Question

My Parent class is :

import java.io.IOException;
public class Parent {
        int x = 0;
        public int getX() throws IOException{
        if(x<=0){
         throw new IOException();
        }
       return x;
      }
 }

I extend this class to write a subclass Child:

public class Child1 extends Parent{
     public int getX(){
        return x+10;
   }
}

Notice while overriding the getX method in the Child class , I have removed the throws clause from the method definition .Now it results in an anomalous behavior by the compiler which is expected :

new Parent().getX() ;

does not compile without enclosing it in a try-catch block , as expected .

new Child().getX() ;

compiles without enclosing it in a try-catch block .

But the below lines of code needs the try-catch block .

Parent p = new Child();
p.getX();

As this could be foreseen i.e. using a parent class reference to invoke a child method during run-time polymorphism , why the designers of Java didn’t make it mandatory to include the throws clause in the method definition while overriding a particular parent class method ? I mean if a parent class method has throws clause in its definition , then while overriding it the overriding method should also include the throws clause , Ain’t it ?

Answer 1

No, this is appropriate – an overridden method can be more restrictive about what it throws (and returns) because this can be useful for callers who know at compile time that they’ll be using the overridden method, and don’t want to bother with exceptions which can’t happen etc. It has to be more restrictive rather than more permissive though, so that it can’t surprise callers who do access it through the parent declaration.

Using the overridden method via a reference of type Parent is never going to violate the contract of “it might throw IOException” – the absence of an exception doesn’t violate the contract. The other way round (if the parent didn’t declare the exception, but the overriding method does) would be a contract violation.