将HashMap减少到另一个HashMap

Question

我开始使用这样的字符串：

"train, Train, bike, BIKE, car...etc"

旅行类型，需要返回一个HashMap<String, Integer>()，如下：

(train, 4), (car, 6), (bike, 2)

该数字表示已经看到旅行类型的次数

这是我所拥有的，将字符串映射到这样的初始hashmap：

(train, 1), (train, 1), (car, 1), (car, 1)...etc

有：

public static HashMap<String, Integer> countTravelTypes(String travelTypes) {
    return Arrays.stream(travelTypes.toLowerCase().replace(" ", "")
            .split(","))
            .map(s -> new HashMap<String, Integer>(){{put(s, 1);}})
            //The line below isn't right but it's as close as I could get
            .reduce(0, map -> map.key, Integer.sum(v))
}

你现在可以帮我减少它吗，所以基本上总结一下所有的列车都显示有4个

Answer 1

我们可以使用Collectors.summingInt()作为Map<String, Integer>类型的输出Map：

Map<String, Integer> result =  
Stream.of("train, Train, bike, BIKE, car, BIKE, Train".split(", "))
      .collect(Collectors.groupingBy(i -> i, Collectors.summingInt(i -> 1)));

Answer 2

你可以使用groupingBy收集器与counting下游收集器一起完成这件事。这是它的外观。

Map<String, Long> result = Arrays.stream(source.split(","))
    .map(String::trim)
    .map(String::toLowerCase)
    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));