Throw Keyword :

• The throw keyword is used to explicitly raise an exception. A raised exception should be handled to prevent the program from exiting abruptly.

• Syntax :

•     throw new Exception_name()
  

• Example :

main() { 
   try { 
      test_age(-2); 
   } 
   catch(e) { 
      print('Age cannot be negative'); 
   } 
}  
void test_age(int age) { 
   if(age<0) { 
      throw new FormatException(); 
   } 
}

Custom Exception :

• As specified above, every exception type in Dart is a subtype of the built-in class Exception. Dart enables creating custom exceptions by extending the existing ones.

• Syntax :

•   class Custom_exception_Name implements Exception { 
       // can contain constructors, variables and methods 
    }
  

• Example :

•      String errMsg() => 'Amount should be greater than zero'; 
    }  
    void main() { 
       try { 
          withdraw_amt(-1); 
       } 
       catch(e) { 
          print(e.errMsg()); 
       }  
       finally { 
          print('Ending requested operation.....'); 
       } 
    }  
    void withdraw_amt(int amt) { 
       if (amt <= 0) { 
          throw new AmtException(); 
       } 
    }