A solution to the duplication puzzle at the Grenoble Dojo

gistfile1.java

package code.puzzles;
import code.puzzles.TheRestOfTheCode.Products;
import code.puzzles.TheRestOfTheCode.Rebates;
import code.puzzles.TheRestOfTheCode.Webservice;
import code.puzzles.TheRestOfTheCode.WebserviceException;

public class WebserviceHelper {
    private Webservice webservice;
    
    public Products getProductsByCategoryAndQuantity(final Integer category, final Integer quantity) throws WebserviceException{
        return getProductsForCategoryAndQuantity(category, quantity)
            .invokeSafely();
    }

    public Rebates getRebates(final Products products) throws WebserviceException{
        return getRebatesFor(products)
            .invokeSafely();
    }

    public WsInvoker<Products> getProductsForCategoryAndQuantity(final Integer category, final Integer quantity) {
        return new WsInvoker<Products>() {
            public Products invoke() {
                return webservice.getProducts(category, quantity);
            }
            
            public String errorMessage() {
                return "failed invoking the webservice with arguments " + category + " and " + quantity;
            }
        };
    }

    public WsInvoker<Rebates> getRebatesFor(final Products products) {
        return new WsInvoker<Rebates>() {
            public Rebates invoke() {
                return webservice.getRebates(products);
            }

            public String errorMessage() {
                return "failed to get rebates for " + products;
            }
        };
    }
}
=======
package code.puzzles;
import code.puzzles.TheRestOfTheCode.WebserviceException;

public abstract class WsInvoker<T> {
    public abstract T invoke();
    public abstract String errorMessage();
    
    public T invokeSafely() throws WebserviceException {
        T t;
        try {
            t = invoke();
        } catch(RuntimeException e) {
            throw new WebserviceException(errorMessage(),  e);
        }
        ifNullThrowError(t);
        return t;
    }

    private void ifNullThrowError(Object object) throws WebserviceException {
        if (object==null)
            throw new WebserviceException("The webservice returned null");
    }
    
}
===========
package code.puzzles;

public class TheRestOfTheCode {
    static class Webservice {
        public Products getProducts(Integer category, Integer quantity) {
            return new Products();
        }
        public Products getProducts(String brand) {
            return new Products();
        }
        public Rebates getRebates(Products products) {
            return new Rebates();
        }
    }
    static class Products {
    }
    static class Rebates {
    }

    @SuppressWarnings("serial")
    public static class WebserviceException extends Exception {

        public WebserviceException(String string, RuntimeException e) {
        }

    }

}