java8中利用stream api将list转成map

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

List cityList= getRegionList();

//转成Map

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

 
或者

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

 
//分组：转成Map>

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

 

//分组：转成Map>

Map> 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

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