簡化stream流等工具類（函數語言程式設計的應用案例）

操作集合

public class CollectionUtils {

    public static boolean containsAny(Object source, Object... targets) {
        return asList(targets).contains(source);
    }

    public static boolean isAnyEmpty(Collection<?>... collections) {
        return Arrays.stream(collections).anyMatch(CollectionUtil::isEmpty);
    }

    public static <T> boolean anyMatch(Collection<T> from, Predicate<T> predicate) {
        return from.stream().anyMatch(predicate);
    }

    public static <T> List<T> filterList(Collection<T> from, Predicate<T> predicate) {
        if (CollUtil.isEmpty(from)) {
            return new ArrayList<>();
        }
        return from.stream().filter(predicate).collect(Collectors.toList());
    }

    public static <T, R> List<T> distinct(Collection<T> from, Function<T, R> keyMapper) {
        if (CollUtil.isEmpty(from)) {
            return new ArrayList<>();
        }
        return distinct(from, keyMapper, (t1, t2) -> t1);
    }

    public static <T, R> List<T> distinct(Collection<T> from, Function<T, R> keyMapper, BinaryOperator<T> cover) {
        if (CollUtil.isEmpty(from)) {
            return new ArrayList<>();
        }
        return new ArrayList<>(convertMap(from, keyMapper, Function.identity(), cover).values());
    }

    public static <T, U> List<U> convertList(T[] from, Function<T, U> func) {
        if (ArrayUtil.isEmpty(from)) {
            return new ArrayList<>();
        }
        return convertList(Arrays.asList(from), func);
    }

    public static <T, U> List<U> convertList(Collection<T> from, Function<T, U> func) {
        if (CollUtil.isEmpty(from)) {
            return new ArrayList<>();
        }
        return from.stream().map(func).filter(Objects::nonNull).collect(Collectors.toList());
    }

    public static <T, U> List<U> convertList(Collection<T> from, Function<T, U> func, Predicate<T> filter) {
        if (CollUtil.isEmpty(from)) {
            return new ArrayList<>();
        }
        return from.stream().filter(filter).map(func).filter(Objects::nonNull).collect(Collectors.toList());
    }

    public static <T, U> List<U> convertListByFlatMap(Collection<T> from,
                                                      Function<T, ? extends Stream<? extends U>> func) {
        if (CollUtil.isEmpty(from)) {
            return new ArrayList<>();
        }
        return from.stream().flatMap(func).filter(Objects::nonNull).collect(Collectors.toList());
    }

    public static <T, U, R> List<R> convertListByFlatMap(Collection<T> from,
                                                         Function<? super T, ? extends U> mapper,
                                                         Function<U, ? extends Stream<? extends R>> func) {
        if (CollUtil.isEmpty(from)) {
            return new ArrayList<>();
        }
        return from.stream().map(mapper).flatMap(func).filter(Objects::nonNull).collect(Collectors.toList());
    }

    public static <K, V> List<V> mergeValuesFromMap(Map<K, List<V>> map) {
        return map.values()
                .stream()
                .flatMap(List::stream)
                .collect(Collectors.toList());
    }

    public static <T, U> Set<U> convertSet(Collection<T> from, Function<T, U> func) {
        if (CollUtil.isEmpty(from)) {
            return new HashSet<>();
        }
        return from.stream().map(func).filter(Objects::nonNull).collect(Collectors.toSet());
    }

    public static <T, U> Set<U> convertSet(Collection<T> from, Function<T, U> func, Predicate<T> filter) {
        if (CollUtil.isEmpty(from)) {
            return new HashSet<>();
        }
        return from.stream().filter(filter).map(func).filter(Objects::nonNull).collect(Collectors.toSet());
    }

    public static <T, K> Map<K, T> convertMapByFilter(Collection<T> from, Predicate<T> filter, Function<T, K> keyFunc) {
        if (CollUtil.isEmpty(from)) {
            return new HashMap<>();
        }
        return from.stream().filter(filter).collect(Collectors.toMap(keyFunc, v -> v));
    }

    public static <T, U> Set<U> convertSetByFlatMap(Collection<T> from,
                                                    Function<T, ? extends Stream<? extends U>> func) {
        if (CollUtil.isEmpty(from)) {
            return new HashSet<>();
        }
        return from.stream().flatMap(func).filter(Objects::nonNull).collect(Collectors.toSet());
    }

    public static <T, U, R> Set<R> convertSetByFlatMap(Collection<T> from,
                                                       Function<? super T, ? extends U> mapper,
                                                       Function<U, ? extends Stream<? extends R>> func) {
        if (CollUtil.isEmpty(from)) {
            return new HashSet<>();
        }
        return from.stream().map(mapper).flatMap(func).filter(Objects::nonNull).collect(Collectors.toSet());
    }

    public static <T, K> Map<K, T> convertMap(Collection<T> from, Function<T, K> keyFunc) {
        if (CollUtil.isEmpty(from)) {
            return new HashMap<>();
        }
        return convertMap(from, keyFunc, Function.identity());
    }

    public static <T, K> Map<K, T> convertMap(Collection<T> from, Function<T, K> keyFunc, Supplier<? extends Map<K, T>> supplier) {
        if (CollUtil.isEmpty(from)) {
            return supplier.get();
        }
        return convertMap(from, keyFunc, Function.identity(), supplier);
    }

    public static <T, K, V> Map<K, V> convertMap(Collection<T> from, Function<T, K> keyFunc, Function<T, V> valueFunc) {
        if (CollUtil.isEmpty(from)) {
            return new HashMap<>();
        }
        return convertMap(from, keyFunc, valueFunc, (v1, v2) -> v1);
    }

    public static <T, K, V> Map<K, V> convertMap(Collection<T> from, Function<T, K> keyFunc, Function<T, V> valueFunc, BinaryOperator<V> mergeFunction) {
        if (CollUtil.isEmpty(from)) {
            return new HashMap<>();
        }
        return convertMap(from, keyFunc, valueFunc, mergeFunction, HashMap::new);
    }

    public static <T, K, V> Map<K, V> convertMap(Collection<T> from, Function<T, K> keyFunc, Function<T, V> valueFunc, Supplier<? extends Map<K, V>> supplier) {
        if (CollUtil.isEmpty(from)) {
            return supplier.get();
        }
        return convertMap(from, keyFunc, valueFunc, (v1, v2) -> v1, supplier);
    }

    public static <T, K, V> Map<K, V> convertMap(Collection<T> from, Function<T, K> keyFunc, Function<T, V> valueFunc, BinaryOperator<V> mergeFunction, Supplier<? extends Map<K, V>> supplier) {
        if (CollUtil.isEmpty(from)) {
            return new HashMap<>();
        }
        return from.stream().collect(Collectors.toMap(keyFunc, valueFunc, mergeFunction, supplier));
    }

    public static <T, K> Map<K, List<T>> convertMultiMap(Collection<T> from, Function<T, K> keyFunc) {
        if (CollUtil.isEmpty(from)) {
            return new HashMap<>();
        }
        return from.stream().collect(Collectors.groupingBy(keyFunc, Collectors.mapping(t -> t, Collectors.toList())));
    }

    public static <T, K, V> Map<K, List<V>> convertMultiMap(Collection<T> from, Function<T, K> keyFunc, Function<T, V> valueFunc) {
        if (CollUtil.isEmpty(from)) {
            return new HashMap<>();
        }
        return from.stream()
                .collect(Collectors.groupingBy(keyFunc, Collectors.mapping(valueFunc, Collectors.toList())));
    }

    // 暫時沒想好名字，先以 2 結尾噶
    public static <T, K, V> Map<K, Set<V>> convertMultiMap2(Collection<T> from, Function<T, K> keyFunc, Function<T, V> valueFunc) {
        if (CollUtil.isEmpty(from)) {
            return new HashMap<>();
        }
        return from.stream().collect(Collectors.groupingBy(keyFunc, Collectors.mapping(valueFunc, Collectors.toSet())));
    }

    public static <T, K> Map<K, T> convertImmutableMap(Collection<T> from, Function<T, K> keyFunc) {
        if (CollUtil.isEmpty(from)) {
            return Collections.emptyMap();
        }
        ImmutableMap.Builder<K, T> builder = ImmutableMap.builder();
        from.forEach(item -> builder.put(keyFunc.apply(item), item));
        return builder.build();
    }

    /**
     * 對比老、新兩個列表，找出新增、修改、刪除的資料
     *
     * @param oldList  老列表
     * @param newList  新列表
     * @param sameFunc 對比函式，返回 true 表示相同，返回 false 表示不同
     *                 注意，same 是透過每個元素的“標識”，判斷它們是不是同一個資料
     * @return [新增列表、修改列表、刪除列表]
     */
    public static <T> List<List<T>> diffList(Collection<T> oldList, Collection<T> newList,
                                             BiFunction<T, T, Boolean> sameFunc) {
        List<T> createList = new LinkedList<>(newList); // 預設都認為是新增的，後續會進行移除
        List<T> updateList = new ArrayList<>();
        List<T> deleteList = new ArrayList<>();

        // 透過以 oldList 為主遍歷，找出 updateList 和 deleteList
        for (T oldObj : oldList) {
            // 1. 尋找是否有匹配的
            T foundObj = null;
            for (Iterator<T> iterator = createList.iterator(); iterator.hasNext(); ) {
                T newObj = iterator.next();
                // 1.1 不匹配，則直接跳過
                if (!sameFunc.apply(oldObj, newObj)) {
                    continue;
                }
                // 1.2 匹配，則移除，並結束尋找
                iterator.remove();
                foundObj = newObj;
                break;
            }
            // 2. 匹配新增到 updateList；不匹配則新增到 deleteList 中
            if (foundObj != null) {
                updateList.add(foundObj);
            } else {
                deleteList.add(oldObj);
            }
        }
        return asList(createList, updateList, deleteList);
    }

    public static boolean containsAny(Collection<?> source, Collection<?> candidates) {
        return org.springframework.util.CollectionUtils.containsAny(source, candidates);
    }

    public static <T> T getFirst(List<T> from) {
        return !CollectionUtil.isEmpty(from) ? from.get(0) : null;
    }

    public static <T> T findFirst(List<T> from, Predicate<T> predicate) {
        return findFirst(from, predicate, Function.identity());
    }

    public static <T, U> U findFirst(List<T> from, Predicate<T> predicate, Function<T, U> func) {
        if (CollUtil.isEmpty(from)) {
            return null;
        }
        return from.stream().filter(predicate).findFirst().map(func).orElse(null);
    }

    public static <T, V extends Comparable<? super V>> V getMaxValue(Collection<T> from, Function<T, V> valueFunc) {
        if (CollUtil.isEmpty(from)) {
            return null;
        }
        assert !from.isEmpty(); // 斷言，避免告警
        T t = from.stream().max(Comparator.comparing(valueFunc)).get();
        return valueFunc.apply(t);
    }

    public static <T, V extends Comparable<? super V>> V getMinValue(List<T> from, Function<T, V> valueFunc) {
        if (CollUtil.isEmpty(from)) {
            return null;
        }
        assert from.size() > 0; // 斷言，避免告警
        T t = from.stream().min(Comparator.comparing(valueFunc)).get();
        return valueFunc.apply(t);
    }

    public static <T, V extends Comparable<? super V>> V getSumValue(List<T> from, Function<T, V> valueFunc,
                                                                     BinaryOperator<V> accumulator) {
        if (CollUtil.isEmpty(from)) {
            return null;
        }
        assert from.size() > 0; // 斷言，避免告警
        return from.stream().map(valueFunc).reduce(accumulator).get();
    }

    public static <T> void addIfNotNull(Collection<T> coll, T item) {
        if (item == null) {
            return;
        }
        coll.add(item);
    }

    public static <T> Collection<T> singleton(T deptId) {
        return deptId == null ? Collections.emptyList() : Collections.singleton(deptId);
    }

}

用法如下，簡化一些較簡單的stream流操作

Set<String> codegenColumnNames = convertSet(codegenColumns, CodegenColumnDO::getColumnName);

MapUtils，操作map

public class MapUtils {

    /**
     * 從雜湊表表中，獲得 keys 對應的所有 value 陣列
     *
     * @param multimap 雜湊表
     * @param keys keys
     * @return value 陣列
     */
    public static <K, V> List<V> getList(Multimap<K, V> multimap, Collection<K> keys) {
        List<V> result = new ArrayList<>();
        keys.forEach(k -> {
            Collection<V> values = multimap.get(k);
            if (CollectionUtil.isEmpty(values)) {
                return;
            }
            result.addAll(values);
        });
        return result;
    }

    /**
     * 從雜湊表查詢到 key 對應的 value，然後進一步處理
     * 注意，如果查詢到的 value 為 null 時，不進行處理
     *
     * @param map 雜湊表
     * @param key key
     * @param consumer 進一步處理的邏輯
     */
    public static <K, V> void findAndThen(Map<K, V> map, K key, Consumer<V> consumer) {
        if (CollUtil.isEmpty(map)) {
            return;
        }
        V value = map.get(key);
        if (value == null) {
            return;
        }
        consumer.accept(value);
    }

    public static <K, V> Map<K, V> convertMap(List<KeyValue<K, V>> keyValues) {
        Map<K, V> map = Maps.newLinkedHashMapWithExpectedSize(keyValues.size());
        keyValues.forEach(keyValue -> map.put(keyValue.getKey(), keyValue.getValue()));
        return map;
    }

}

使用案例：找到value進行其他操作

            MapUtils.findAndThen(userMap,NumberUtils.parseLong(assignee),
                    assignUser-> bpmTaskSubSignRespVO.setAssigneeUser(convert3(assignUser)));