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NOTE: This is written for me to recap and relearn what I learnt before….

This article is written in order for me to recap what I have learnt and used before, though I know there are already many sites which talked about all these.

Visitor Pattern defines a new operation to deal with the classes of the elements without changing their structures.

E.g.,

Create a IProcess interface,

public interface IProcessor {
    public void process(IMessage message);
}

IProcessor accepts IMessage as the parameter.

Create a IMessage interface,

public interface IMessage {
    public String parse();
}

Create a MessageProcessor class which implements IProcessor

public class MessageProcessor implements IProcessor{

    private String messageType;

    public void process(IMessage message) {
        messageType = message.parse();
    }

    public String toString() {
        return "Processor for " + messageType ;
    }
}

Create SMSMessage and MMSMessage class which implement IMessage

public class SMSMessage implements IMessage {

    public String parse() {
        return "SMS Message";
    }
}

public class MMSMessage implements IMessage {

    public String parse() {
        return "MMS Message";
    }
}

You can create another message processor depending on your need.

public class AnotherMessageProcessor {
    private String messageType;

    public void process(IMessage message) {
        messageType = message.parse();
    }

    public String toString() {
        return "Another Processor for " + messageType;
    }
}

To test it, use the following code stub

public class TestPattern {

    public static void main(String[] args) {
        MMSMessage mmsMessage = new MMSMessage();
        SMSMessage smsMessage = new SMSMessage();

        MessageProcessor messageProcessor = new MessageProcessor();
        AnotherMessageProcessor
                   anotherMessageProcessor =
                                   new AnotherMessageProcessor();

        messageProcessor.process(mmsMessage);
        System.out.println(messageProcessor.toString());

        messageProcessor.process(smsMessage);
        System.out.println(messageProcessor.toString());

        anotherMessageProcessor.process(mmsMessage);
        System.out.println(anotherMessageProcessor.toString());

        anotherMessageProcessor.process(smsMessage);
        System.out.println(anotherMessageProcessor.toString());
    }
}

The output

Processor for MMS Message
Processor for SMS Message
Another Processor for MMS Message
Another Processor for SMS Message