Question

I have a superclass named Message with the field String msgType and a sub-class named ConnectMessage that extends it, which contains the field String pos_X.

The constructor of ConnectMessage is:

public ConnectMessage(String msgType, String x){
    super(msgType);
    this.pos_X = x;}

Inside the main thread i call: clientMessage = Message.fromString(inputString);
where fromString creates a ConnectMessage instance like this:

ConnectMessage cm = new ConnectMessage(str1, str2);
return cm;

But when in the main thread i call s = clientMessage.pos_X i get a compiler error, saying that there is no variable pos_X in class Message. How does inheritance work here and how can i fix that, so that the statement clientMessage.pos_X will mean pos_X defined in class ConnectMessage?

Answer 1

It’s pretty obvious:

you declared clientMessage as Message class. This class does not have a filed named pos_x.

If you want to access that field, you have to either:

• pull up the pos_x field to the base class
• cast clientMessage to ConnectMessage i.e. ((ConnectMessage)message).pos_x

Of course, I assume the field is public.

Regards