Java Interview Answers: programming

Showing posts with label programming. Show all posts

Saturday 31 August 2013

Top Most How to Find Runtime of a Process in UNIX and Linux

So you checked your process is running in Linux operating system and it's running find, by using ps command. But now you want to know, from how long process is running, What is start date of that process etc. Unfortunately ps command in Linux or any UNIX based operating system doesn't provide that information. But as said, UNIX has commands for everything, there is a nice UNIX tip, which you can use to check from how long a process is running. It’s been a long time, I have posted any UNIX or Linux command tutorial, after sharing some UNIX productivity tips . So I thought to share this nice little tip about finding runtime of a process in UNIX based systems e.g. Linux and Solaris. In this UNIX command tutorial, we will see step by step guide to find, since when a particular process is running in a server.

Top Most How to compare Arrays in Java – Equals vs deepEquals Example

java.util.Arrays class provides equals() and deepEquals() method to compare two Arrays in Java. Both of these are overloaded method to compare primitive arrays e.g. int, long, float, double and Object arrays e.g. Arrays.equals(Object[] , Object[]). Arrays.equals() returns true if both Arrays which it is comparing are null, If both array pointing to same Array Object or they must be of same length and contains same element in each index. In all other cases it returns false. Arrays.equals() calls equals() method of each Object while comparing Object arrays. One of the tricky question in Java related to Array comparison is Difference between Arrays.equals() and Arrays.deepEquals() method. Since both equals and deepEquals is used for array comparison, what is difference between them becomes important. Short answer of this questions is that, Array's equals() method does not perform deep comparison and fails logical comparison in case of nested Array, on other hand deepEquals() perform deep comparison and returns logical comparison in case of nested array.

Top Most How to append text into File in Java – FileWriter Example

Some times we need to append text into File in Java instead of creating new File. Thankfully Java File API is very rich and it provides several ways to append text into File in Java. Previously we have seen how to create file and directory in Java and how to read and write to text file in Java and in this Java IO tutorial we will see how to append text into file in Java. We are going to use standard FileWriter and BufferedWriter approach to append text to File. One of the key point to remember while using FileWriter in Java is to initialize FileWriter to append text i.e. writing bytes at the end of File rather than writing on beginning of the File. In next section we will see complete Java program to append text into File in Java. By the way you can also use FileOutputStream instead of FileWriter if you would like to write bytes, instead of text. Similar to FileWriter, FileOutputStream constructoralso takes a boolean append argument to open connection to append bytes into File in Java.

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 How to add, subtract days, months, years, hours from Date and Time in Java

Adding days, hours, month or years to dates is a common task in Java. java.util.Calendar can be used to perform Date and Time arithmetic in Java. Calendar class not only provides date manipulation but it also support time manipulation i.e. you can add, subtract hours, minutes and seconds from current time. Calendar class automatically handles date transition or month transition for example if you ask date after 30 days it will return you date based on whether current month is 30 or 31 days long. Same is true in case of adding and subtracting years, Calendar takes care whether current or following year is a leap year or not. For example 2012 is a leap year and it has February with 29 days, if you ask Calendar day before 365 it will return 24th July (assuming current date 23^rd July) which shows it take care of leap year. By the way there are couple of more date and time related articles e.g. How to find current date and time in Java and How to convert Date to String in Java. If you haven’t read them already, It’s worth checking to know more about Date and Time in Java.

Top Most What is static import in Java 5 with Example

Static import in Java allows to import static members of class and use them, as they are declared in the same class. Static import is introduced in Java 5 along with other features like Generics, Enum, Autoboxing and Unboxing and variable argument methods. Many programmer think that using static import can reduce code size and allow you to freely use static field of external class without prefixing class name on that. For example without static import you will access static constant MAX_VALUE of Integer class as Integer.MAX_VALUE but by using static import you can import Integer.MAX_VALUE and refer it as MAX_VALUE. Similar to regular importstatements, static import also allows wildcard * to import all static members of a class. In next section we will see Java program to demonstrate How to use static import statements to import static fields.

Top Most java.lang.classcastexception cannot be cast to in Java - cause and solution

As name suggests ClassCastException in Java comes when we try to type cast an object and object is not of the type we are trying to cast into. In fact ClassCastException in Java is one of most common exception in Java along with java.lang.OutOfMemoryErrorand ClassNotFoundException in Java before Generics was introduced in Java 5 to avoid frequent instances of java.lang.classcastexception cannot be cast to while working with Collection classes like ArrayListand HashMap, which were not type-safe before Java 5. Though we can minimize and avoid java.lang.ClassCastException in Java by using Generics and writing type-safe parameterized classes and method, its good to know real cause of ClassCastException and How to solve it. In this Java tutorial we will see Why java.lang.ClassCastException comes and how to resolve ClassCastException in Java. We will also see how to minimize or avoid ClassCastException in Java by using Generics, as prevention is always better than cure.

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 How to attach source in eclipse for Jars, debugging and code look-up – JDK Example

Attaching source of any Jar in Eclipse e.g. JDK or open source libraries like Spring framework is good idea because it help during debugging and code development. As a Java programmer at least you should attach source of JDK in Eclipse IDE to find out more about JDK classes. Though Eclipse IDE is pretty good on code assist, sometime You want to know what's going inside a library or a JDK method, rather than just reading documentation. Interview questions like How HashMap works in Java or How substring cause memory leak can only be answered if you are familiar with source code of these classes. Once you attach source code of Java or Spring in Eclipse IDE, You can check code with just a single click. Some one may argue for Java decompiler like JAD which can create source from .class file which is also a smart way to look code for open source library, but decompiled source file is not same as original source file, as you lost comment and readability. As per my experience attaching source code corresponding to JAR file is much better than decompiling a class file using JAD decompiler.

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 Recursion in Java with example – Programming Techniques Tutorial

Recursion is one of the tough programming technique to master. Many programmers working on both Java and other programming language like C or C++ struggles to think recursively and figure out recursive pattern in problem statement, which makes it is one of the favorite topic of any programming interview. If you are new in Java or just started learning Java programming language and you are looking for some exercise to learn concept of recursion than this tutorial is for you. In this programming tutorial we will see couple of example of recursion in Java programs and some programming exercise which will help you to write recursive code in Java e.g. calculating Factorial, reversing String and printing Fibonacci series using recursion technique. For those who are not familiar with recursion programming technique here is the short introduction: "Recursion is a programming technique on which a method call itself to calculate result". Its not as simple as it look and mainly depends upon your ability to think recursively. One of the common trait of recursive problem is that they repeat itself, if you can break a big problem into small junk of repetitive steps then you are on your way to solve it using recursion.

Top Most SQL Query to find all table names on database in MySQL and SQL Server Examples

How do you find names of all tables in a database is a recent SQL interview questions asked to one of my friend. There are many ways to find all table names form any database like MySQL and SQL Server. You can get table names either from INFORMATION_SCHEMA or sys.tables based upon whether you are using MySQL or Sql Server database. This is not a popular question like when to use truncate and delete or correlated vs noncorrelated subquery which you can expect almost all candidate prepare well but this is quite common if you are working on any database e.g. MySQL. In this SQL tutorial we will see examples of getting names of all tables from MySQL and SQL Server database. In MySQL there are two ways to find names of all tables, either by using "show" keyword or by query INFORMATION_SCHEMA. In case of SQL Server or MSSQL, You can either use sys.tables or INFORMATION_SCHEMA to get all table names for a database. By the way if you are new in MySQL server and exploring it , you may find this list of frequently used MySQL server commands handy.

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 Oracle Pagination SQL query example for Java programmer

Many time we need SQL query which return data page by page i.e. 30 or 40 records at a time, which can be specified as page size. In fact Database pagination is common requirement of Java web developers, especially dealing with largest data sets. In this article we will see how to query Oracle 10g database for pagination or how to retrieve data using paging from Oracle. Many Java programmer also use display tag for paging in JSP which supports both internal and external paging. In case of internal paging all data is loaded in to memory in one shot and display tag handles pagination based upon page size but it only suitable for small data where you can afford those many objects in memory. If you have hundreds of row to display than its best to use external pagination by asking databaseto do pagination. In pagination, ordering is another important aspect which can not be missed. It’s virtually impossible to sort large collection in Java using Comparator or Comparablebecause of limited memory available to Java program, sorting data in database using ORDER BY clause itself is good solution while doing paging in web application.

Top Most How to convert milliseconds to Date in Java - tutorial example

Do you want to convert milliseconds to Date in Java ? Actually java.util.Date is internally specified in milliseconds from epoch. So any date is number of millisecond passed since January 1, 1970, 00:00:00 GMT and Date provides constructorwhich can be used to create Date from milliseconds. Knowing the fact that Date is internally maintained in milliseconds allows you to store date in form of millisecond in Server or in your Classbecause that can be effectively expressed with a long value. In fact many experienced Java programmer store Date as long value while writing Immutable class which requires Date, one of the reason for that is Date being mutable and long value of Date can be very handy. By the ways this is next in Date related article, we have already discussed How to convert String to Date and How to get current month, year and day of week from Date in Java. If you haven’t read them already, you may find them useful. In this Java tutorial we will see example of converting millisecond into Date in Java.

Top Most 3 Example to print array values in Java - toString and deepToString from Arrays

Printing array values in Java or values of array element in Java would have been much easier if arrays are allowed to directly prints its values whenever used inside System.out.println() or format and printf method, Similar to various classes in Java do this by overriding toString() method. Despite being an object, array in Java doesn't print any meaningful representation of its content when passed to System.out.println() or any other print methods. If you are using array in method argument or any other prominent place in code and actually interested in values of array then you don't have much choice than for loop until Java 1.4. Things has been changed since Java 5 because it introduced two extremely convenient methods for printing values of both primitive and object arrays in Java. Arrays.toString(array) and Arrays.deepToString(twoDimensionArray) can print values of any array. Main difference between Arrays.toString() and Arrays.deepToString is that deepToString is used to print values of multidimensional array which is far more convenient than nesting of multiple for loops. In this Java tutorial we will see 3 different ways of printing array values in Java or value of element from Array in Java.

Top Most Spring - java.lang.ClassNotFoundException: org.springframework.web.context.ContextLoaderListener Quick Solution

If you have worked in Spring MVC than you may be familiar with
java.lang.ClassNotFoundException: org.springframework.web.context.ContextLoaderListener,
which is common problem during deployment. Spring MVC throws java.lang.ClassNotFoundException: org.springframework.web.context.ContextLoaderListener ,
when its not able to find org.springframework.web.context.ContextLoaderListener class which is used to load spring MVC configuration files like application-context.xml and other Spring Framework configuration files defined in context-param element of web.xmlin an Spring MVC web application as:

Top Most JSTL foreach tag example in JSP - looping ArrayList

JSTL foreach loop in JSP

JSTL foreach tag is pretty useful while writing Java free JSP code. JSTL foreach tag allows you to iterate or loop Array List, HashSetor any other collection without using Java code. After introduction of JSTL and expression language(EL) it is possible to write dynamic JSP code without using scriptlet which clutters jsp pages. JSTL foreach tag is a replacement of for loop and behaves similarly like foreach loop of Java 5 but still has some elements and attribute which makes it hard for first-timers to grasp it. JSTL foreach loop can iterate over arrays, collections like List, Setand print values just like for loop. In this JSP tutorial we will see couple of example of foreach loop which makes it easy for new guys to understand and use foreach loop in JSP. By the way this is our second JSP tutorial on JSTL core library, in last tutorial we have seen How to use core <c:set> tag in JSP page.

Java Interview Answers