How to override properties if exist otherwise keep them in java objects spring boot

Question

if we have two obejcts a and b of two classes A and B,

    public class A{
    String prop1;
    String prop2;
    String prop3;
    String prop4;
    
   public A(String prop1, String prop2, String prop3, String prop4) {
        this.prop1 = prop1;
        this.prop2 = prop2;
        this.prop3 = prop3;
        this.prop4 = prop4;
    }

    public A() {
    }
   //... setters and getters
    }




    public class B{
    String prop2;
    String prop3;

public B(String prop2, String prop3) {
        this.prop1 = prop1;
        this.prop2 = prop2;
        this.prop3 = prop3;
        this.prop4 = prop4;
    }

    public B() {
    }

    }

A a = new A("prop1","prop2","prop3","prop4");
B b = new B();
b.setProp3("p");

How can we copy not null properties from b to a ? in the example above, how we can copy prop3 which is not null and ignore prop2 which is null.

There is not automatism to do so, although there might be libraries (mapstruct is one that comes to my mind) that could do (part of) the job. To give a more specific answer, please provide a clear problem statement. Do you want to convert an `B` into an `A`? Or do you want to update an existing `A` with information from `B`? — Turing85, Dec 29 '22 at 15:20
Why is the `null`-check needed at all? If the values are not initialized explicitly, they are implicitly initialized with `null`, so "overriding" `null` with `null` does no harm. — Turing85, Dec 29 '22 at 15:48
in the example all properties of a are not null, but if we copy b properties to a prop2 will be null, we would keep the original values of prop2 since the comming from b is null — Med, Dec 29 '22 at 16:14
[This post and its answers](https://stackoverflow.com/questions/46669594/map-struct-when-source-is-null-target-should-not-be-set-to-null) shows how it can be done with MapStruct. — Turing85, Dec 29 '22 at 16:23

YLombardi · Answer 1 · 2022-12-29T16:09:03.573

2

You can use MapStruct.
It's a library that is very easy to use and you can copy all the properties from a class to another.

@Mapper
public interface AToBMapper {
    A toB(A a);
    B toA(B b);
}

A a = new A("prop1","prop2","prop3","prop4");
B = mapper.toB(a);

This is very useful when you need to copy the properties from a DTO to an Entity.

If you don't want to create a new object but update an existing one, it's also possible as you can see here : MapStruct: How to map to existing target?

More exemple here : https://www.baeldung.com/mapstruct

edited Dec 29 '22 at 16:09

answered Dec 29 '22 at 16:01

YLombardi

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mapstruct will remove the properties that are not commmon – Med Dec 29 '22 at 16:16
This does not work if we want to update some `B` with information of some `A`, especially if we want to only consider values of the instance of `A` that are not-`null`. – Turing85 Dec 29 '22 at 16:20
1

@Med Of course it will, MapStruct maps "by convention". We can, however, always [define explicit mappings](https://mapstruct.org/documentation/stable/reference/html/#basic-mappings). – Turing85 Dec 29 '22 at 16:23

score 1 · Answer 2 · answered Dec 29 '22 at 15:34

It would be possible to accomplish this with reflection or a library like BeanUtils but maybe consider using the KISS principle and just write a simple method for it:

public void copyToA(A a, B b) {
  if (b.getProp2() != null) a.setProp2(b.getProp2());
  if (b.getProp3() != null) a.setProp3(b.getProp3());
}

With BeanUtils you could extend BeanUtilsBean to overwrite copyProperty and check if the value is null and then just use it like this:

var beanUtilsBean = new YourBeanUtilsBean();
beanUtilsBean.copyProperties(a, b);

For more information check out this answer https://stackoverflow.com/a/3521314/11480721:

How to override properties if exist otherwise keep them in java objects spring boot

2 Answers2