StreamPlactice.java

class StreamPlactice {
   public static void main(String[] args) {
        List<Bean> list = Arrays.asList(
                new Bean(1, "a"),
                new Bean(2, "b"),
                new Bean(3, "c"),
                new Bean(1, "aa"),
                new Bean(1, "aaa"),
                new Bean(2, "bb")
        );
        // collect groupingBy
        Map<Integer, List<Bean>> map =
                list.stream().collect(groupingBy(Bean::getKey));
        System.out.println(map);

        // flat化
        List<String> flattenList = map.entrySet().stream()
                .map(e -> e.getValue().stream()
                        .map(b -> b.getValue())
                        .reduce("",(src, dest) -> src + dest)
                ).collect(toList());
        System.out.println(flattenList);
    }

    static class Bean {
        private final int key;
        private final String value;
        public Bean(int key, String value) {
            this.key = key;
            this.value = value;
        }
        public int getKey() {
            return key;
        }
        public String getValue() {
            return value;
        }
        @Override
        public String toString() {
            return "Bean:{" +
                    "key=" + key +
                    ", value='" + value + '\'' +
                    '}';
        }
    }
}