Thread: Overriding Methods

You have a class A that contains someMethod() which throws a NullPointerException. So any code calling that method knows it must catch the NPE. You then have a class B that extends A and overrides someMethod()- but that someMethod() throws an IllegalArgumentException instead. That results in this code-

A bInstance = new B();
try{
   bInstance.someMethod();
}
catch(NullPointerException e){
   //whatever
}

Do you see what's wrong with allowing that?

Originally Posted by tcstcs

Please explain in more detail.

What? How much more detail do you need? What didn't you understand? Did you write a similar program yourself to try it out?

Code that calls a method that is declared to throw exceptions must be explicitly coded to either handle those exceptions or itself be declared to throw them on. If you could override a method in a subclass to throw different exceptions, code that expected the superclass implementation could receive a subclass implementation and try to call that method, but wouldn't know about the different exceptions, so wouldn't know to handle them or throw them on. That would break the rule that they must be explicitly handled or thrown, so it isn't allowed.

The exceptions declared to be thrown by a method are part of the method signature. When you override a method, you must use the same method signature.

Check out Exceptions in Java.