exam sample, curious if i got it right

Question

The question is:

Suppose o is a reference of type Object that is pointing to a type A object that contains a f method and a toString method. Both toString and f have no parameters. show the statement that calls the toString method and the statement that calls the f method.

is the answer:

 f();
 toString();

Answer 1

No, that's not right. First of all, you're not using the instance o to invoke the methods. Without specifying an instance the compiler would cause those methods to be implicitly called on this.

Second, you can't invoke o.f() since f is not a method of Object. An explicit cast is required to tell the compiler that o is of type A.

Object o = new A();
String s = o.toString();
((A)o).f();

See Also

• Inheritance (the Java Tutorials)

Answer 2

Depends on the scope in which you are invoking the function. If you are calling f and toString in instance methods of the A class, then you are correct.

If you are calling f and toString in static methods of the A class, or any method of other classes, then you will need to instantiate a new A object, then call the functions on it, like so:

A myA = new A();  // Assuming the existence of a no-args constructor
myA.f();
myA.toString();

If your reference is strictly of the type Object, then you cannot invoke f, unless you first cast it to type A.