Overriding, and Your First Sight of Polymorphism

Java picks a method from the object's actual class, not the variable's declared type. That's polymorphism — and @Override is what turns a broken attempt at it into a compile error instead of a silent bug.

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A subclass can replace a method it inherited. Give the method the same name and the same parameters, and mark it @Override:

class BaseTest {
    void setUp() {
        System.out.println("[BASE] Opening browser");
    }
}
class ApiTest extends BaseTest {
    @Override
    void setUp() {
        System.out.println("[API ] No browser needed - starting REST client");
    }
}

BaseTest t = new ApiTest();
t.setUp();

[API ] No browser needed - starting REST client

Look closely at those two lines of calling code, because something important happened. The variable is declared as BaseTest, but the object created was an ApiTest — and the method that ran was ApiTest's. Java chooses the method based on the actual object, not the declared type of the variable. That behavior is called polymorphism, it is the engine of the whole next chapter, and this is your first sight of it.

@Override deserves a moment. It is optional, and you should never omit it. It tells the compiler "I intend to replace a parent method" — and if you have made a mistake, so that no parent method matches, the compiler refuses to build. Without it, that same mistake compiles happily and produces a silent bug — which is exactly what the next article is about. One annotation, an entire class of defect prevented.

Given the BaseTest/ApiTest above: in BaseTest t = new ApiTest();, which class's setUp runs? What must be the first statement in a subclass constructor that calls its parent? What does @Override protect you from?

Answer — ApiTest's, because Java picks the method from the actual object. super(...). And @Override protects you from a method you meant as an override that matches no parent method — the compiler rejects it instead of silently creating a new one.

Overriding looks simple in an example this small. The trap that catches almost everyone shows up the moment the replacement method's signature drifts even slightly from the original.