Throwable method overriding in Java

Question

First, sorry for my bad english. Question: If I have a subclass that extends a method which throws a CHECKED Exception, why Java's allow's me to throw a RuntimeException in the subclass's overriding method like the example below:

public class A {

    public void doSomething() throws FileNotFoundException {
        System.out.println("doing something...");
    }
}

And then...

public class B extends A {
    public void doSomething() throws RuntimeException { // <- my question
            System.out.println("doing something here too...");
    }
}

Answer 1

Any method can throw RuntimeException or Error - the unchecked exceptions base classes. So the throws RuntimeException is irrelevant to anything else.

You can override a method with a narrower throws clause. The throws FileNotFoundException does not imply the method must throw the exception. The method in the base class may throw it; the method in the derived method does not in this case.

You can't widen the throws clause because client code with a reference to the base class would not be expecting it.

This is similar to covariant return types where you can narrow the return type of method in a derived class/interface.

Answer 2

The whole idea about these restrictions is that a subclass should not force callers to catch another type of checked exception than the one(s) specified by the overridden method declaration (be it a broader one or a completely unrelated one).

In your case:

• throws FileNotFoundException is completely omitted in B.doSomething: this is fine (it would also be fine if the subclass's method declared to throw a subclass of FileNotFoundException). There would be a problem if B.doSomething() declared throws IOException or something completely unrelated, like throws SqlException.
• throws RuntimeException is declaring that it can throw an unchecked exception. Callers are not forced to catch unchecked exceptions anyway, so this wouldn't break any code written to call A.doSomething()