java generic cast question

Question

I want to implement a sort of transfer object pattern. So, i have a method that fills object´s properties via BeanUtils.copyProperties(..,..) (apache common).

Here is the code:

public abstract class BaseTO<TObject> {

    public Long id;

    /***
     * Obtains id of the object that´s transfered
     * @return object´s id
     */
    public Long getId() {
        return id;
    }

    /****
     * set transfered object´s id 
     * @param id object´s id
     */
    public void setId(Long id) {
        this.id = id;
    }

    /***
     * Fill transfer object properties.
     * @param entity entity to be transfered
     * @return self reference
     */
    public BaseTO<TObject> build(TObject entity){
        try {
            BeanUtils.copyProperties(this, entity);
        } catch (IllegalAccessException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        customDataTransformation(entity);
        return this;
    }

    protected void customDataTransformation(TObject entity) {
    }


}

CustomerTO Class

public class CustomerTO extends BaseTO<Customer> {

    private String name;
    private String surname;
    private String email;
    private String sex;
    private DocumentType documentType;
    private String documentNumber;

    --- getters and setters

    @Override
    protected void customDataTransformation(Customer entity) {
        this.sex = Sex.MALE.equals(entity.getSex()) ? "M" : "F";
    }


}

the problem

CustomerTO toEdit = (CustomerTO) (customerId!=null ? new CustomerTO().build(entityObject):new CustomerTO());

as you can see here have to cast to (CustomerTO). I want if it´s possible avoid that, to make the code simpler.

Is it posible that public BaseTO build(TObject entity) can return the object of the subclass??

I hope to be clear.

Thanks in advance.

Did you write all this just because you want to skip 1 little cast? — toto2
– toto2, Commented Jul 24, 2011 at 15:04
@toto: I think that the question is more than about just casting but is also about compile-time type checking. +1 to the question. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Jul 24, 2011 at 15:09
I'm not an expert on this pattern but I don't think this is how it's intended to be used. It's for tranfering data, not ease of inheritance. It looks like you want an object factory that stores properties for all possible objects and spits out subclasses on demand, with the type determined at runtime. What's wrong with the good ol' copy constructor? CustomerTO toEdit = new CustomerTO(entityObject) You'll get compile-time checking and a much simpler codebase to maintain. I think you're trying to be too clever. — Paul
– Paul, Commented Jul 24, 2011 at 15:34
@Paul i´m trying to separete business logic from presentation. I will change the code using constructors. Thanks for answering — Müsli
– Müsli, Commented Jul 24, 2011 at 16:36

zacheusz · Accepted Answer · 2011-07-24 15:47:33Z

Maybe try this:

    class BaseTO<TObject, R extends BaseTO<TObject,R>> {

    public R build(TObject entity) {

and then CustomerTO:

class CustomerTO extends BaseTO<Customer, CustomerTO> {

or less restrictively, only change the build signature:

public <X extends BaseTO<TObject>> X build(TObject entity) {

But IMHO better approach will be simply adding constructor to TO with TObject parameter.

public BaseTO(TObject entity) {
        try {
            BeanUtils.copyProperties(this, entity);
        } catch (IllegalAccessException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        customDataTransformation(entity);
}

then in each extending class create simple constructor

     public CustomerTO(Customer entity) {
        super(entity);
    }

and forget about the build method and use it simply

CustomerTO toEdit = (customerId!=null ? new CustomerTO(entityObject):new CustomerTO());

toto2 · Accepted Answer · 2011-07-24 15:42:05Z

1

This compiles:

public class BaseTO<T> {

    public BaseTO<T> build(T entity) {
       return this;
    }

    public static class CustomerTO extends BaseTO<String> {

       @Override public CustomerTO build(String string) {
           return (CustomerTO) super.build(string);
       }
}

but you will have to override build for all subclasses of BaseTO. You write explicitly the cast only once instead of every time you call build.

EDIT: see the point raised by @Paul in the comments above. You might be suffering from "give a man a hammer and everything looks like a nail to him."

edited Jul 24, 2011 at 15:42

answered Jul 24, 2011 at 15:33

toto2

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