In this article we will learn how to create user defined exceptions (checked and unchecked), and why and when to use it.
Custom exceptions are useful:
– To transform an exception and give the user a proper user readable error.
– To group similar exceptions. Say you can group business errors into BUS_ERR_XX error code, database errors into DB_ERR_XX error code.
Steps:
– Create a class which extends Exception (for checked exceptions) or RuntimeException (for unchecked exceptions).
– Call super method from the new class’s constructor, and pass the error code or error message.
In the case of unchecked exceptions, throwable is optional but it can be passed to super if you want to get the original exception in the chain of exceptions. (Refer example below for more information)
– Throw the custom exception.
Example 1: Custom checked exception
class BusinessException extends Exception
{
private static final long serialVersionUID = 1L;
public BusinessException( String errorMessage )
{
super( errorMessage );
}
}
Example 2: Custom unchecked exception
class UnexpectedException extends RuntimeException
{
private static final long serialVersionUID = 1L;
public UnexpectedException( String errorMessage, Throwable throwable )
{
super( errorMessage, throwable );
}
}
Example 3: validateYear()
validateYear() method validates year input and throws custom exceptions exceptions.
private void validateYear( String yearStr ) throws BusinessException
{
int year = 0;
try
{
year = Integer.parseInt( yearStr );
}
catch ( Exception e )
{
throw new UnexpectedException( String.format( “SYS_ERR_01: Invalid year – %s”, yearStr ), e );
}
if ( year < 1950 || year > 2021 )
{
throw new BusinessException( “BUS_ERR_01: Year should be between 1950 and 2021” );
}
}
Example 4: Main
Main method gets user year input and triggers validateYear method
import java.util.Scanner;
…
…
public static void main( String args[] )
{
CustomExceptionTest test = new CustomExceptionTest();
String yearStr;
try ( Scanner scanner = new Scanner( System.in ) )
{
yearStr = scanner.nextLine();
}
try
{
test.validateYear( yearStr );
}
catch ( BusinessException e )
{
System.out.printf( “BusinessException: %s %n %n” , e.getMessage() );
}
catch ( UnexpectedException e )
{
System.out.printf( “UnexpectedException: %s %n %n” , e.getMessage() );
e.printStackTrace();
}
}
Output: For input year '3'
BusinessException: BUS_ERR_01: Year should be between 1950 and 2021
Output: For input year 'abc' (without Throwable)
UnexpectedException: SYS_ERR_01: Invalid year – abc
com.codebonneamie.demo.UnexpectedException: SYS_ERR_01: Invalid year – abc
at com.codebonneamie.demo.CustomExceptionTest.validateYear(CustomExceptionTest.java:43)
at com.codebonneamie.demo.CustomExceptionTest.main(CustomExceptionTest.java:20)
Output: For input year 'abc' (with Throwable)
UnexpectedException: SYS_ERR_01: Invalid year – abc
com.codebonneamie.demo.UnexpectedException: SYS_ERR_01: Invalid year – abc
at com.codebonneamie.demo.CustomExceptionTest.validateYear(CustomExceptionTest.java:43)
at com.codebonneamie.demo.CustomExceptionTest.main(CustomExceptionTest.java:20)
Caused by: java.lang.NumberFormatException: For input string: “abc”
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65) at java.lang.Integer.parseInt(Integer.java:580)
at java.lang.Integer.parseInt(Integer.java:615)
at com.codebonneamie.demo.CustomExceptionTest.validateYear(CustomExceptionTest.java:39)
… 1 more
Note: In this example to demonstrate custom exception I have passed error message as input. Ideally you should pass error code, and arguments, and get the error message at the time of display to support internationalization.