Collectors的mapping和collectingAndThen

  //对于Map的值，先把PutTarget转换为CoursePutApply，再调用toList()
    Map<String, List<CoursePutApply>> result1 = this.putTargets.stream()
      .collect(
        Collectors.groupingBy(
          PutTarget::getCity, Collectors.mapping((PutTarget putTarget) -> new CoursePutApply(), toList())
        )
      );

    //对于Map的值，先调用toList(), 再把PutTarget转换为CoursePutApply
    Map<String, CoursePutApply> result2 = this.putTargets.stream()
      .collect(
        Collectors.groupingBy(
          PutTarget::getCity, Collectors.collectingAndThen(toList(), (List<PutTarget> list) -> newCoursePutApply(this.courseId, list))
        )
      );