Java 8：递归流映射

Question

In my Library class there is a Set<Library> items field. Hence there in the LibraryDTO I have Set<LibraryDTO> items . When I build a Library from the LibraryDTO , I realized that I will enter an infinite loop in converting the DTO to an entity in this items field, example:

private Library buildLibraryFromLibraryDTO(LibraryDTO libraryDTO) {

return Library.builder()        
        .items(libraryDTO.getItems().stream().map(library -> Library.builder()                      
                .items(library.getItems().stream().map ... repeats the same DTO to entity conversion process from the above level)                  
                .build()).collect(Collectors.toSet()))

        .build();
}

I thought about making a recursive method going through the items stream but I'm having difficulties with logic. How could I do it? I researched and saw some examples of recursion with stream but was unable to replicate for my case. Thank you

Answer 1

You need to use a base case when you don't need to call recursively to avoid an infinite loop. Means when libraryDTO.getItems() is null then don't call again.

You can use ternary operator to check if libraryDTO.getItems() is not null then call buildLibraryFromLibraryDTO for every item else null.

private Library buildLibraryFromLibraryDTO(LibraryDTO libraryDTO) {
return Library.builder()
        .items(libraryDTO.getItems() != null 
                       ? libraryDTO.getItems()
                         .stream()
                         .map(library -> buildLibraryFromLibraryDTO(library))
                         .collect(Collectors.toSet())
                       : null)
        .build();
}