java8 stream转换操作 List转Map、List转List、List转Map的多种方式

以下为个人在工作中的stream转换操作的使用经验，如有失误，请指正！

一、List转Map

1、List的元素为对象，Map的key为对象的某个属性，Map的value为整个对象

使用lambda表达式

Map<String,BaseUserProperty> userIdMap=Optional.ofNullable(BaseUserList).orElse(new ArrayList<BaseUserProperty>())
        .stream().collect(Collectors.toMap(BaseUserProperty::getUserId,Function.identity()));

使用箭头函数

Map<String,BaseUserProperty> userIdMap=Optional.ofNullable(BaseUserList).orElse(new ArrayList<BaseUserProperty>())
        .stream().collect(Collectors.toMap(item->item.getUserId(),item->item));

如果key有重复时的处理方式

Map<String,BaseUserProperty> userIdMap=Optional.ofNullable(BaseUserList).orElse(new ArrayList<BaseUserProperty>())
        .stream().collect(Collectors.toMap(item->item.getUserId(),item->item,(key1,key2)->key1));

指定生成LinkedHashMap

Map<String,BaseUserProperty> userIdMap=Optional.ofNullable(BaseUserList).orElse(new ArrayList<BaseUserProperty>())
        .stream().collect(Collectors.toMap(BaseUserProperty::getUserId,Function.identity(),(key1,key2)->key1,LinkedHashMap::new));

2、List的元素为Map，需要转换的Map的key为元素Map的某个key，需要转换的Map的value为整个元素Map

使用箭头函数

List<Map<String,Object>> mapList=new ArrayList<Map<String,Object>>();
Map<String,Map<String,Object>> objMap=Optional.ofNullable(mapList).orElse(new ArrayList<Map<String,Object>>())
        .stream().collect(Collectors.toMap(map->String.valueOf(map.get("key")),item->item));

重复key处理

List<Map<String,Object>> mapList=new ArrayList<Map<String,Object>>();
Map<String,Map<String,Object>> objMap=Optional.ofNullable(mapList).orElse(new ArrayList<Map<String,Object>>())
        .stream().collect(Collectors.toMap(map->String.valueOf(map.get("key")),item->item,(key1,key2)->key1));

指定生成LinkedHashMap

Map<String,Object> classTypeKeyColumnClassMap= Optional.ofNullable(classTypeKeyList).orElse(new ArrayList<String>())
        .stream().collect(Collectors.toMap(item->item,item->"0",(key1,key2)->key1,LinkedHashMap::new));

二、List<T>转List<T2>

1、T为对象，T2为对象的某个属性

使用lambda表达式

List<String> userIds=Optional.ofNullable(weekScheduleInfoDTOList).orElse(new ArrayList<ScheduleInfoDTO>())
        .stream().map(ScheduleInfoDTO::getUserId).collect(Collectors.toList());

使用箭头函数

List<String> userIds=Optional.ofNullable(weekScheduleInfoDTOList).orElse(new ArrayList<ScheduleInfoDTO>())
        .stream().map(item ->  item.getUserId()).collect(Collectors.toList());

2、T为Map，T2为Map的某个key

List<String> vacationKeyList=Optional.ofNullable(vacationMapList).orElse(new ArrayList<Map<String,Object>>()).stream()
        .map(tempMap -> String.valueOf(tempMap.get("key"))).collect(Collectors.toList());

三、List<T>转Map<k,List<T>>

1、T为对象，k为对象的某个属性

使用lambda表达式

Map<String,List<ScheduleInfoDTO>> userScheduleListMap=Optional.ofNullable(weekScheduleInfoDTOList).orElse(new ArrayList<ScheduleInfoDTO>()).stream()
        .collect(Collectors.groupingBy(ScheduleInfoDTO::getUserId));

使用箭头函数

Map<String,List<ScheduleInfoDTO>> userScheduleListMap=Optional.ofNullable(weekScheduleInfoDTOList).orElse(new ArrayList<ScheduleInfoDTO>()).stream()
        .collect(Collectors.groupingBy(item->item.getUserId()));

指定生成LinkedHashMap

Map<String,List<ScheduleInfoDTO>> userScheduleListMap=Optional.ofNullable(weekScheduleInfoDTOList).orElse(new ArrayList<ScheduleInfoDTO>()).stream()
        .collect(Collectors.groupingBy(ScheduleInfoDTO::getUserId,LinkedHashMap::new,Collectors.toList()));

2、T为Map，k为Map的key

使用箭头函数

List<Map<String,Object>> weekScheduleInfoDTOList1=new ArrayList<Map<String,Object>>();
Map<String,List<Map<String,Object>>> userScheduleListMap2=Optional.ofNullable(weekScheduleInfoDTOList1).orElse(new ArrayList<Map<String,Object>>()).stream()
        .collect(Collectors.groupingBy(item-> String.valueOf(item.get("key"))));

指定生成LinkedHashMap

List<Map<String,Object>> weekScheduleInfoDTOList1=new ArrayList<Map<String,Object>>();
Map<String,List<Map<String,Object>>> userScheduleListMap2=Optional.ofNullable(weekScheduleInfoDTOList1).orElse(new ArrayList<Map<String,Object>>()).stream()
        .collect(Collectors.groupingBy(item-> String.valueOf(item.get("key")),LinkedHashMap::new,Collectors.toList()))