What is the difference between STRINGBUFFER and STRING?

What is the difference between STRINGBUFFER and STRING?

String object is immutable. i.e , the value stored in the String object cannot be changed. For e.g.

String myString = “Hello”; myString = myString + ” Guest”; System.out.println("myString " + myString);

When you run System.out on myString  the output will be “Hello Guest”. Although we made use of the same object (myString), internally a new object was created in the process because -  String is an immutable class in java . That’s a performance issue.

StringBuffer/StringBuilder objects are  mutable   StringBuffer/StringBuilder objects are mutable i.e. we can make changes to the value stored in the object. This means is that string operations such as append would be more efficient if performed using StringBuffer/StringBuilder objects than String objects.

String str = “Be Happy With Your Salary.'' str += “Because Increments are a myth"; StringBuffer strbuf = new StringBuffer(); strbuf.append(str); System.out.println(strbuf);

The Output of the code snippet would be: Be Happy With Your Salary. Because Increments are a myth. But here the difference is that only one  object of stringbuffer is created in the example. That improves space complexity of the program. 

A sample ant build script and its explaination

A sample build script is given below . Please refer comments for explaination

<?xml version="1.0" ?>
<!-- Default compilation action for ant build -->
<project name="<Your application>" default="compile">
<path id="compile.classpath">
<fileset dir="WEB-INF/lib">
<include name="*.jar"/>
</fileset>
</path>
<!-- init target for ant build -->
<!-- creates classes , conf , dist folders. -->
<target name="init">
<delete dir="WEB-INF/classes"/>
<mkdir dir="WEB-INF/classes/conf"/>
<mkdir dir="dist" />
</target>
<!-- As specified in project tag default action in build will be called -->
<target name="compile" depends="init" >
<!-- Compile all classes and copy them to WEB-INF/classes -->
<javac destdir="WEB-INF/classes" debug="true" srcdir="src">
<classpath refid="compile.classpath"/>
</javac>

<copy todir="WEB-INF/classes">
<fileset dir="conf"/>
</copy>
</target>
<!-- Compile and package the classes to a war -->
<target name="war">
<war destfile="dist/s31.war" webxml="WEB-INF/web.xml">
<fileset dir="../dist"/>
<lib dir="WEB-INF/lib"/>
<classes dir="WEB-INF/classes"/>
</war>
</target>
</project>

Linux Commands

File Commands
1.	ls	Directory listing
2.	ls -al	Formatted listing with hidden files
3.	ls -lt	Sorting the Formatted listing by time modification
4.	cd dir	Change directory to dir
5.	cd	Change to home directory
6.	pwd	Show current working directory
7.	mkdir dir	Creating a directory dir
8.	cat >file	Places the standard input into the file
9.	more file	Output the contents of the file
10.	head file	Output the first 10 lines of the file
11.	tail file	Output the last 10 lines of the file
12.	tail -f file	Output the contents of file as it grows,starting with
		the last 10 lines
13.	touch file	Create or update file
14.	rm file	Deleting the file
15.	rm -r dir	Deleting the directory
16.	rm -f file	Force to remove the file
17.	rm -rf dir	Force to remove the directory dir
18.	cp file1 file2	Copy the contents of file1 to file2
19.	cp -r dir1 dir2	Copy dir1 to dir2;create dir2 if not present
20.	mv file1 file2	Rename or move file1 to file2,if file2 is an existing
		directory
21.	ln -s file link	Create symbolic link link to file
Process management
1.	ps	To display the currently working processes
2.	top	Display all running process

3.	kill pid	Kill the process with given pid
4.	killall proc	Kill all the process named proc
5.	pkill pattern	Will kill all processes matching the pattern
6.	bg	List stopped or background jobs,resume a stopped
		job in the background
7.	fg	Brings the most recent job to foreground
8.	fg n	Brings job n to the foreground

Searching

1.	grep pattern file	Search for pattern in file
2.	grep -r pattern dir	Search recursively for pattern in dir
3.	command | grep	Search pattern in the output of a command
	pa
4.	locate file	Find all instances of file
5.	find . -name filename	Searches in the current directory (represented by
		a period) and below it, for files and directories with
		names starting with filename
6.	pgrep pattern	Searches for all the named processes , that
		matches with the pattern and, by default, returns
		their ID

System Info

1.	date	Show the current date and time
2.	cal	Show this month's calender
3.	uptime	Show current uptime
4.	w	Display who is on line
5.	whoami	Who you are logged in as

6.	finger user	Display information about user
7.	uname -a	Show kernel information
8.	cat /proc/cpuinfo	Cpu information
9.	cat proc/meminfo	Memory information
10.	man command	Show the manual for command
11.	df	Show the disk usage
12.	du	Show directory space usage
13.	free	Show memory and swap usage
14.	whereis app	Show possible locations of app
15.	which app	Show which applications will be run by default

Compression

1.	tar cf file.tar file	Create tar named file.tar containing file
2.	tar xf file.tar	Extract the files from file.tar
3.	tar czf file.tar.gz files	Create a tar with Gzip compression
4.	tar xzf file.tar.gz	Extract a tar using Gzip
5.	tar cjf file.tar.bz2	Create tar with Bzip2 compression
6.	tar xjf file.tar.bz2	Extract a tar using Bzip2
7.	gzip file	Compresses file and renames it to file.gz
8.	gzip -d file.gz	Decompresses file.gz back to file

Network

1.	ping host	Ping host and output results
2.	whois domain	Get whois information for domains
3.	dig domain	Get DNS information for domain
4.	dig -x host	Reverse lookup host
5.	wget file	Download file
6.	wget -c file	Continue a stopped download

Shortcuts
1.	ctrl+c	Halts the current command
2.	ctrl+z	Stops the current command, resume with fg in the
		foreground or bg in the background
3.	ctrl+d	Logout the current session, similar to exit
4.	ctrl+w	Erases one word in the current line
5.	ctrl+u	Erases the whole line
6.	ctrl+r	Type to bring up a recent command
7.	!!	Repeats the last command
8.	exit	Logout the current session