Java Generic Class Mapper Many to one

Question

I've realized a little framework to Convert a generic Entity of type A to one of Type B.

So I created an interface:

public interface IConvert<A,B> {
    public B convert (A entity);
    public List<B> convertList(List<A> entitylist);
}

And an abstract class to generalize the convertList method:

public abstract class AbstractConverter<A,B> implements IConvert<A, B> {

    @Override
    public abstract B convert(A entity) ;

    @Override
    public List<B> convertList(List<A> entitylist) {
        List<B> bs=new ArrayList<B>();
        for (A a : entitylist) {
            bs.add(convert(a));
        }
        return bs;
    }    
}

Suppose we have to map a BClass instance to an AClass instance:

public class AClassConverter extends AbstractConverter<BClass, AClass>{

    @Override
    public AClass convert(BClass entity) {
        return new AClass(); //BClass to AClass mapping
    }
}

I can simply define a new class AClassConverter, extending the abstract one and implement the specific convert method. The convertList method is free.

What I'm trying to understand is how to generalize an m to one type converter without have to always reimplement the convertList method for entity:

public class AClassConverter extends AbstractConverter<BClass, AClass> {

    @Override
    public AClass convert(BClass entity) {
    return new AClass(); //BClass to AClass mapping
    }

    public AClass convert(CClass entity) {
        return new AClass(); //CClass to AClass mapping
    }

    public List<AClass> convert(List<CClass> entityList) {
        //foreach > call convert(c);
        //return aClassList;
    }
}

Consider that AClass, BClass, and CClass don't extend any common class (apart from Object). And AClass, BClass, and CClass don't have cross reference (AClass is unaware of BClass so I can't define in BClass something as convertToAClass...).

I'd need something as:

public class AClassConverter extends AbstractConverter<BClass, AClass> , AbstractConverter<CClass, AClass>

!!!

Regards, and sorry for the confusion.

Answer 1

Due to the lack of multiple inheritance in java it cannot be done quite that easily.

However, at the cost of a slightly less elegant solution, you could refactor your code pulling the convertList method into a static helper class.

Maybe like so:

public final class ConverterHelper {
   private ConverterHelper() {}

   public static <A,B> List<B> convertList(IConvert<A,B> converter, List<A> list)
   {
      List<B> bs=new ArrayList<B>();
      for (A a : list) {
         bs.add(converter.convert(a));
      }
      return bs;
   }
}

You would then always have to use the static helper method if you wanted to convert lists.

Answer 2

@cyon is right that you need to pull convertList out to a helper class.

Here is my solution to what I think you are after.

public interface IConverter_Bi<T, T1> {
    public T convert (T1 entity);
}

public class Multi_Converter { // here m = 3
    public IConvert_Bi<T, T1> of_Bi (T1 entity){// impl}; // you can annoymously implement
    public IConvert_Bi<T, T2> of_Bi (T2 entity){// impl}; 
    public IConvert_Bi<T, T3> of_Bi (T3 entity){// impl};   
}

public class Util{

    public List<T> convertList(List<T1> entitylist, IConvert_Bi<T,T1> converter);
}

// here is some client code

// get the master converter
Multi_Converter master = ... impl    

// transform t2List (List<T2>) to List<T>

// get a bi-converter for T2. here is one of the possible ways
T2 aT2 = t2List.get(0);
t2Converter = master.of_Bi(t2List.get(0));

Uti.convertList(t2List, t2Converter)