对你来说， GroupingBy 可能是多余的。您只需将DTO收集到 Map 并归还 values 收藏。
代码段：

public static Collection<IsolatedTrafficPerSession> groupBasedOnSessionId(List<IsolatedDetailQueryResponseDTO> list) {
    return list.stream()
               .collect(Collectors.toMap(
                       IsolatedDetailQueryResponseDTO::getIsolateSessionId,
                       dto -> new IsolatedTrafficPerSession()
                               .setSession_id(dto.getIsolateSessionId())
                               .setTimeSpent(dto.getIsolateType() == 0 ? dto.createdTimestamp : 0),
                       ( v1, v2) -> v1.setTimeSpent(v1.getTimeSpent() + v2.getTimeSpent())))
               .values();
}

测试：

public static void main(String[] args) {
    List<IsolatedDetailQueryResponseDTO> input = Arrays.asList(
            new IsolatedDetailQueryResponseDTO("123", "0", 1, 0),
            new IsolatedDetailQueryResponseDTO("123", "1", 2, 0),
            new IsolatedDetailQueryResponseDTO("124", "0", 2, 0),
            new IsolatedDetailQueryResponseDTO("123", "0", 0, 10),
            new IsolatedDetailQueryResponseDTO("123", "1", 0, 5),
            new IsolatedDetailQueryResponseDTO("124", "0", 0, 2));

    Collection<IsolatedTrafficPerSession> result = groupBasedOnSessionId(input);
    result.forEach(System.out::println);
}

输出：

123 15
124 2

注：
使用流畅的样式设置器 IsolatedTrafficPerSession 如下所示。

public IsolatedTrafficPerSession setSession_id(String sessionId) {
        this.sessionId = sessionId;
        return this;
    }

用于 IsolatedDetailQueryResponseDTO :

public IsolatedDetailQueryResponseDTO(String isolateSessionId, String isolateTabId, int isolateType, int createdTimestamp) {
       this.isolateSessionId = isolateSessionId;
       this.isolateTabId = isolateTabId;
       this.isolateType = isolateType;
       this.createdTimestamp = createdTimestamp;
   }