无法将java.util.Optional与stream转换

Question

@Entity
@Data @AllArgsConstructor @NoArgsConstructor
public class Student {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String name;

Stream.of("John","Sophie","emilia").forEach(s->{
            studentRepository.save(new Student(s));
        });

int[] empIds = { 1, 2, 3 };
        List<Student> students= Stream.of(empIds)
                .map(studentRepository::findById).collect(Collectors.toList());

Answer 1

第一个问题是stream.of将创建int arrays流，而不是Integer流

例如：

Stream.of(empIds).forEach(System.out::println);      //[I@7c3e4b1a
IntStream.of(empIds).forEach(System.out::println);   //1 2 3

因此，请使用IntStream.of或Arrays.Stream()

数组.流

List<Student> students= Arrays.stream(empIds)
            .mapToObj(studentRepository::findById)
            .filter(Optional::isPresent)
            .map(Optional::get)
            .collect(Collectors.toList());

IntStream.of

List<Student> students= IntStream.of(empIds)
            .mapToObj(studentRepository::findById)
            .filter(Optional::isPresent)
            .map(Optional::get)
            .collect(Collectors.toList());

在当前方法中，您将返回列表 >

List<Optional<Student>> students= IntStream.of(empIds)
            .map(studentRepository::findById).collect(Collectors.toList());