Why can't overriding methods throw exceptions

Question

I was going through SCJP 6 book by Kathe sierra and came across this explanations of throwing exceptions in overridden method. I quite didn't get it. Can any one explain it to me ?

The overriding method must NOT throw checked exceptions that are new or broader than those declared by the overridden method. For example, a method that declares a FileNotFoundException cannot be overridden by a method that declares a SQLException, Exception, or any other non-runtime exception unless it's a subclass of FileNotFoundException.

Answer 1

Well java.lang.Exception extends java.lang.Throwable. java.io.FileNotFoundException extends java.lang.Exception. So if a method throws java.io.FileNotFoundException then in the override method you cannot throw anything higher up the hierarchy than FileNotFoundException e.g. you can't throw java.lang.Exception. You could throw a subclass of FileNotFoundException though. However you would be forced to handle the FileNotFoundException in the overriden method. Knock up some code and give it a try!

The rules are there so you don't lose the original throws declaration by widening the specificity, as the polymorphism means you can invoke the overriden method on the superclass.

Answer 2

To understand this let's consider an example where we have a class Mammal which defines readAndGet method which is reading some file, doing some operation on it and returning an instance of class Mammal.

class Mammal {
    public Mammal readAndGet() throws IOException {//read file and return Mammal`s object}
}

Class Human extends class Mammal and overrides readAndGet method to return the instance of Human instead of the instance of Mammal.

class Human extends Mammal {
    @Override
    public Human readAndGet() throws FileNotFoundException {//read file and return Human object}
}

To call readAndGet we will need to handle IOException because its a checked exception and mammal's readAndMethod is throwing it.

Mammal mammal = new Human();
try {
    Mammal obj = mammal.readAndGet();
} catch (IOException ex) {..}

And we know that for compiler mammal.readAndGet() is getting called from the object of class Mammal but at, runtime JVM will resolve mammal.readAndGet() method call to a call from class Human because mammal is holding new Human().

Method readAndMethod from Mammal is throwing IOException and because it is a checked exception compiler will force us to catch it whenever we call readAndGet on mammal

Now suppose readAndGet in Human is throwing any other checked exception e.g. Exception and we know readAndGet will get called from the instance of Human because mammal is holding new Human().

Because for compiler the method is getting called from Mammal, so the compiler will force us to handle only IOException but at runtime we know method will be throwing Exception exception which is not getting handled and our code will break if the method throws the exception.

That's why it is prevented at the compiler level itself and we are not allowed to throw any new or broader checked exception because it will not be handled by JVM at the end.

There are other rules as well which we need to follow while overriding the methods and you can read more on Why We Should Follow Method Overriding Rules to know the reasons.