Dynamic method dispatch

Question

There is lot of info on Dynamic dispatch in the internet, I feel like a chicken as I am not able to implement it. Please help me. Here is what I am trying to do.

ClassA{

    public void createReq(){
    }

    public String postReq(){
    }

}

ClassB{

@Test
public void myTest(){
Class A = new ClassA();
a.createReq();
String test = a.getResponse();

/* Not sure how do i do this part */

}

So, I get a string 'test' in myTest method. I want to create a ClassC that extends ClassB and write a method that would verify the string returned in myTest soon after the step (a.getResponse()).

If there is no ClassC implemented, I would just want to simply end the test. If only ClassC exists and implements a method for verification, I want the verification to be done.

How do I do this? Please help. Thanks.

Answer 1

You could create a Dispatcher interface which simply defines a method dispatch(String) (or whatever you try to achieve). The base class (ClassB) uses a NullPattern which implements the interface while the child class (ClassC) implements the interface according your needs.

The interface is quite simple:

public interface Dispatcher
{
    public void dispatch(String message);
}

The NullPattern is implemented like this:

public class NullDispatcher implements Dispatcher
{
    public void dispatch(String message)
    {
        // do nothing
    }
}

ClassB should be modified like this:

public class ClassB
{
    private Dispatcher dispatcher;

    public ClassB()
    {
        dispatcher = new NullDispatcher();
    }

    public void setDispatcher(Dispatcher dispatcher)
    {
        // change this to your needs
        if (dispatcher == null)
            dispatcher = new NullDispatcher();
        else
            this.dispatcher = dispatcher;
    }

    @Test
    public void myTest()
    {
        ClassA a = new ClassA();
        a.createRequest();
        String test = a.getResponse();

        dispatcher.dispatch(test);
    }
}

Here a new Dispatcher can be set using the setDispatcher(Dispatcher) method. This dispatcher will be used in myTest to dispatch the result of a.getResponse().

The extending class just needs to set a specific implementation of the Dispatcher. F.e. to print the response to the console you could implement a ConsoleDispatcher like this:

public class ConsoleDispatcher implements Dispatcher
{
    public void dispatch(String message)
    {
        System.out.println(message);
    }
}

To use the ConsoleDispatcher instead of the NullDispatcher in ClassC you might use a code similar to:

public class ClassC extends ClassB
{
    public ClassC()
    {
        this.setDispatcher(new ConsoleDispatcher());
    }
}

As ClassC extends ClassB you will have access to myTest which uses the defined dispatcher to dispatch the message accordingly.

HTH