Tuesday, May 18, 2010

SCJP Exam Watch 5 - Object Orientation (1)

If a method is overridden but you use a polymorphic (supertype) reference to refer to the subtype object with the overriding method, the compiler assumes you're calling the supertype version of the method.

If the supertype version declares a checked exception, but the overriding subtype method does not, the compiler still thinks you are calling a method that declares an exception.

Let's take a look at an example:

class Animal {
    public void eat() throws Exception {
        // throws an Exception

class Dog extends Animal {
    public void eat() {
        /*no Exceptions*/

    public static void main(String[] args) {
        Animal a = new Dog();
        Dog d = new Dog();
        d.eat();        // OK
        a.eat();        // Compiler error - unreported exception

This code will not compile because of the exception declared on the Animal eat() method. This happens even though, at runtime, the eat() method used would be the Dog version, which does not declare the exception.

Refrence: SCJP Sun® Certified Programmer for Java™ 6 Study Guide Exam (310-065)

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