Is there an easy way to parse an object from class A to Object from class B, that fields are a subset of A?

Question

example:

class A{
    String aa;
    String bb;
    String cc;
    String dd;
}

class B{
    String aa;
    String bb;
}

OF course my real classes are bigger with more than 15 fields, that are also complex Object on their own. by now I am doing

B parseToB(A a){
    B b = new b();
    b.setAa(a.getAa());
    b.setBb(a.getBb());
    return b;
}

Is there an easy way to parse A to B? A Library or a builtin mapper class? It looks kind of like a weird spot in my code where I am just getsett-ing 15 lines of code.

you can't parse the two, since there is no hierarchical relation. Just because those variables have the same name, does not mean they are related. You can map them, though. — Stultuske, Feb 11 '20 at 07:45
At the very least you should provide a copy constructor or similar instead of having a dozen public setters. Or for an automatic solution search for "object to object mapper". https://stackoverflow.com/questions/3319002/automapper-for-java — Voo, Feb 11 '20 at 07:48

score 1 · Accepted Answer · answered Feb 11 '20 at 08:13

I personally use MapStruct in my projects for this sort of thing, it provides a nice flexible way to declare simple mappings and also provides good extensions for complex mapping types.

So for your example you could simply define an interface (or abstract class)

@Mapper
public interface ObjectMapper {

    ObjectMapper INSTANCE = Mappers.getMapper(ObjectMapper.class);

    A mapToA(B b);

    B mapToB(A a);
}

You will need to configure an annotation processor during your build cycle (if you are using maven), and then you can simply call the mapper in your code

A firstObject = new A(); //Set your variables

B secondObject = ObjectMapper.INSTANCE.mapToB(firstObject);

score 0 · Answer 2 · answered Feb 11 '20 at 07:56

You can for example provide a constructor in B class that accept A as parameter:

class B {
    String aa;
    String bb;

    public B(A a) {
        this.aa = a.getAa();
        this.bb = a.getAa();
    }
 }

And then you can create a new instance of B starting from A simply calling the constructor:

B newObject = new B(a);

Remember that if you want to create a new instance of B class without parameter you have to add also the empty constructor (which can be omitted if there are no other constructors):

class B {
    String aa;
    String bb;


    public B() {
    }

    public B(A a) {
        this.aa = a.getAa();
        this.bb = a.getAa();
    }
 }

score 0 · Answer 3 · answered Feb 11 '20 at 07:57

You can use reflection. Try this:

static class A {

        String aa, bb;
        Integer cc;
        Boolean dd;
    }

    static class B {

        String aa, bb;
        Integer cc;
        Integer dd;

        @Override
        public String toString() {
            return "B{" + "aa=" + aa + ", bb=" + bb + ", cc=" + cc + ", dd=" + dd + '}';
        }


    }

    public static void main(String []args) throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
        A a = new A();
        a.aa = "aa";
        a.bb = "bb";
        a.cc = 10;
        a.dd = true;
        B b = new B();
        for (Field declaredField : b.getClass().getDeclaredFields()) {
            if (a.getClass().getDeclaredField(declaredField.getName()) != null // both class have a field with the same name      
              && a.getClass().getDeclaredField(declaredField.getName()).getType()
                  .equals(declaredField.getType())) { // and the fields are of the same type
                declaredField.set(b, a.getClass().getDeclaredField(declaredField.getName()).get(a));
            }
        }
        System.out.println(b);

    }

Is there an easy way to parse an object from class A to Object from class B, that fields are a subset of A?

3 Answers3