list转map(根据某个或多个属性分组)

需要将对应的list换成本地list，和对象换成本地对象

1、List转Map<String,List>

// 根据一个字段分组
Map<String, List<String>> map = objectLists.stream()
                .collect(Collectors.groupingBy(
                        Object::getName,
                        Collectors.mapping(Object::getId, Collectors.toList())));

// 根据多个字段分组
Map<String, List<Object>> groupMap = objectLists.stream().collect(
                Collectors.groupingBy(s->s.getId()+s.getName()));


// 把某属性作为key，对应的Object对象作为value     	   
Map<String, List<Object>> groupMap = objectLists.stream().collect(
                Collectors.groupingBy(Object::getName));

2、List转Map<String,List>

Map<String, List<String>> studentMap = studentList.stream()
                .collect(Collectors.groupingBy(
                        Object::getId,
                        Collectors.mapping(Object::getName, Collectors.toList())
                ));