problem with inheritance / polymorphism

[Alison Brown]

I've got an incoming XML message. One of the fields inside it tells me what sort of message has to be generated from it, there are currently 5 different ones I've got to deal with, so I've got 1 base class (with a build method) & 5 subclasses for this. I can't think of a nice way to handle the subclass constructors though. I've thought of 2 variations which are basically the same, but both are pretty bad. There has to be a clever way of doing it but I don't know what it is.

1. declare msg of class baseclass
if xml says its baseclass 1 then
msg = new baseclass1 ()
else if msg says its baseclass2 then
msg = new baseclass2 ()
else if ....
else
raise error as unrecognised message, return
end if;
msg.build ();

2. have a hash table of message codes to integers returning 0 if not found
switch (on the int returned from the hash look up)
case 1 : msg = new baseclass1 ()
case 2 : msg = new basseclass2 ()
...
default : raise error as unrecognised message, return

end switch;
msg.build ();

With both of these, this code has to change whenever a new subclass is introduced, which is what I want to avoid.
Any ideas?
:confused

[Anonymous]

How about the following scenario -- suppose you have N messages represented by N derived classes. Suppose you stick the following associations in a properties file (or a preferences object)

MSG_1 = org.foo.MessageClass1
MSG_2 = org.foo.MessageClass2
...     ...
MSG_N = org.foo.MessageClassN

Given a 'MSG_i' string, it is easy to find the associated fully qualified classname (if it exists). Given this name, the following snippet instantiates the corresponding object:

String message= "MSG_i";
String className;
// lookup className given a message text.
BaseMessage= Class.forName(className).newInstance();

et voila. If a new message is introduced, simply add a new derived class for it in your jar and add a line to the properties (of preferences) file.
kind regards

[Alison Brown]

Thanks very much, that's exactly what I needed to know.

[Michael Morris]

Hi Alison,
Looks like a job for the good old Factory pattern to me. Something like this:

public class XMLMessageFactory {
    public static final int TYPE_1 = 1;
    public static final int TYPE_2 = 2;
    public static final int TYPE_3 = 3;
    public static final int TYPE_4 = 4;
    public static final int TYPE_5 = 5;
    private XMLMessageFactory() {}
    public static XMLMessage createMessage(int typ) throws InvalidMessageType {
        XMLMessage msg;
        switch (typ) {
        case TYPE_1 :
            msg = new XML1Message();
            break;
        case TYPE_2 :
            msg = new XML2Message();
            break;
            
        case TYPE_3 :
            msg = new XML3Message();
            break;
            
        case TYPE_4 :
            msg = new XML3Message();
            break;
            
        case TYPE_5 :
            msg = new XML3Message();
            break;
        default :
           throw new InvalidMessageType("unrecoginzed message type");
        }
        return msg;
    }
}

XMLMessage would be either an abstract base class or maybe just an interface. So you would just call

    msg = XMLMessageFactory.createMessage(mstTyp);

Hope this helps,
Michael Morris