## How to Find Missing Number on Integer Array of 1 to 100 - BitSet Example

One of the most frequently asked question on programming interviews is, write a program to find the missing number in an array in Java, C# or any other language; depending upon which language you choose. This kind of coding interview questions are not only asked in small start-ups but also on some of the biggest technical companies like Google, Amazon, Facebook, Microsoft, mostly when they visit the campus of reputed universities to hire graduates. Simplest version of this question is to find missing elements in an area of 100 integers, which contains numbers between 1 and 100. This can easily be solved by calculating the sum of the series using n(n+1)/2, and this is also one of the quickest and efficient ways, but it cannot be used if the array contains more than one missing numbers or if the array contains duplicates.

## Does column width 80 make sense in 2014?

One of the oldest coding practice is to keep line width 80, Why? I believe it was to make your code more readable in the age of small monitors so that whole content can fit in screen, or it might have origin from the age of punch card, which was used to be 80 column wide; but do you think this rule make sense in 2014? We are now living in the age where most of the developers has got large monitors, which can show up-to 180 characters, doesn't this is wastage of precious monitor space? It also make your code unnecessary long, than it actually is. I first come to know about line wrapping at 80, while reading Oracle Code Conventions for the Java Programming Language, which was last revised at April 20, 1999, which under indentation says
4.1 Line Length
Avoid lines longer than 80 characters, since they're not handled well by many terminals and tools.

Note: Examples for use in documentation should have a shorter line length-generally no more than 70 characters.

source : http://www.oracle.com/technetwork/java/javase/documentation/codeconventions-136091.html#248

## Don't use System.exit() on Java Web Application

I have recently come across a code snippet, where programmer was using System.exit() if application failed to acquire necessary resource after couple of retry. His reasoning was that since, application cannot function, if essential resources like database is not available or there is no disk space to write records in File system. Ok, I hear you; but System.exit() in Java Web application, which runs inside either web server or application server, which itself is Java program is not a good idea at all. Why? because invoking System.exit() kills your JVM, invoking this from Tomcat or Jetty, will not only kill your application but most likely server itself. This can be potentially dangerous, if that server also host other critical application, which is not uncommon at all. As per my experience, System.exit() calls are quite common in overly broad try-catch blocks in web application start-up code that loads environment variables, properties files, connect to MQ Series, establishes database connection, opens socket connections, etc. This is still ok, if you are writing core Java based server, where each application has their own JVM, but with web application deployed on Tomcat, JBoss, WebSphere, Weblogic or any other application server, using System.exit() is big mistake. In worst case can result in outage for lots of other critical application. On the other hand, there are ways to prevent your web application from someone else’s mistake, by enabling Security Manager. System.exit() and Runtime.exit() both goes through the security manager. Enabling Security manager will catch these calls and reduce them into an exception rather than shutting down the whole VM. It's not difficult to enable the security manager in most application servers, Tomcat, JBoss both has documented steps to enable security Manager.

## Strategy Design Pattern in Java using Enum - Tutorial Example

I have said this before that Java Enum is very versatile and can do lot more than you normally expect from it. We have seen lot of examples of Enum in my earlier posts e.g. writing thread-safe Singleton using Enum and 10 ways to use Enum in Java. In this article, we will  learn a new way to use Enum, for implementing Strategy design pattern. Strategy pattern is one of the famous pattern, which takes advantage of polymorphism, to remove switch cases and strive for open close design principle. Formally it encapsulate related algorithm, known as strategy and make them interchangeable. So your Client, also known as Context, can use different algorithm or strategy, without any modification. One of the key advantage of Strategy pattern is it's extensibility, i.e. introducing new Strategy is as easy as writing a new class and implementing Strategy interface, with Enum, instead of creating separate class, you creates a separate Enum instance, which means less number of classes and full benefit if Strategy pattern.

## Why Use Interface in Java or Object Oriented Programming

Many times, I have seen questions like why should we use interface in Java, if we can not define any concrete methods inside interface? Or even more common, What is the real use of interface in Java? I can understand beginners asking this question, when they just see name of the method inside interface and nothing else. It takes time to realize real goodness or actual use of interface or abstraction in Java or any object oriented programming. One reason of this is lack of experience in really modelling something real in program using object oriented analysis and design. In this article, I will try to answer this question and give you couple of reason to use interface in your code. If you have good understanding of Object oriented basics e.g. Polymorphism, then you know that it allows you to write flexible code. Interface or abstraction are key to achieve polymorphism, when a caller use interface for calling a method, he introduce flexibility and dynamism in code, as that code will work with any implementation of that interface, not just the present concrete implementation. You will never going to get this flexibility, if you use concrete classes for calling methods, we will see this in more details in next section. Also Programming for Interfaces, is also well recognized and one of the key object oriented design principle for coding. Another use of interface in Java is that, it opens new opportunities for other goodies e.g. design patterns. Lot's of design patterns are heavily relied on interfaces and Polymorphism e.g. Decorator, Composite, Proxy or Adapter pattern, all implements same interface, as there target, and because they are based on interfaces, they can be used interchangeably.

## java.io.IOException: Map failed and java.lang.OutOfMemoryError: Map failed

While working with memory mapped file, you may get java.io.IOException: Map failed error, which is mainly caused by Caused by: java.lang.OutOfMemoryError: Map failed error as shown below. This error usually comes while mapping a big file in memory e.g. trying to map a file greater than 1 or 2GB

java.io.IOException: Map failed
at sun.nio.ch.FileChannelImpl.map(FileChannelImpl.java:888)
Caused by: java.lang.OutOfMemoryError: Map failed
at sun.nio.ch.FileChannelImpl.map0(Native Method)
at sun.nio.ch.FileChannelImpl.map(FileChannelImpl.java:885)
... 6 more

## How to Make and Run batch .bat file in Windows 8

batch files and shell scripts are developers best friend, they help to improve productivity a lot. They not only help you to automate boring, time consuming task but also saves lot of time doing and doing them again. Batch file also reduce chances of error, once tested and encourage re-usability by sharing them across team or people with similar needs. Though not every programmer, developer or computer user has mindset to use batch file, something it's too difficult to write, some just not able to fight with initial resistance and many of them are not even aware of batch file. As a Java programmer, you really don't need to learn these stuff, as almost everybody who use windows operating system for more than year, knows about it. Anyway knowing is not enough, main thing is adapting and taking full advantage of them, developing mentality towards scripting, automation and writing batch files. Remember, shell scripting is one of the important skill to get a programming job and more important to support application running on UNIX operating sytem in Investment banking domain. Writing batch scripts are not full blown scripting, but it's lighter form of it, It works best to train yourself and develop that scripting mentality to improve productivity. In this article, we will learn how to write a batch file in windows 8 operating system and revisit some important things about batch file.