Java 8：集合类转换（List转换为Map..）

集合转换

List转换为Map

• value为List<Apple>

// List<Apple> 根据color转换为map
Map<String, List<Apple>> appleMap = apples.stream().collect(groupingBy(Apple::getColor));

• value为Apple

// List<Apple> 根据color转换为map<String, Apple>,重复的Color取后者
Map<String, Apple> appleMap = apples.stream().collect(toMap(Apple::getColor, a -> a, (a1, a2) -> a2));

• value为Apple的属性

// List<Apple> 根据color转换为map<String, Integer>,重复的Color取后者
Map<String, Integer> map2 = apples.stream().collect(toMap(Apple::getColor, Apple::getWeight, (a1, a2) -> a2));

Map转List

Map<String, Apple> appleMap = apples.stream().collect(toMap(Apple::getColor, a -> a, (a1, a2) -> a2));
// map转List
List<String> colors = appleMap.values().stream().map(Apple::getColor).collect(toList());

Map转Set

Map<String, Apple> appleMap = apples.stream().collect(toMap(Apple::getColor, a -> a, (a1, a2) -> a2));
// map转set
Set<String> set = appleMap.values().stream().map(Apple::getColor).collect(toSet());