Java8 集合操作

集合分组

方式一

public static Map<String, List<Apple>> getAppleByOne(List<Apple> apples) {
    Map<String, List<Apple>> map = new HashMap<>();
    for (Apple a : apples) {
        List<Apple> list = map.get(a.getColor());
        if (list == null) {
            list = new ArrayList<>();
            map.put(a.getColor(), list);
        }
        list.add(a);
    }
    return map;
}

方式二

private static Map<String,List<Apple>> getByOptioal(List<Apple> apples){
    Map<String, List<Apple>> map = new HashMap<>();
    apples.stream().forEach((Apple a) -> {
        List<Apple> list1 = Optional
        .ofNullable(map.get(a.getColor()))
        .orElseGet(() -> {
            List<Apple> list = new ArrayList<>();
            map.put(a.getColor(), list); 
            return list;
        });
        list1.add(a);
    });
    return map;
}

方式三

private static Map<String,List<Apple>> getBycollection(List<Apple> apples){
    return apples.stream()
        .collect(Collectors.groupingBy(Apple::getColor));
}

参考：java8之Collector分组