Thursday, May 26, 2011

10 example of using ArrayList in Java >>> Java ArrayList Tutorial

ArrayList in Java is most frequently used collection class after HashMap in Java. Java ArrayList represents an automatic re-sizable array and used in place of array. Since we can not modify size of an array after creating it, we prefer to use ArrayList in Java which re-size itself automatically once it gets full. ArrayList in Java implements List interface and allow null. Java ArrayList also maintains insertion order of elements and allows duplicates opposite to any Set implementation which doesn't allow duplicates. ArrayList supports both Iterator and ListIterator for iteration but it’s recommended to use ListIterator as it allows the programmer to traverse the list in either direction, modify the list during iteration, and obtain the Iterator's current position in the list. But while using ListIterator you need to be little careful because ListIterator has no current element; its cursor position always lies between the element that would be returned by a call to previous () and the element that would be returned by a call to next (). In this Java ArrayList tutorial we will see how to create Java ArrayList and perform various operations on Java ArrayList. This collection class is also favorites on many core Java interviews with questions like Difference between ArrayList and Vector  or LinkedList vs ArrayList.

Friday, May 20, 2011

Why wait notify and notifyAll called from synchronized block or method in Java


Why wait (), notify () and notifyAll () must be called from synchronized block or method in Java

Why wait(), notify() and notifyAll() method must be called from synchronized block or method in JavaMost of Java developer knows that wait() ,notify() and notifyAll() method of object class must have to be called inside synchronized method or synchronized block in Java but how many times we thought why ? Recently this questions was asked to in Java interview to one of my friend, he pondered for a moment and replied that if we don't call wait () or notify () method from synchronized context we will receive IllegalMonitorStateException in java. He was right in terms of behavior of language but as per him interviewer was not completely satisfied with the answer and wanted to explain more about it. After the interview he discussed the same questions with me and I thought he might have told about race condition between wait () and notify () in Java that could exists if we don't call them inside synchronized method or block. Let’s see how it could happen:

Saturday, May 14, 2011

Top 30 UNIX command Interview Questions asked in Investment Banks

UNIX command Questions Answers asked in Interview
UNIX or Linux operating system has become default Server operating system and for whichever programming job you give interview you find some UNIX command interview questions there. These UNIX command interview questions are mostly asked during Java development and Support role interviews on various investment banks mostly because most of electronic trading systems or stock trading system works on Unix servers. As we know that high volume low latency systems which wants to take advantage of little bit of volatility in market for Equity , Futures and options or Foreign exchange trading need a stable server side operating system and Redhat Linux is doing great job there. with the advent of Algorithmic trading this speed factor becomes more important so getting someone who has good knowledge of operating system and commands on which these trading system runs is definitely required. but these UNIX command interview questions are equally applicable for any job interview which requires some work on Unix Operating System. With the growing use of Linux in form of RedHat, Solaris and IBM AIX its must to keep you familiar with essential Linux commands available on various platforms. 

Unix and Linux Command Interview Questions and AnswersLong back I had once asked one of my friend why are you preparing Unix Command interview questions if you going for a Java Interview and he told me that this job doesn't only require knowledge of Java but also knowledge of Unix, Linux, SQL and other scripting language , which is quite true. After that I thought to collect various UNIX command interview questions asked to Java developers or trading system support interviews and this is the result of that compilation. This list of UNIX command interview questions are by means complete and would be great if you guys contribute some genuine and good Unix Command Interview questions and answers asked during interviews. I have divided the questions on three categories for sake of managing and keeping this list of Unix Interview questions up to date.


Beginners UNIX Interview Questions Answers

1. Write command to list all the links from a directory?
In this UNIX command interview questions interviewer is generally checking whether user knows basic use of "ls" "grep" and regular expression etc
You can write command like:
ls -lrt | grep "^l"


2. Create a read-only file in your home directory?
This is a simple UNIX command interview questions where you need to create a file and change its parameter to read-only by using chmod command you can also change your umask to create read only file.
touch file
chmod 400 file
3. How will you find which operating system your system is running on in UNIX?
By using command "uname -a" in UNIX

4. How will you run a process in background? How will you bring that into foreground and how will you kill that process?
For running a process in background use "&" in command line. For bringing it back in foreground use command "fg jobid" and for getting job id you use command "jobs", for killing that process find PID and use kill -9 PID command. This is indeed a good Unix Command interview questions because many of programmer not familiar with background process in UNIX.

5. How do you know if a remote host is alive or not?
You can check these by using either ping or telnet command in UNIX. This question is most asked in various Unix command Interview because its most basic networking test anybody wants to do it.


6. How do you see command line history in UNIX?
Very useful indeed, use history command along with grep command in unix to find any relevant command you have already executed. Purpose of this Unix Command Interview Questions is probably to check how familiar candidate is from available tools in UNIX operation system.

7. How do you copy file from one host to other?
Many options but you can say by using "scp" command. You can also use rsync command to answer this UNIX interview question or even sftp would be ok.

8. How do you find which process is taking how much CPU?
By using "top" command in UNIX, there could be multiple follow-up UNIX command interview questions based upon response of this because “TOP” command has various interactive options to sort result based upon various parameter.

9. How do you check how much space left in current drive ?
By using "df" command in UNIX. For example "df -h ." will list how full your current drive is. This is part of anyone day to day activity so I think this Unix Interview question will be to check anyone who claims to working in UNIX but not really working on it.

10. What is the difference between Swapping and Paging?
Swapping:
Whole process is moved from the swap device to the main memory for execution. Process size must be less than or equal to the available main memory. It is easier to implementation and overhead to the system. Swapping systems does not handle the memory more flexibly as compared to the paging systems.
Paging:
Only the required memory pages are moved to main memory from the swap device for execution. Process size does not matter. Gives the concept of the virtual memory. It provides greater flexibility in mapping the virtual address space into the physical memory of the machine. Allows more number of processes to fit in the main memory simultaneously. Allows the greater process size than the available physical memory. Demand paging systems handle the memory more flexibly.


Intermediate UNIX Interview Questions Answers

1. What is difference between ps -ef and ps -auxwww?
UNIX interview questions answers, UNIX Linux questionsThis is indeed a good Unix Interview Command Question and I have faced this issue while ago where one culprit process was not visible by execute ps –ef command and we are wondering which process is holding the file.
ps -ef will omit process with very long command line while ps -auxwww will list those process as well.

2. How do you find how many cpu are in your system and there details?
By looking into file /etc/cpuinfo for example you can use below command:
cat /proc/cpuinfo

3. What is difference between HardLink and SoftLink in UNIX?
I have discussed this Unix Command Interview questions  in my blog post difference between Soft link and Hard link in Unix

4. What is Zombie process in UNIX? How do you find Zombie process in UNIX?
When a program forks and the child finishes before the parent, the kernel still keeps some of its information about the child in case the parent might need it - for example, the parent may need to check the child's exit status. To be able to get this information, the parent calls 'wait()'; In the interval between the child terminating and the parent calling 'wait()', the child is said to be a 'zombie' (If you do 'ps', the child will have a 'Z' in its status field to indicate this.)
Zombie : The process is dead but have not been removed from the process table.

5. What is "chmod" command? What do you understand by this line “r-- -w- --x?

6. There is a file some where in your system which contains word "UnixCommandInterviewQuestions” How will find that file in Unix?
By using find command in UNIX for details see here 10 example of using find command in Unix

7. In a file word UNIX is appearing many times? How will you count number?
grep -c "Unix" filename

8. How do you set environment variable which will be accessible form sub shell?
By using export   for example export count=1 will be available on all sub shell.

9. How do you check if a particular process is listening on a particular port on remote host?
By using telnet command for example “telnet hostname port”, if it able to successfully connect then some process is listening on that port. To read more about telnet read networking command in UNIX

10. How do you find whether your system is 32 bit or 64 bit ?
Either by using "uname -a" command or by using "arch" command.


Advanced UNIX Interview Questions and Answers

1. How do you find which processes are using a particular file?
By using lsof command in UNIX. It wills list down PID of all the process which is using a particular file.

2. How do you find which remote hosts are connecting to your host on a particular port say 10123?
By using netstat command execute netstat -a | grep "port" and it will list the entire host which is connected to this host on port 10123.

3. What is nohup in UNIX?

4. What is ephemeral port in UNIX?
Ephemeral ports are port used by Operating system for client sockets. There is a specific range on which OS can open any port specified by ephemeral port range.

5. If one process is inserting data into your MySQL database? How will you check how many rows inserted into every second?
Purpose of this Unix Command Interview is asking about "watch" command in UNIX which is repeatedly execute command provided with specified delay.

6. There is a file Unix_Test.txt which contains words Unix, how will you replace all Unix to UNIX?
You can answer this Unix Command Interview question by using SED command in UNIX for example you can execute sed s/Unix/UNIX/g fileName.

7. You have a tab separated file which contains Name, Address and Phone Number, list down all Phone Number without there name and Addresses?
To answer this Unix Command Interview question you can either you AWK or CUT command here. CUT use tab as default separator so you can use
cut -f3 filename.

8. Your application home directory is full? How will you find which directory is taking how much space?
By using disk usage (DU) command in Unix for example du –sh . | grep G  will list down all the directory which has GIGS in Size.

9. How do you find for how many days your Server is up?
By using uptime command in UNIX

10. You have an IP address in your network how will you find hostname and vice versa?
This is a standard UNIX command interview question asked by everybody and I guess everybody knows its answer as well. By using nslookup command in UNIX, you can read more about Convert IP Address to hostname in Unix here.

I hope this UNIX command interview questions and answers would be useful for quick glance before going for any UNIX or Java job interview. Please share any interesting UNIX command interview you have come across and I will add into this list. If you are going for any Unix interview on brokerage firm or stock trading company or any Investment bank you can have a quick look here, though most of questions you might already know but its good to review it. if you like this you can see my other unix command tutorial for beginners as well 


Some of my favorite interview questions post , you may find interesting


Enjoy :)

Friday, May 6, 2011

10 points about Java Heap Space or Java Heap Memory

10 Points about Java heap memory
When I started java programming I didn't know what is java heap or heap space in Java, I was even not aware of where does object in Java gets created, it’s when I started doing professional programming I came across error java.lang.OutOfMemoryError in Tomcat then I realized What is Heap in Java or Java Heap Space. Its happens with most of programmer because learning language is easy but learning basics is difficult since there is no formal process which can teach you every basics of programming its experience and work which reveals the secret of programming. For Java developer knowledge of Heap in Java, setting size of java heap space, dealing with java heap space OutOfMemoryError, analyzing heap dumps is very important. This Java Heap tutorial is for my beginner brothers who are new in programming and learning it. It makes too much difference if you know the basics and underlying, until you know that object is created in heap, you won't be able to think why OutOfMemoryError occurs in Heap. I am trying to provide as much information about Heap in Java as I know but would like you guys to contribute and share your knowledge about Heap in Java to benefit all. By the way if you are confused between heap and stack, which is where your local variables get created, then, you can also check difference between heap and stack memory in Java.

Thursday, May 5, 2011

Tibco Tutorials for beginners


This Tibco Tutorial is collection of all my previous Tibco rendezvous and Tibco EMS tutorials. I have written this compilation post to provide all tibco tutorials at one place for easy navigation and access. Anyone who is started working on Tibco on any investment bank or brokerage house for there electronic trading systems or algorithmic trading system can benefit from these tibco tutorials because these are based on my experience in stock trading systems which uses Tibco Rendezvous for all there Front-End backend communication. Tibco Certified messaging is used by Electronic trading system or Order Management system to receive orders and Send Execution in FIX Protocol.

Monday, May 2, 2011

Top 10 tips on logging in Java - Tutorial

Java logging or logging in java is as much an art as science. knowing write tools and API for java logging is definitely science part but choosing format of java logs , format of messages, what to log in java , which logging level to use for which kind of messages are purely an experienced based things and sort of art which you learn by applying logging in java. Since its proven fact that java logging severely affects performance and I have seen latency of online stock trading application goes multiple times if they run in DEBUG mode than in WARN or higher level mode. Since latency and speed is major concern for any electronic trading system or high volume low latency stock trading system, it becomes absolutely necessary to understand and learn java logging in great details and best practices and tips available on logging in java. This is not just for finance and investment banking domain but also to any java server or client application which requires speed and java logging at same time. 


This Java logging tutorial is in continuation of my article How HashMap works in Java  and difference between HashMap and Hashtable in Java  , How Garbage collection works in Java and How Synchronization works in Java if you haven’t read already you may find some useful information based on my experience in Java .