How to write custom exception in Java?

A custom exception is also known as a user-defined exception are derived classes of Java Exception classes. In order to create your Custom exception in Java following point must be taken care of.

• All exceptions must be a child of Throwable.
• If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.
• If you want to write a runtime exception, you need to extend the RuntimeException class.

Custom exception example in Java

class InvalidAgeException extends Exception{  
 InvalidAgeException(String s){  
  super(s);  
 }  
}  

class CustomExceptionTest{  
  
   static void validate(int age)throws InvalidAgeException{  
     if(age<18)  
      throw new InvalidAgeException("not valid");  
     else  
      System.out.println("welcome to vote");  
   }  
     
   public static void main(String args[]){  
      try{  
      validate(13);  
      }catch(Exception m){System.out.println("Exception occured: "+m);}  
  
      System.out.println("rest of the code...");  
  }  
} 

Above is an example of a custom exception in Java that checks the age of voter. If age is less than 18 years than it throws an InvalidAgeException.