Core Java Programs to improve your skill set

Problem Description: Print a multiplication table for the given number with appropriate allignment upto the given number.

Ex: Multiplication table upto number 5.
 * |   1    2    3    4     5
-----------------------------
 1 |   1    2    3    4     5
 2 |   2    4    6    8   10
 3 |   3    6    9  12   15
 4 |   4    8  12  16   20
 5 |   5  10  15  20   25

Source can be found here.

Problem Description: Print a calendar for the given month in a year. Use the below hints to generate the logic and don't use any built in functions.

1. Odd Days:
We are supposed to find the day of the week on a given date.
For this, we use the concept of 'odd days'.
In a given period, the number of days more than the complete weeks are called odd days.
2. Leap Year:
i. Every year divisible by 4 is a leap year, if it is not a century.
ii. Every 4th century is a leap year and no other century is a leap year.
Note: A leap year has 366 days.
Examples:
i. Each of the years 1948, 2004, 1676 etc. is a leap year.
ii. Each of the years 400, 800, 1200, 1600, 2000 etc. is a leap year.
iii. None of the years 2001, 2002, 2003, 2005, 1800, 2100 is a leap year.
3. Ordinary Year:
The year which is not a leap year is called an ordinary years. An ordinary year has 365 days.
4. Counting of Odd Days:
4.1. 1 ordinary year = 365 days = (52 weeks + 1 day.)
1 ordinary year has 1 odd day.
4.2. 1 leap year = 366 days = (52 weeks + 2 days)
1 leap year has 2 odd days.
4.3. 100 years = 76 ordinary years + 24 leap years
= (76 x 1 + 24 x 2) odd days = 124 odd days.
= (17 weeks + days) 5 odd days.
Number of odd days in 100 years = 5.
Number of odd days in 200 years = (5 x 2) 3 odd days.
Number of odd days in 300 years = (5 x 3) 1 odd day.
Number of odd days in 400 years = (5 x 4 + 1) 0 odd day.
Similarly, each one of 800 years, 1200 years, 1600 years, 2000 years etc. has 0 odd days.
5. Day of the Week Related to Odd Days:

Solution can be found here.

Problem description: Write a Java program to accepet variable number of String paramters which are in the form of numbers and calculate the sum of all number. Handle the exception, when a String not able to cnvert as Number and exclude the same while sum calculation.

Solution can be found here.

Problem description: Throw Module specific exception i.e., InvalidCredentialsException to identify quickly related to Login functionality errors.

This user defined(Module Specific) exception should match with the following criteria.

1. When user key in blank in username field:
   -   Throw exception along with the message to convey as username should not be blank.
   
2. When user key in bank in password field:
   -   Throw exception along with the message to convey as password should not be blank.
   
3. When user key in non-numeric string in username field:
   -   Throw exception along with the actual root cause i.e., NumberFormatException.
   
4. When user name and password not matched with the criteria:
   -   Throw exception along with the message to convey as username and password are incorrect.

Solution can be found here.