How do I call a method from an extend Class in Java?

I have a general Class called "Message":

public abstract class Message {

    private int type;
    private String origin;
    private String destination;
    
    public Message(int type, String origin, String destination) {
        this.type = type;
        this.origin = origin;
        this.destination = destination;
    }

    public int getType() {
        return type;
    }

    public String getOrigin() {
        return origin;
    }

    public String getDestination() {
        return destination;
    }
    
}

Also I have 2 more classes that extend from that one. "Conffirmation Connection Message"

public class ConnectionConffirmationMessage extends Message{

    public ConnectionConffirmationMessage(int type, String origin, String destination) {
        super(type, origin, destination);
    }

}

and "Connection Message"

public class ConnectionMessage extends Message{
    
    User info;

    public ConnectionMessage(int type, String origin, String destination, User info) {
        super(type, origin, destination);
        this.info = info;
    }

    public User getInfo() {
        return info;
    }
    
}

My problem is that now when I want to ask for the info when the Connection Message arrives I can’t. Because the general class "Message" doesn’t have the method "getInfo()".

Message msg = (Message) fin.readObject();

Any idea of what can I do?

Also, I can’t simply change the type because I have this in a while with a switch, so if another type of Message arrives I have to process it.

>Solution :

You can cast you message into the specific sub-type

Before Java 14

if (msg instanceof ConnectionMessage) {
   User user = ((ConnectionMessage) message).getInfo(); 
   // do something with the user
}

After Java 14

if (msg instanceof ConnectionMessage connectionMessage) { 
    User user = connectionMessage.getInfo(); 
    // do something with the user
}

In this way, you will call the correct method.

Other solutions would be:

• adding an abstract method to your Message class. There you could call method without casting. Disadvantage: You need to implement the abstract method to every subclass of Message.
• using reflections and test for the existance of the getInfo and call it via reflections. Generally speaking, you should not use reflections, if there is an native java way (like casting)