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Tuesday, October 18, 2016

Most Common mistakes in Java Coding.

1. Copy & Paste

Its one of the most powerful tool and technique in the typical programmer. But at the same time its too dangerous that you ought to forgot the code according to you requirements. So always be cautious about what piece of code you are copying. Read at least two to three times to avoid the tricky for fishy things while your unit testing.

2. Exception Catch Block.

Always give priority to write some thing useful and meaning full statements in the exception catch block.It really worth when you troubling shooting the issues. Never leave this block with empty or sop's. And always throw the exception to the calling method.

3. String operations

Since String is immutable object, when you do any operation on string that results a new object. So after you do any modifications on String and do assign the string some object and use it.

4. Null pointers

Don't perform the operations on objects without checking the Object null condition. And while checking the null condition always provide 'null' key word as the first operand. e.g. if(null != object) . Especially while printing any calling any methods on an object.

5. Result Set next() operation.

Be in mindful that when ever you call resultset.next() operation cursor moves from one row to its next row.




CRON Expressions

 
CRON expression.
 
Cron-Expressions are used to configure instances of CronTrigger. Cron-Expressions are strings that are actually made up of seven sub-expressions, that describe individual details of the schedule. These sub-expression are separated with white-space, and represent:

  1. Seconds
  2. Minutes
  3. Hours
  4. Day-of-Month
  5. Month
  6. Day-of-Week
  7. Year (optional field)
 
 
 
+-------------------- second (0 - 59)
|  +----------------- minute (0 - 59)
|  |  +-------------- hour (0 - 23)
|  |  |  +----------- day of month (1 - 31)
|  |  |  |  +-------- month (1 - 12)
|  |  |  |  |  +----- day of week (0 - 6) (Sunday=0 or 7)
|  |  |  |  |  |  +-- year [optional]
|  |  |  |  |  |  |
*  *  *  *  *  *  * command to be executed 
 
 
Examples.
 
1. For every 10 seconds 
 
0/10 * * 1/1 * ? 
 
2. For every Sunday midnight at 23:00 
 
0 0 23 ? * SUN
 
Some of the frequently used schedules
 
 
You can play with http://www.cronmaker.com/ website for more number of combinations. 

Cron Expression Examples
Cron Expression Example
Creates Trigger that Fires at
0 0 12 * * ?
12 pm (noon) every day
0 15 10 ? * *
10:15 am every day
0 15 10 * * ?
10:15 am every day
0 15 10 * * ? *
10:15 am every day
0 15 10 * * ? 2005
10:15 am every day during the year 2005
0 * 14 * * ?
Every minute starting at 2 pm and ending at 2:59 pm, every day
0 0/5 14 * * ?
Every 5 minutes starting at 2 pm and ending at 2:55 pm, every day
0 0/5 14,18 * * ?
Every 5 minutes starting at 2 pm and ending at 2:55 pm, AND fires every 5 minutes starting at 6 pm and ending at 6:55 pm, every day
0 0-5 14 * * ?
Every minute starting at 2 pm and ending at 2:05 pm, every day
0 10,44 14 ? 3 WED
2:10 pm and at 2:44 pm every Wednesday in the month of March.
0 15 10 ? * MON-FRI
10:15 am every Monday, Tuesday, Wednesday, Thursday and Friday
0 15 10 15 * ?
10:15 am on the 15th day of every month
0 15 10 L * ?
10:15 am on the last day of every month
0 15 10 ? * 6L
10:15 am on the last Friday of every month
0 15 10 ? * 6L
10:15 am on the last Friday of every month
0 15 10 ? * 6L 2002-2005
10:15 am on every last friday of every month during the years 2002, 2003, 2004 and 2005
0 15 10 ? * 6#3
10:15 am on the third Friday of every month
0 0 12 1/5 * ?
12 pm (noon) every 5 days every month, starting on the first day of the month
0 11 11 11 11 ?
Every November 11th at 11:11 am.