Java中List转MapList实现的几种姿势

// 比如将下面的列表，按照字符串长度进行分组
List<String> list = new ArrayList<>();
list.add("hello");
list.add("Word");
list.add("come");
list.add("on");
Map<Integer, List<String>> ans = new HashMap<>();

for(String str: list) {
    List<String> sub = ans.get(str.length());
    if(sub == null) {
        sub = new ArrayList<>();
        ans.put(str.length(), sub);
    }
    sub.add(str);
}
System.out.println(ans);

for (String str : list) {
    ans.computeIfAbsent(str.length(), k -> new ArrayList<>()).add(str);
}

Map<Integer, List<String>> ans = list.stream().collect(Collectors.groupingBy(String::length));

public static <K, V> Map<K, List<V>> toMapList(List<V> list, KeyFunc<V, K> keyFunc) {
    Map<K, List<V>> result = new HashMap<>();
    for (V item: list) {
        K key = keyFunc.geTKEy(item);
        if (!result.containsKey(key)) {
            result.put(key, new ArrayList<>());
        }
        result.get(key).add(item);
    }
    return result;
}

public static interface KeyFunc<T, K> {
    K getKey(T t);
}

public static void main(String[] args) {
    List<String> list = new ArrayList<>();
    list.add("hello");
    list.add("word");
    list.add("come");
    list.add("on");
    Map<Integer, List<String>> res = toMapList(list, new KeyFunc<String, Integer>() {
        @Override
        public Integer getKey(String s) {
            return s.length();
        }
    });
    System.out.println(res);
}

public static <K, V> Map<K, List<V>> toMapList(List<V> list, Function<V, K> func) {
    return list.stream().collect(Collectors.groupingBy(func));
}

public static void main(String[] args) {
    List<String> list = new ArrayList<>();
    list.add("hello");
    list.add("word");
    list.add("come");
    list.add("on");
    Map<Integer, List<String>> res = toMapList(list, (Function<String, Integer>) String::length);
    System.out.println(res);
}

public static <K, V> Map<K, List<V>> toMapList(List<V> list, Function<V, K> func) {
    return list.stream().collect(Collectors.groupingBy(func));
}


public static <K, I, V> Map<K, List<V>> toMapList(List<I> list, Function<I, K> keyFunc, Function<I, V> valFunc) {
    return list.stream().collect(Collectors.groupingBy(keyFunc, Collectors.mapping(valFunc, Collectors.toList())));
}

List<String> list = new ArrayList<>();
list.add("hello");
list.add("word");
list.add("come");
list.add("on");
list.add("on");
HashMultimap<Integer, String> map = HashMultimap.create();
for (String item: strList) {
    map.put(item.length(), item);
}
System.out.println(map);