child class method overwrites parent class method

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

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+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.
+
+