Removing blank spaces from a String in Java

sText= sText.replaceAll("\\s+", "");


Reference -
http://www.mkyong.com/java/how-to-remove-whitespace-between-string-java/

using Comparator to sort a List in JAVA

Sorting a List based on one of its attributes is achieved by using the Comparator interface.

For example,

public class EmpSortByName implements Comparator{

public int compare(Employee o1, Employee o2) {
return o1.getName().compareTo(o2.getName());
}
}

import java.util.*;

public class TestEmployeeSort {

public static void main(String[] args) {

List coll = Util.getEmployees();
//Collections.sort(coll);
//use Comparator implementation
Collections.sort(coll, new EmpSortByName());
printList(coll);
}

private static void printList(List list) {
System.out.println("EmpId\tName\tAge");
for (Employee e: list) {
System.out.println(e.getEmpId() + "\t" + e.getName() + "\t" + e.getAge());
}
}
}

This sorts the list based of employee names.

Calculate distance between 2 geo points

Given the latitude and longitude of 2 points, we can find the distance between then using the Haversine formula.

The Java function to do the same is given below :


public static double getDistance(double lat1, double lon1, double lat2, double lon2)
{
int R = 6371; // km
double dLat = Math.toRadians(lat2-lat1);
double dLon = Math.toRadians(lon2-lon1);
double lat1mod = Math.toRadians(lat1);
double lat2mod = Math.toRadians(lat2);

double a = Math.sin(dLat/2) * Math.sin(dLat/2) +
Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(lat1mod) * Math.cos(lat2mod);
double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
double d = R * c;

return d;
}

auto shutdown Windows machine

shutdown /s /t seconds

/s - shutdown, there are other options like /r - restart
/t seconds - specifies time in seconds, after which system will be shutdown

complexity of algorithms

big theta - gives both an upper and lower bound on the running time of an algorithm. So, if we say running time of algorithm A is theta(n^2), we mean that given any input, the algorithm would take f(n^2) time to complete.

big O - gives an upper bound on the running time of an algorithm. So, if we say run time of A is O(n^2), we mean, given any input size, it will not take more than f(n^2) time to execute A. It may take lesser.

big omega - gives a lower bound. if run time of A is omega(n^2), we mean that it takes no less than f(n^2) time to complete. It may take more.

So, when we say worst case complexity of quick sort is theta(n^2), what we mean is, in the worst case, quick sort always takes f(n^2) time, however large the input size is. It does not say anything about the best case, or the average case !!!

Some C++ stuff

The const keyword in C++

Virtual Functions

replace text in file linux

sed 's/old_text/new_text/' filename > outputfile

old_text - text to be replaced in file 'filename'

new_text - text to be substituted file 'filename'

the above command can be used to remove text from a file, by leaving the new_text blank :

sed 's/old_text//' filename > outputfile

Nutch, Solr, ......

I am working on developing a custom search using Apache Nutch in Java. Being bombarded with so much new stuff - nutch, lucene, servlet container, tomcat, jetty. Had never even heard these before !!!! Its a bit confusing, and a bit intimidating, but hope to make good sense of all these terms.

Charging a cell phone battery when charger/charging port does not work !

Do not want to buy a new charger if your charger does not work? Or has your cell phone had a gr8 fall, and the charging port is damaged?

DO NOT WORRY !!!

You can charge your cell phone battery using a USB cable. All you need to do is strip the cable on one side, and locate the red(+ve) and black(-ve) wires. Now, connect the red wire to the +ve terminal of your battery, and the black one to the -ve, while the battery is plugged into the phone.

Better yet, watch this video on Youtube : http://www.youtube.com/watch?v=9pYcgmLhUcQ

WARNING : I do not know if this damages your battery or not, but it is worth trying ;)

Persisting Environment variables in Linux

I was tired of changing the JAVA_HOME path everyday, before starting work at office. So I dug in a bit and found that you could make environment variables persistent by editing the file /etc/environment. If you locate the file and it opens in read-only mode, try 'sudo gedit /etc/environment' at the command prompt.