在Java中从一个对象映射到另一个（传输对象）时忽略空字段？

Question

I'm writing a transfer object which simply maps an object coming back from a webservice to my own object: Car A to Car B.

a Car class has a mileage property.

CarB.setMileage(CarA.getMileage()); //if the CarA.getMileage() is null, then my setter fails and I get a nullPointerException

I have 50 fields, and am I correct that I just need write 50 separate checks for null before setting my fields?

ie,

if (CarA.getMileage() != null) {
   CarB.setMileage(CarA.getMileage());
}

Is there a way to avoid writing 50 separate if !=null check statements?

Answer 1

Try using Dozer mapping. It is a good tool and it can help ignoring null values too. It can be used to map two object programatically and using XML. You do not to specify any mapping between two fields of objects if attribute names in them are same. Following is the link:

http://dozer.sourceforge.net/documentation/faq.html

Answer 2

You can create a static factory method in CarB :

// carA is of type CarA returned by the web service
CarB carB = CarB.fromCarA(carA);

All null checks etc would then be done in that factory method. Sample code:

public class CarB
{
    //....

    public static CarB fromCarA(final CarA carA)
    {
        if (carA == null)
            throw new NullPointerException("Where is my car???");

        final CarB ret = new CarB();

        // Supposing CarA returns an Integer...
        final Integer mileage = carA.getMileage();
        if (mileage != null)
            ret.setMileage(mileage);

        // etc etc, then
        return ret;
    }

    // etc
}

Answer 3

Little common but the better way is a common method in your bean Class

function String checkForNull(String str){

  //check for null and return corresponding
}

and then

CarB.setMileage(checkForNull(CarA.getMileage()));