Week 5: Class hierarchies/09A. Calling parent class methods [ENCAPSULATION]

A child class can call parent class's methods. 
This is useful, for example, when a method is reimplemented in a child class, 
but you want to use the CHILD class IMPLEMENTATION as a BASE.


Consider the example of the class Message:
class Message {
    protected String sender;
    protected String msg;
    
    public Message(String sender, String msg) {
        this.sender = sender;
        this.msg = msg;
    }
    
    public String getMsg() {
        return "Sender: " + sender + "\n\n" + msg;
    }   
}
 

The 'SecretMessage' class inherits the 'Message' class. 
The class has been reimplemented with the method getMsg. 
However, the reimplemented method requests a message from the child/inheriting class only, which is then "encrypted":

class SecretMessage extends Message {
    public SecretMessage(String sender, String msg) {
        super(sender, msg);
    }
    
    // Overwritten method, which calls parent class's corresponding
    // method and then "encypts" the message.
    public String getMsg() {
        String content = super.getMsg();
        String secretmsg = "";
        for (int i=0; i<content.length(); i++) {
            // ADD '1' to each character of the string
            // then append to the 'secretmsg' STRING
            secretmsg += (char) (content.charAt(i) + 1);
        }
        return secretmsg;
    }
}


An example of calling classes:
public static void main(String[] args) {
    Message m = new Message("Pete Public", "Hello.");
    SecretMessage sm = new SecretMessage("Sam Secret", "Hello.");
    
    System.out.println("Class message:");
    System.out.println(m.getMsg());
    
    System.out.println();
    System.out.println("Class SecretMessage :");
    System.out.println(sm.getMsg());
}


Program outputs:
Class message:
Sender: Pete Public
Hello.

Class SecretMessage:
Ifmmp/


 

Note that CLIENTS of a CHILD class have NO ACCESS to a feature of the PARENT class 
if it has been REIMPLEMENTED (i.e. OVERWRITTEN) in the child class. 
This follows the basic principle of encapsulation: the class decides which features the client has access to.


Thus, clients of an entity of type SecretMessage cannot call the method getMsg of the Message class in any way.