How can I convert List<SomeClass> to Map<SomeClass::getField1, Set<SomeClass::getField2>>

Question

I have a class that has two fields as shown below I want to convert this class into Map<SomeClass::getField1, Set<SomeClass::getField2>> Any idea how to write a lambda for this? I'll be grateful for any help

@Getter
@Setter
@Builder
@AllArgsConstructor
@NoArgsConstructor
public SomeClass {

  @NotBlank
  String field1;

  @NotBlank
  String field2;
}

Answer 1

You need to combine Collectors.groupingBy with Collectors.mapping and Collectors.toSet:

List<SomeClass> list = List.of();
Map<String, Set<String>> map = list.stream()
        .collect(Collectors.groupingBy(
                SomeClass::getField1,
                Collectors.mapping(
                        SomeClass::getField2, // convert
                        Collectors.toSet()    // then collect as set
                )
        ));