Why String is Immutable in Java ?

January 24, 2015 by vinaybysani

String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference.

The following code will create only one string object in the heap.

1.String string1 = “abcd”;
2.String string2 = “abcd”;

If string is not immutable, changing the string with one reference will lead to the wrong value for the other references.

Get previous month’s last date thru javascript

January 24, 2015 by vinaybysani

var d = new Date(); // Wed Jan 07 2015
d = new Date(d.getFullYear(),d.getMonth(),0); //Wed Dec 31 2014

adding zero in date column will negative the date by 1day

SAAS vs PAAS vs IAAS

January 14, 2015 by vinaybysani

SaaS (Software as a Service) model you are provided with access to application softwares often referred to as on-demand softwares. You don’t have to worry about the installation, setup and running of the application. Service provider will do that for you. You just have to pay and use it through some client. Examples: Google Apps, Microsoft Office 365.

PaaS (Platform as a Service), as the name suggests, provides you computing platforms which typically includes operating system, programming language execution environment, database, web server etc. Examples: AWS Elastic Beanstalk, Windows Azure, Heroku, Force.com, Google App Engine, Apache Stratos.

IaaS (Infrastructure as a Service), as the name suggests, provides you the computing infrastructure, physical or (quite often) virtual machines and other resources like virtual-machine disk image library, block and file-based storage, firewalls, load balancers, IP addresses, virtual local area networks etc. Examples: Amazon EC2, Windows Azure, Rackspace, Google Compute Engine

Hyperlinks

January 4, 2015 by vinaybysani

epub to pdf convertor – http://www.epub-to-pdf.com/

Enum use in java

January 2, 2015 by vinaybysani

Enums are lists of constants. When you need a predefined list of values which do not represent some kind of numeric or textual data, you should use an enum.

http://crunchify.com/why-and-for-what-should-i-use-enum-java-enum-examples/

SQL to MongoDB Aggregation Mapping Chart

December 25, 2014 by vinaybysani

http://docs.mongodb.org/manual/reference/sql-aggregation-comparison/

Ugly Numbers Program

December 25, 2014 by vinaybysani

Ugly numbers are numbers whose only prime factors are 2, 3 or 5. The sequence
1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, …
shows the first 11 ugly numbers. By convention, 1 is included.
Write a program to find and print the 150’th ugly number.

# include
# include
 
/*This function divides a by greatest divisible 
  power of b*/
int maxDivide(int a, int b)
{
  while (a%b == 0)
   a = a/b; 
  return a;
}   
 
/* Function to check if a number is ugly or not */
int isUgly(int no)
{
  no = maxDivide(no, 2);
  no = maxDivide(no, 3);
  no = maxDivide(no, 5);
   
  return (no == 1)? 1 : 0;
}    
 
/* Function to get the nth ugly number*/
int getNthUglyNo(int n)
{
  int i = 1; 
  int count = 1;   /* ugly number count */
 
  /*Check for all integers untill ugly count 
    becomes n*/
  while (n > count)
  {
    i++;      
    if (isUgly(i))
      count++; 
  }
  return i;
}
 
/* Driver program to test above functions */
int main()
{
    unsigned no = getNthUglyNo(150);
    printf("150th ugly no. is %d ",  no);
    getchar();
    return 0;
}

This method is not time efficient as it checks for all integers until ugly number count becomes n, but space complexity of this method is O(1)

Methods to use involve dynamic programming, google it