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 Exce         


        

Answer 1

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

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

Propagate the required exception to the Father.. to me this is against encapsulation, inheritance and general OOP ( the father potentially throw and exception that will never happen )

Au contraire: This is good OO. Image the caller side:

Father f = factory.getSomeImplementation();
f.method1();
// user has no chance to see the `Exception` of Child coming...

The factory can return an instance of Father or Child or something completely different like Brother. But the contract of method1 must be the same in all cases. And this contract includes checked exceptions. This is the Liskov substitution principle, one of the basic rules of OO.

So if the exception is part of the business contract of method1 it must be declared at the root. If it is not (e.g. a simple argument check) then a RuntimeException is the route to go anyways (i.e. even without inheritance).

Answer 4

The "throws" part is a part of the method signature.
This is why the method at the "child" class is not an override of the method of the parent class.

Answer 5

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.