Question

Currently I have a class, TransactionData, which is just a little bit more than a POJO. I build the object from an HTTPServletRequest. What I do:

public class TransactionData
{

    // ...

    public TransactionData(HttpServletRequest request) throws IOException
    {
        // do actual work here
    }

}

There many WTF here, the most disturbing one is that the object TransactionData is tightly coupled to HTTPServletRequest. What I thought: create an interface, TransactionDataExtractor, with an extract() method, so that I might implement different classes to build the object.

public interface TransactionDataExtractor
{
    public TransactionData extract();
}

But how do I pass the stuff needed to build the TransactionData to every implementation? The firt thing that came to mind was to use the different constructors, like this:

public class TransactionDataExtractorRequest implements TransactionDataExtractor
{
    private HttpServletRequest httpRequest;

    public TransactionDataExtractorRequest(HttpServletRequest httpRequest)
    {
        this.httpRequest = httpRequest;
    }

    public TransactionData extract()
    {
        // do whatever is required
    }

}    

But in this case whenever I need to build a new TransactionData object I have to create a new TransactionDataExtractorRequest. An implicit dependency I don’t like at all.
The other alternative I could think of was passing an Object parameter to extract() and cast it whenever required, giving up the type safety and introducing a lot of boiler plate ugly code

    public TransactionData extract(Object o)
    {
        HttpServletRequest h;
        if (o instanceof HttpServletRequest)
        {
             h = (HttpServletRequest)o;
        } 
        //...
    }

I don’t know if I have made myself clear. I do feel like I’m missing something, I know the solution is very simple but I can’t get hold of it.
Any thoughts?
TIA.

EDIT: the problem might even be that my hunch is completely wrong and I may dismiss it without any regret

Answer 1

If your only problem is ensuring type safety when passing the source object to extract(), you can use generics:

public interface TransactionDataExtractor<E> {
    public TransactionData extract(E source); 
} 

public class TransactionDataExtractorRequest 
    implements TransactionDataExtractor<HttpServletRequest> {
    public TransactionData extract(HttpServletRequest source) { ... }
}