List<T > to Map<S,T >
Map<String,DemoEntity> map = stats.stream().collect(Collectors.toMap(DemoEntity::getKey,
c -> c));
List<T > to Map<S,T > (Filter duplicate key
)
Map<String,DemoEntity> result = items.stream().collect(Collectors.toMap(DemoEntity::getKey,
c -> c,(e1,e2) -> e1));
List<T > to Map<S,S >
Map<String,String> map = stats.stream().collect(Collectors.toMap(DemoEntity::getKey,
DemoEntity::getStringValue));
List<T > to Map<S,List<T > >
Map<String,List<DemoEntity>> map = vars.stream().collect(Collectors.groupingBy(DemoEntity::getKey));
doc