What Restrictions Are Placed On Method Overriding


  • 29 Dec, 2011

    The argument list must exactly match that of the overridden method. If they don't match, you can end up with an overloaded method you didn't intend.

    → The return type must be the same as, or a subtype of, the return type declared in the original overridden method in the superclass. (More on this in a few pages when we discuss covariant returns.)

    → The access level can't be more restrictive than the overridden method's.

    → The access level CAN be less restrictive than that of the overridden method.

    → Instance methods can be overridden only if they are inherited by the subclass. A subclass within the same package as the instance's superclass can override any superclass method that is not marked private or final. A subclass in a different package can override only those non-final methods marked public or protected (since protected methods are inherited by the subclass).

    → The overriding method CAN throw any unchecked (runtime) exception, regardless of whether the overridden method declares the exception. (More in Chapter 5.)

    → 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.

    → The overriding method can throw narrower or fewer exceptions. Just because an overridden method "takes risks" doesn't mean that the overriding subclass' exception takes the same risks. Bottom line: an overriding method doesn't 106 Chapter 2: Object Orientation have to declare any exceptions that it will never throw, regardless of what the overridden method declares.

    → You cannot override a method marked final.

    → You cannot override a method marked static. We'll look at an example in a few pages when we discuss static methods in more detail.

    → If a method can't be inherited, you cannot override it. Remember that overriding implies that you're reimplementing a method you inherited! For example, the following code is not legal, and even if you added an eat() method to Horse, it wouldn't be an override of Animal's eat() method. public class TestAnimals {
    public static void main (String [] args) {
    Horse h = new Horse();
    h.eat(); // Not legal because Horse didn't inherit eat()
    }
    }
    class Animal {
    private void eat() {
    System.out.println("Generic Animal Eating Generically");
    }
    }
    class Horse extends Animal { }

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