java8中利用stream api将list转成map

Region{
    string: code;
    string: label;
    string: parentCode;
    //set/get 
}

List<Region> cityList= getRegionList();

//转成Map<regionCode, Region>

Map<String, Region> map = cityList.stream().collect(Collectors.toMap(Region::getCode, item->item));

 
或者

Map<String, Region> map = cityList.stream().collect(Collectors.toMap(Region::getCode, Function.identity()));

 
上面两个转换在code重复时会报错，改成：

Map<String, Region> map = cityList.stream().collect(Collectors.toMap(Region::getCode, Function.identity(), (k1, k2)->k2));

 
或者转成指定实现类的map：

Map<String, Region> map = cityList.stream().collect(Collectors.toMap(Region::getCode, Function.identity(), (k1, k2)->k2, LinkedHashMap::new));

 
//分组：转成Map<parentCode, List<Region>>

Map<String, List<Region>> map2 = cityList.stream().collect(Collectors.groupingBy(Region::getParentCode));

 

//分组：转成Map<parentCode, List<regionCode>>

Map<String, List<String>> map3 = cityList.stream().collect(Collectors.groupingBy(Region::getParentCode, Collectors.mapping(Region::getCode, Collectors.toList())));

 

//分组：计算每个parentCode对应的集合元素个数：Map<parentCode, Long>

Map<String, Long> map3 = cityList.stream().collect(Collectors.groupingBy(Region::getParentCode, Collectors.counting()));