Exception and Inheritance in Java

Question

Suppose we have this problem

public class Father{
    public void method1(){...}
}

public class Child1 extends Father{
    public void method1() throws Exception{
    super.method1();
    ... 
    }

}

Child1 extends Father and overrides method1 but given the implementation Child1.method1 now throws a exception. This won't compile as the overriding method can't throw new exceptions.

What is the best solution?

• Propagate the required exception to the Father. To me this is against encapsulation, inheritance and general OOP (the Father potentially throws an exception that will never happen).
• Use a RuntimeException instead? This solution won't propagate the Exception to the Father, but Oracle docs and other sources state that class of exceptions should be used when "Client code cannot do anything". This is not that case, this exception will be useful to recover blablabla (why is it wrong to use RuntimeException instead?).
• Other..

Answer 1

Using RTE is not a bad idea. This is the methodology of Spring framework and it works quite fine. If you are implementing application probably use this solution.

If however you are implementing library that exposes API IMHO you should use checked exception. In this case you should create your own exception for example BaseException. Method method() of Father will throw it. The define ChildException extends BaseException and declare method1() of child class to throw it.

This does not break encapsulation: base class throws base exception. It does not have any idea about the concrete exception. Child class throws concrete exception that however extends base exception and therefore can be treated by client code as base exception.

As an example I can give you IOException and FileNotFoundException that extends it. You can work with input stream catching IOException while the concrete stream is FileInputStream and it throws FileNotFoundException. But client does not know this. It catches IOException.

Answer 2

Depends on what throws the Exception in Child1. If its some preconditions etc, you can always use any of the subclasses of RuntimeException such as IllegalArgumentException.

However, if there is some sort of CheckedException, then logic suggests that you should handle it that method itself and bubble up the message in some other way.

I think the general rule of thumb is that

if you know how to handle it.. use a checked exception else unchecked exception

Answer 3

If super class method does not declare an Exception then subclass overridden method can not declare checked exception. So you can only use Unchecked exception.

Other option is to allow Super class to declare ParentException and then child overridden methods can declare any exceptions which are child of ParentException