Java流与泛型的使用

Question

我有一个下面的课：

@Data
public class PagedSchoolDto {
private final Integer count;
private final Map<String, List<School>> content;
private final String pagingState;
private final Boolean hasNext;

public PagedSchoolDto(final Slice<School> slice) {
    this.content = slice.getContent().stream().collect(
            Collectors.groupingBy(School::getId, () -> new TreeMap<>(new UUIDComparator()), Collectors.toList()));
    this.count = slice.getContent().size();
    this.hasNext = slice.hasNext();
    this.pagingState = getPagingState(slice);
}

@Nullable
private static String getPagingState(final Slice<School> slice) {
    if (slice.hasNext()) {
        CassandraPageRequest pageRequest = (CassandraPageRequest) slice.nextPageable();
        return pageRequest.getPagingState().toString();
    } else {
        return null;
    }
}

}

@Data
public class PagedDto<T> {
private final Integer count;
private final Map<String, List<T>> content;
private final String pagingState;
private final Boolean hasNext;

public PagedDto(final Slice<T> slice) {
    this.content = slice.getContent().stream().collect(
            Collectors.groupingBy(<T>::getId, () -> new TreeMap<>(new UUIDComparator()), Collectors.toList()));
    this.count = slice.getContent().size();
    this.hasNext = slice.hasNext();
    this.pagingState = getPagingState(slice);
}

@Nullable
private static <T> String getPagingState(final Slice<T> slice) {
    if (slice.hasNext()) {
        CassandraPageRequest pageRequest = (CassandraPageRequest) slice.nextPageable();
        return pageRequest.getPagingState().toString();
    } else {
        return null;
    }
}

我的所有类都有一个名为ID的属性，它是类型UUID，所以比较器应该可以在所有泛型类型中正常工作。问题是我不知道如何编写collectors.groupingby的函数和supplier代码，因为第二个代码片段给出了编译错误。

Answer 1

泛型类型T未指定，例如'Object'，它不提供getId方法。为了访问泛型代码中的getId，T必须用getId方法扩展一个接口，该方法由任何与pageddto一起使用的类实现：

public interface WithId {
    UUID getId();
}


public class School implements WithId {

    private UUID id;

    @Override
    public UUID getId() {
        return id;
    }


public class PagedDto<T extends WithId> {
...