Lambda表达式之list收集集合某一元素到另一集合

List<BelongInstitutions> belongInstitutionsList = getBelongInstitutionsList(courseBelongInstitutions, courseBelongChannels);
List<String> courseIdList = belongInstitutionsList.stream().map((a) -> a.getCmoocAllId()).collect(Collectors.toList());