to follow best practices to deal with passwords or Social security numbers. here I will list down some of the points I learned and take care while doing authentication and authorization or working with password. I recommend to read more on this topic and have a checklist of things based on your application requirement. Anyway here are few points which make sense to me:

Top Most Difference between EnumMap and HashMap in Java

HashMap vs EnumMap in Java

What is difference between EnumMap and HashMap in Java is the latest Java collection interview question which has been asked to couple of my friends. This is one of the tricky Java question, specially if you are not very much familiar with EnumMap in Java, which is not uncommon, given you can use it with only Enum keys. Main difference between EnumMap and HashMap is that EnumMap is a specialized Map implementation exclusively for Enum as key. Using Enum as key, allows to do some implementation level optimization for high performance which is generally not possible with other object's as key. We have seen lot of interview questions on HashMap in our article How HashMap works in Java but what we missed there is this question which is recently asked to some of my friend. Unlike HashMap, EnumMap is not applicable for every case but its best suited when you have Enum as key. We have already covered basics of EnumMap and some EnumMap example in my last article What is EnumMap in Java and In this post we will focus on key differences between HashMap and EnumMap in Java.

Top Most How to reverse String in Java using Iteration and Recursion

How to reverse string in java is popular core java interview question and asked on all levels from junior to senior java programming job. since Java has rich API most java programmer answer this question by using StringBuffer reverse() method which easily reverses an String in Java and its right way if you are programming in Java but most interview doesn't stop there and they ask interviewee to reverse String in Java without using StringBuffer or they will ask you to write an iterative reverse function which reverses string in Java. In this tutorial we will see how to reverse string in both iterative and recursive fashion. This example will help you to prepare better for using recursion in java which is often a weak area of Java programmer and exposed during a programming interview.

Top Most How to Stop Thread in Java Code Example

Thread is one of important Class in Java and multi-threading is most widely used feature,but there is no clear way to stop Thread in Java. Earlier there was a stop method exists in Thread Class but Java deprecated that method citing some safety reason. By default a Thread stops when execution of run() method finish either normally or due to any Exception.In this article we will How to Stop Thread in Java by using a boolean State variable or flag. Using flag to stop Thread is very popular way of stopping thread and its also safe, because it doesn't do anything special rather than helping run() method to finish it self.

Top Most Difference between trustStore and keyStore in Java - SSL

trustStore vs keyStore in Java

trustStore and keyStore are used in context of setting up SSL connection in Java application between client and server. TrustStore and keyStore are very much similar in terms of construct and structure as both are managed by keytool command and represented by KeyStore programatically but they often confused Java programmer both beginners and intermediate alike. Only difference between trustStore and keyStoreis what they store and there purpose. In SSL handshake purpose of trustStore is to verify credentials and purpose of keyStore is to provide credential. keyStore in Java stores private key and certificates corresponding to there public keys and require if you are SSL Server or SSL requires client authentication. TrustStore stores certificates from third party, your Java application communicate or certificates signed by CA(certificate authorities like verisign, Thawte, Geotrust or GoDaddy) which can be used to identify third party. This is second article on setting up SSL on Java program, In last post we have seen How to import SSL certificates into trustStore and keyStore and In this Java article we will some differences between keystore and truststore in Java, which will help to understand this concept better.

Top Most Difference between equals method and "==" operator in Java - Interview Question

Both equals() and "==" operator in Java is used to compare objects to check equality but main difference between equals method and == operator is that former is method and later is operator. Since Java doesn’t support operator overloading, == behaves identical for every object but equals() is method, which can be overridden in Java and logic to compare objects can be changed based upon business rules. Another notable difference between == and equals method is that former is used to compare both primitive and objects while later is only used for objects comparison. At the same time beginners struggle to find when to use equality operator (==) and when to use equals method for comparing Java objects. In this tutorial we will see how equals() method and == operator works in Java and what is difference between "==" and equals method in Java and finally when to use "==" and equals() to compare objects.

Top Most Difference between java and javaw executable commands

difference between java.exe and javaw.exe commands was the recent java question asked to one of my friend. while running java program on windows you might have noticed that they will appear in task manager as either java.exe or javaw.exe, also in JAVA_HOME/bin we see these two java commands java.exe and javaw.exe, do you know difference between java.exe and javaw.exe ? surprisingly not many java programmer know answer of this java interview question, may be because java developers has not paid enough attention to these commands. any way here we will see few differences between java and javaw commands:

Top Most How to Set Classpath for Java on Windows Unix and Linux

What is CLASSPATH in Java
Classpath in Java is path to directory or list of directory which is used by ClassLoaders to find and load class in Java program. Classpath can be specified using CLASSPATH environment variable which is case insensitive, -cp or -classpath command line option or Class-Path attribute in manifest.mf file inside JAR file in Java. In this Java tutorial we will learn What is Classpath in Java, how Java resolve classpath and How Classpath works in Java along side How to set classpath for Java in windows and UNIX environment. I have experience in finance and insurance domain and Java is heavily used in this domain for writing sophisticated Equity, Fixed income trading applications. Most of these investment banks has written test as part of there core Java interview questions and I always find at least one question related to CLASSPATH in Java on those interviews. Java CLASSPATH is one of the most important concepts in Java, but, I must say mostly overlooked. This should be the first thing you should learn while writing Java programs because without correct understanding of Classpath in Java you can't understand how Java locates your class files. Also don't confuse Classpath with PATH in Java, which is another environment variable used to find java binaries located in JDK installation directory, also known as JAVA_HOME. Main difference between PATH and CLASSPATH is that former is used to locate Java commands while later is used to locate Java class files.

Top Most Difference between IdentityHashMap and HashMap in Java

IdentityHashMap in Java was added in Java 1.4 but still its one of those lessor known class in Java. Main difference between IdentityHashMap and HashMap in Java is that IdentityHashMap is a special implementation of Map interface which doesn't use equals() and hashCode() method for comparing object unlike other implementation of Map e.g. HashMap. Instead IdentityHashMap use equality operator "==" to compare keys and values in Java which makes it faster compare to HashMap and suitable where you need reference equality check and instead of logical equality. By the way IdentityHashMap is special implementation of Map interface much like EnumMap but it also violates general contract of Map interface which mandates using equals method for comparing Object. Also IdentityHashMap vs HashMap is a good Java question and have been asked couple of times. Though this question is not as popular as How HashMap works in Java or Difference between Hashtable and HashMap, it’s still a good question to ask. In this Java tutorial we will see example of IdentityHashMap and explores some key differences between IdentityHashMap and HashMap in Java.

Servlet Questions Asked in Interview

Question 1: In web.xml file <load-on-startup>1</load-on-startup> is defined between <servlet></servlet> tag what does it means.

Ans: whenever we request for any servlet the servlet container will initialize the servlet and load it which is defined in our config file called web.xml by default it will not initialize when our context is loaded .defining like this <load-on-startup>1</load-on-startup> is also known as pre initialization of servlet means now the servlet for which we have define this tag has been initialized in starting when context is loaded before getting any request.When this servlet question was asked to me in an interview few years back , I was not even aware of this element but this questions pointed me to look DTD of web.xml and understand other elements as well..

Question 2: How can we create deadlock condition on our servlet?

Ans: one simple way to call doPost() method inside doGet() and doGet()method inside doPost() it will create deadlock situation for a servlet. This is rather simple servlet interview questions but yet tricky if you don’t think of it J

Question 3: For initializing a servlet can we use constructor in place of init ().

Ans: No, we can not use constructor for initializing a servlet because for initialization we need an object of servletConfig using this object we get all the parameter which are defined in deployment descriptor for initializing a servlet and in servlet class we have only default constructor according to older version of java so if we want to pass a

Config object we don’t have parametrized constructor and apart from this servlet is loaded and initialized by container so ots a job of container to call the method according to servlet specification they have lifecycle method so init() method is called firstly.

More important Java doesn't allow interfaces to declare constructors. These kinds of servlet interview questions are quite popular on service based companies who just want to dig one level more.

Question 4: Why super.init (config) wiil be the first statement inside init(config) method.

Ans: This will be the first statement if we are overriding the init(config ) method by this way we will store the config object for future reference and we can use by getServletConfig () to get information about config object if will not do this config object will be lost and we have only one way to get config object because servlet pass config object only in init method . Without doing this if we call the servletConfig method will get NullPointerException.

Question5: Can we call destroy() method inside the init() method is yes what will happen?

Ans:Yes we can call like this but if we have not override this method container will call the default method and nothing will happen.after calling this if any we have override the method then the code written inside is executed.

Question 6: How can we refresh servlet on client and server side automatically?

Ans: On client side we can use Meta http refresh and server side we can use server push.

Question 7: How can you get the information about one servlet context in another servlet?

Ans: In context object we can set the attribute which we want on another servlet and we can get that attribute using their name on another servlet.

Context.setAttribute (“name”,” value”)

Context.getAttribute (“name”)

Question 8: Why we need to implement Single Thread model in case of Servlet.

Ans: In J2EE we can implement our servlet on two different ways either by using:

1. Single Thread Model

2. Multithread Model

Depending upon our scenario, if we have implemented single thread means only one instance is going handle one request at a time no two thread will concurrently execute service method of servlet.

Example in banking account where sensitive data is handle mostly this scenario was used this interface is deprecated in Servlet API version 2.4.

As the name signifies multi thread means a servlet is capable to handle multiple requests at same time. This servlet interview question was quite popular few years back on entry level but now its loosing its shine.

Question 9: what is servlet collaboration?

Ans communication between two servlet is called servlet collaboration which is achieved by 3 ways.

1. RequestDispatchers include () and forward() method .

2. Using sendRedirect()method of Response object.

3. Using servlet Context methods

Question 10: What is the difference between ServletConfig and ServletContext?

Ans: ServletConfig as the name implies provide the information about configuration of a servlet which is defined inside the web.xml file or we can say deployment descriptor.its a specific object for each servlet.

ServletContext is application specific object which is shared by all the servlet belongs to one application in one JVM .this is single object which represent our application and all the servlet access application specific data using this object.servlet also use their method to communicate with container.

These Servlet interview questions are good for quick recap of important concept before appearing on any J2EE interview. Please share if you have come across any other interesting interview question on Servlets.

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Top Most How to Convert and Print Byte array to Hex String in Java

We often needs to convert byte arrays to Hex String in Java, In order to print byte array contents in readable format. Since many cryptographic algorithm e.g. MD5 return hash value as byte array, In order to see and compare those byte array, you need to convert byte array to Hex String. As we have seen, while generating MD5 check sum of File, there are multiple ways to convert byte array to Hexadecimal String in Java. You can either write your own method, or you can use open source library e.g. Apache commons codec to create Hex String from byte array in Java. Commons codec provides a utility class Hex, which contains a encodeHexString() method to create Hex String from byte array. It's one of the best option to generate Hex String, if you are already using this library to generate MD5 hash values. In this Java tutorial we will see what is issue with printing byte array as normal String, and 2 examples to convert byte array into Hexadecimal String in Java.

Top Most How to find middle element of LinkedList in Java in one pass

How do you find middle element of LinkedList in one pass is a programming question often asked to Java and non Java programmers in telephonic Interview. This question is similar to checking palindrome or calculating factorial, where Interviewer some time also ask to write code. In order to answer this question candidate must be familiar with LinkedList data structure i.e. In case of singly LinkedList, each node of Linked List contains data and pointer, which is address of next Linked List and last element of Singly Linked List points towards null. Since in order to find middle element of Linked List you need to find length of LinkedList, which is counting elements till end i.e. until you find the last element on Linked List. What makes this data structure Interview question interesting is that you need to find middle element of LinkedList in one pass and you don’t know length of LinkedList. This is where candidates logical ability puts into test, whether he is familiar with space and time trade off or not etc. As if you think carefully you can solve this problem by using two pointers as mentioned in my last post on How to find length of Singly Linked List in Java. By using two pointers, incrementing one at each iteration and other at every second iteration. When first pointer will point at end of Linked List, second pointer will be pointing at middle node of Linked List. In fact this two pointer approach can solve multiple similar problems e.g. How to find 3^rd element from last in a Linked List in one Iteration or How to find nth element from last in a Linked List. In this Java programming tutorial we will see a Java program which finds middle element of Linked List in one Iteration.

Top Most What is difference between java.sql.Time, java.sql.Timestamp and java.sql.Date - JDBC interview Question

Difference between java.sql.Time, java.sql.Timestamp and java.sql.Date is most common JDBC question appearing on many core Java interviews. As JDBC provides three classes java.sql.Date, java.sql.Time and java.sql.Timestamp to represent date and time and you already have java.util.Date which can represent both date and time, this question poses lot of confusion among Java programmer and that’s why this is one of those tricky Java questions which is tough to answer. It becomes really tough if differences between them is not understood correctly. We have already seen some frequently asked or common JDBC questions like why JDBC has java.sql.Date despite java.util.Date and Why use PreparedStatement in Java in our last tutorials and we will see difference between java.sql.Date, java.sql.Time and java.sql.Timestamp in this article. By the way apart from these JDBC interview questions, if you are looking to get most from JDBC you can also see 4 JDBC performance tips and 10 JDBC best practices to follow. Those article not only help you to understand and use JDBC better but also help on interviews. Let’s come back to difference sql time, timestamp and sql date.

Top Most How to create thread safe Singleton in Java - Java Singleton Example

Thread safe Singleton means a Singleton class which returns exactly same instance even if exposed to multiple threads. Singleton in Java has been a classical design pattern like Factory method pattern or Decorator design pattern and has been used a lot even inside JDK like java.lang.Runtime is an example of Singleton class. Singleton pattern ensures that exactly one instance of class will remain in Java program at any time. In our last post 10 Interview questions on Singleton in Java we have discussed many different questions asked on Singleton pattern, One of them was writing Thread safe singleton in Java. Prior to Java 5 double checked locking mechanism is used to create thread-safe singleton in Java which breaks if one Thread doesn't see instance created by other thread at same time and eventually you will end up with more than one instance of Singleton class. From Java 5 onwards volatile variable guarantee can be used to write thread safe singleton by using double checked locking pattern. I personally don't prefer that way as there are many other simpler alternatives to write thread-safe singleton is available available like using static field to initialize Singleton instance or using Enum as Singleton in Java. Let’s see example of both ways to create thread-safe Singleton in Java.

Top Most Why Java does not support Operator overloading

Unlike C++, Java doesn't support operator overloading. Java doesn't provide freedom to programmer, to overload the standard arithmetic operators e.g. +, -, * and / etc. If you have worked previously in C++, than you know that, Java has left lot of feature supported in C++ e.g. Java doesn't support multiple inheritance, no pointers in Java, and no pass by reference in Java. Rarely this question asked in Java interviews, to check how programmer thinks about certain features, which is not supported in Java. Another similar questions is regarding Java being pass by reference, which is mostly appear as, whether Java is pass by value or reference. Though I don't know the real reason behind it, I think following observation make sense on, why Operator overloading is not supported in Java.

1) Simplicity and Cleanliness

Simple and clear design was one of the goals of Java designers. They, just don't want to replicate the language, but wanted to have a clear, truly object oriented language. Adding Operator overloading would have definitely made design more complex than without it, and it might have lead to more complex compiler or slows the JVM ,because it needs to do extra work to identify the actual meaning of operators, and reduce the opportunity to optimize the language by guarantee behavior of operators in Java.

2) Avoid Programming Errors

Java doesn't allow user defined operator overloading, because if you allow programmer to do operator overloading, they will come up with multiple meanings for same operator, which will make the learning curve of any developer hard and things more confusing and messy. Its been observed that, there is increase in programming errors, when language supports operator overloading, which in turn increase e development and delivery time. Since Java and JVM has taken most of developers responsibility, in memory management by providing garbage collector, it doesn't really make sense to left this feature to pollute the code, and as a loop hole for programming errors.

3) JVM Complexity

From JVM perspective, supporting operator overloading is more difficult, and if the same thing can be achieved, by using method overloading in more intuitive and clean way, it does make sense to not support operator overloading in Java. A complex JVM, may result in slower JVM, than a relatively simpler JVM ,and reduce the opportunity of optimization by taking out guaranteed behavior of operators in Java.

4) Easy Development of Tools

This is an additional benefit of not supporting operator overloading in Java. Omission of operator overloading has kept the language easier to handle and process, which in turn makes it easier to develop the tools, that process the language e.g. IDE or re-factoring tool. Re-factoring tools in Java are far better than C++.

In conclusion many things, which can be achieved by operator overloading, can also be achieved using method overloading using more intuitive and easy way and that might be the reason java designer thought that supporting operator overloading will not be a big benefit for language, but in fact only Java designer can answer real motivation of, why Java doesn't support operator overloading, like some other questions as Why Java doesn't support multiple inheritance or Why String is immutable in Java.

Monday 26 August 2013

Top Most How to check if a number is a palindrome or not in Java - Example

How to check if a number is a palindrome or not is a variant of popular String interview question how to check if a String is a palindrome or not. A number is said to be a palindrome if number itself is equal to reverse of number e.g. 313 is a palindrome because reverse of this number is also 313. On the other hand 123 is not a palindrome because reverse of 123 is 321 which is not equal to 123, i.e. original number. In order to check if a number is a palindrome or not we can reuse the logic of How to reverse number in Java. Since in most of interview, you are supposed to solve this question without taking help from API i.e. only using basic programming construct e.g. loop, conditional statement, variables, operators and logic. I have also seen programmer solving this question by first converting integer to String and than reversing String using reverse() method of StringBuffer and than converting String back to Integer, which is not a correct way because you are using Java API. Some programmer may think that this is just a trivial programming exercise but it’s not. Questions like this or Fibonacci series using recursion can easily separate programmers who can code and who can’t. So it’s always in best interest to keep doing programing exercise and developing logic.

Top Most Inner class and nested Static Class in Java with Example

Inner class and nested static class in Java both are classes declared inside another class, known as top level class in Java. In Java terminology, If you declare a nested class static, it will called nested static class in Java while non static nested class are simply referred as Inner Class. Inner classes are also a popular topic in Java interviews. One of the popular question is difference between inner class and nested static class , some time also refereed as difference between static and non static nested class in Java. One of the most important question related to nested classes are Where should you use nested class in Java? This is one of the tricky Java question, If you have read Effective Java from Joshua Bloch than in one chapter he has suggested that if a class can only be useful to one particular class, it make sense to keep that inside the class itself otherwise declare it as top level class. Nested class improves Encapsulationand help in maintenance. I actually look JDK itself for examples and if you look HashMap class, you will find that Map.Entry is a good example of nested class in Java. Another popular use of nested classes are implementing Comparator in Java e.g. AgeCompartor for Person class. Since some time Comparator is also tied with a particular class, it make sense to declare them as nested static class in Java. In this Java tutorial we will see what is Inner class in Java, different types of Inner classes, what is static nested classand finally difference between static nested class and inner class in Java.

Top Most How to get Key from Value in Hashtable, HashMap or Map in Java

It's not easy to get key from value in Hashtable or HashMap, as compared to getting value from key, because Hash Map or Hashtable doesn't enforce one to one mapping between key and value inside Map in Java. infact Map allows same value to be mapped against multiple keys inside HashMap, Hashtable or any other Map implementation. What you have in your kitty is Hashtable.containsValue(String value) or Hashtable.containsKey(String key) to check whether key or value exists in Hashtable or not, but some time we want to retrieve value from Map corresponding to any key and there is no API method to do in Map. We can still do this, but it highly depends data in your Map because Hashtable and HashMap both allows duplicate values mapped to different key. In this Java tutorial, we will see example of how to get key from value in Hashtable in Java.

Java Interview Answers

Saturday 31 August 2013

Top Most How to create read only List, Map and Set in Java – unmodifiable example