Exception handling in Inheritance

Question

This is in continuation of question in link: I am learning the Exception handling in java (basically in inheritance) that child class method must throw the exception which is subclass of parent class method.

"When a subclass overrides a method in super class then subclass method definition can only specify all or subset of exceptions classes in the throws clause of the parent class method(or overridden method"

This is a rule. but i am not able to figure out whats the reason behind making this rule, Why vice versa is not allowed. Why this rule is created

Already answered by Steve McLeod in the previous question: http://stackoverflow.com/a/3520613/685806 — Pino, Sep 08 '14 at 14:59
This question is not answered, the answer above explains the rule but not the reason for the rule which is what OP is asking for. — Ross Drew, Sep 08 '14 at 15:06

Ross Drew · Accepted Answer · 2014-09-12T16:14:08.017

For polymorphism to work, the method signatures need to remain the same where expected so that they conform to the same interface.

For example if you have a Car which has a method drive() which throws UnknownIgnitionException and you have a Ford that extends that class, you will sometimes want to treat it like a car (and not specifically a Ford type Car) so you would expect the method drive() to look and act exactly the same in all cars.

This means if you wanted to test drive a bunch of cars (Ford, Fiat, Porche) you could have

 for (Car c : listOfCars) //list contains Fords, Porches, Fiats, etc
 {
   try {
      c.drive(); //All sub-classed Cars contain this exact method 
   }
   catch (UnknownIgnitionException e)
   {
   //As all subclassed cars are restricted to the same method signature, you can be sure what exceptions all Cars throw
   }
 }

If they all threw standalone exceptions then you couldn't do this.

Exception handling in Inheritance

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