Question

1-Explain protected member access?

2-Explain how to invoke a superclass method from a subclass method for the case in which the subclass method overrides a superclass method and the case in which the subclass method does not override a superclass method

Answer 1

NOTE: Since you have not mentioned the language of your preference, I am providing the coding examples in Java.

1)

Variables, methods, and constructors, which are declared protected in a superclass can be accessed only by the subclasses in other package or any class within the package of the protected members' class.

The protected access modifier cannot be applied to class and interfaces. Methods, fields can be declared protected, however methods and fields in a interface cannot be declared protected.

Protected access gives the subclass a chance to use the helper method or variable, while preventing a nonrelated class from trying to use it.

Example

The following parent class uses protected access control, to allow its child class override openSpeaker() method −

class AudioPlayer {
   protected boolean openSpeaker(Speaker sp) {
      // implementation details
   }
}

class StreamingAudioPlayer {
   boolean openSpeaker(Speaker sp) {
      // implementation details
   }
}

Here, if we define openSpeaker() method as private, then it would not be accessible from any other class other than AudioPlayer. If we define it as public, then it would become accessible to all the outside world. But our intention is to expose this method to its subclass only, that’s why we have used protected modifier.

2)

In order to invoke a superclass method from a subclass method for the case in which the subclass method overrides a superclass method, the super keyword is used. See the example below for more explanation.

Example:

public class CrunchifyObjectOverriding {

public static void main(String args[]) {

Company a = new Company(); // Company reference and object

Company b = new eBay(); // Company reference but eBay object

a.address();// runs the method in Company class

b.address();// Runs the method in eBay class

}

}

class Company {

public void address() {

System.out.println("This is Address of Crunchify Company...");

}

}

class eBay extends Company {

public void address() {

super.address(); // invokes the super class method

System.out.println("This is eBay's Address...");

}

}

OUTPUT

This is Address of Crunchify Company...

This is Address of Crunchify Company...

This is eBay's Address...

The case in which the subclass method does not override a superclass method, we can just call the method without the "super" keyword.

Example

public class CrunchifyObjectOverriding {

public static void main(String args[]) {

Company a = new Company(); // Company reference and object

Company b = new eBay(); // Company reference but eBay object

a.address();// runs the method in Company class

b.address();// Runs the method in eBay class

}

}

class Company {

public void streetAddress() {

System.out.println("36-A Street");

}

}

class eBay extends Company {

public void fullAddress() {

streetAddress(); // invokes the super class method

System.out.println("This is eBay's Address...");

}

}