can't map property. Consider to declare/implement a mapping method. Can't generate mapping method from non-iterable type to iterable type

Question

I can't seem to use mapstruct correctly

@Mapping(target = "products", source = "itemBookType")
SearchBookingResult backToTp(ItemBook itemBook);

When running this code I get the following error:

Can't map property "ProductType itemBookType" to "List<ProductOverview> products". Consider to declare/implement a mapping method: "List<ProductOverview> map(ProductType value)".

I added the following code at the bottom:

List<ProductOverview> map(ProductType value);

but still it returns me the following error:

Can't generate mapping method from non-iterable type to iterable type from java stdlib.

Itembook class:

public class ItemBook {
    private ProductType itemBookType; //ProductType class
    private Integer idref;
    private String reference;
}

ProductType class:

public enum ProductType {
    BOOK, PHONE, GAME
}

SearchBookingResult class:

public class SearchBookingResult extends BaseResponse<SearchBookingResult> {
    private String reference;
    private List<ProductOverview> products;
}

the Mapper

 @Mapper(componentModel = "spring")
    public interface ItemBookMapper {
        ItemBookTpMapper INSTANCE = Mappers.getMapper(ItemBookTpMapper.class);
        @Mapping(target = "reference", source = "idref")
        @Mapping(target = "products", source = "itemBookType")
        SearchBookingResult backToTp(ItemBook itemBook);
        List<ProductOverview> map(ProductType value);
    
    }

ProductOverView class is abstract:

public abstract class ProductOverview implements Serializable {

    private ProductType productType;

    public ProductOverview(ProductType productType) {
        this.productType = productType;
    }

    public ProductType getProductType() {
        return productType;
    }
}

map reference work but products return many error.

Bigmouth

You need to implement the method yourself as MapStruct cannot deduce how to convert from your single element to a list element. – Flightless

and how? can you give me directions? – Unconventionality

By writing the method? How else would you do it? Instead of only the declaration List<ProductOverview> map(ProductType value); write a proper implementation of that method. If you have an interface use a default method else use an abstract class that implements it (and read the documentation of MapStruct which explains this as well). – Flightless

can you give the documentation about it? – Unconventionality

What is it you don't understand about writing a method? That is all you need to do? You have the signature already now write the implementation. – Flightless

I said i wrote it but it still returns error – Unconventionality

No you didn't... You wrote the declaration not the implementation. – Flightless

but the problem is that I don't know how to write the implementation, how, where? – Unconventionality

You know how to write java don't you? Just instead of a method declaration write the actual method as you would write it in any other java class. It is no different then anything else you write. – Flightless

Answer 1

MapStruct can't generate mapping method from non-iterable type to iterable type because it's impossible to create a generic mapping.

The only solution, as suggested by the exception, is to create a custom method where you can implement your own mapping algorithm.

@Mapper(componentModel = "spring")
public interface ItemBookMapper {

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

    @Mapping(target = "reference", source = "idref")
    @Mapping(target = "products", source = "itemBookType", qualifiedByName = "mapProducts")
    SearchBookingResult backToTp(ItemBook itemBook);

    @Named("mapProducts") 
    default List<ProductOverview> mapProducts(ProductType value){
        List<ProductOverview> products = new ArrayList<>();
        
        //add your custom mapping implementation

        return products;
    }

}

• Official documentation: adding-custom-methods
• Baeldung tutorial: mapstruct-custom-mapper

Shemeka