public static <T> List<T> toList(Collection<T> collection) {
    if (collection == null) {
        return new ArrayList<>();
    }
    if (collection instanceof List) {
        return (List<T>) collection;
    }
    return collection.stream().collect(Collectors.toList());
}
public static <T> Set<T> toSet(Collection<T> collection) {
    if (collection == null) {
        return new HashSet<>();
    }
    if (collection instanceof Set) {
        return (Set<T>) collection;
    }
    return collection.stream().collect(Collectors.toSet());
}

@Test//將集合 Collection 轉(zhuǎn)化為 List
public void testToList() {
    Collection<OrderItem> collection = coll;
    List<OrderItem> list = toList(coll);
}
@Test//將集合 Collection 轉(zhuǎn)化為 Set
public void testToSet() {
    Collection<OrderItem> collection = coll;
    Set<OrderItem> set = toSet(collection);
}

public static <T, R> List<R> map(List<T> collection, Function<T, R> mapper) {
    return collection.stream().map(mapper).collect(Collectors.toList());
}
public static <T, R> Set<R> map(Set<T> collection, Function<T, R> mapper) {
    return collection.stream().map(mapper).collect(Collectors.toSet());
}
public static <T, R> List<R> mapToList(Collection<T> collection, Function<T, R> mapper) {
    return collection.stream().map(mapper).collect(Collectors.toList());
}
public static <T, R> Set<R> mapToSet(Collection<T> collection, Function<T, R> mapper) {
    return collection.stream().map(mapper).collect(Collectors.toSet());
}

@Test
public void testMapToList() {
    Collection<OrderItem> collection = coll;
    List<OrderItem> list = toList(coll);
    List<Long> orderIdList = map(list, (item) -> item.getOrderId());
}
@Test
public void testMapToSet() {
    Collection<OrderItem> collection = coll;
    Set<OrderItem> set = toSet(coll);
    Set<Long> orderIdSet = map(set, (item) -> item.getOrderId());
}
@Test
public void testMapToList2() {
    Collection<OrderItem> collection = coll;
    List<Long> orderIdList = mapToList(collection, (item) -> item.getOrderId());
}
@Test
public void testMapToSetV2() {
    Collection<OrderItem> collection = coll;
    Set<Long> orderIdSet = mapToSet(collection, (item) -> item.getOrderId());
}

public static <T, K> Map<K, T> toMap(Collection<T> collection, Function<? super T, ? extends K> keyMapper) {
    return toMap(collection, keyMapper, Function.identity());
}
public static <T, K, V> Map<K, V> toMap(Collection<T> collection,
                                        Function<? super T, ? extends K> keyFunction,
                                        Function<? super T, ? extends V> valueFunction) {
    return toMap(collection, keyFunction, valueFunction, pickLast());
}
public static <T, K, V> Map<K, V> toMap(Collection<T> collection,
                                        Function<? super T, ? extends K> keyFunction,
                                        Function<? super T, ? extends V> valueFunction,
                                        BinaryOperator<V> mergeFunction) {
    if (CollectionUtils.isEmpty(collection)) {
        return new HashMap<>(0);
    }
    return collection.stream().collect(Collectors.toMap(keyFunction, valueFunction, mergeFunction));
}

@Test
public void testToMap() {
    Collection<OrderItem> collection = coll;
    Set<OrderItem> set = toSet(collection);
    Map<Long, OrderItem> map = toMap(set, OrderItem::getOrderId);
}
@Test
public void testToMapV2() {
    Collection<OrderItem> collection = coll;
    Set<OrderItem> set = toSet(collection);
    Map<Long, Double> map = toMap(set, OrderItem::getOrderId, OrderItem::getActPrice);
}

public static <K, V, C> Map<K, C> convertMapValue(Map<K, V> map, 
                        BiFunction<K, V, C> valueFunction,
                        BinaryOperator<C> mergeFunction) {
    if (isEmpty(map)) {
        return new HashMap<>();
    }
    return map.entrySet().stream().collect(Collectors.toMap(
            e -> e.getKey(),
            e -> valueFunction.apply(e.getKey(), e.getValue()),
            mergeFunction
    ));
}
public static <K, V, C> Map<K, C> convertValue(Map<K, V> originMap, BiFunction<K, V, C> valueConverter) {
    return convertValue(originMap, valueConverter, Lambdas.pickLast());
}
public static <T> BinaryOperator<T> pickFirst() {
    return (k1, k2) -> k1;
}
public static <T> BinaryOperator<T> pickSecond() {
    return (k1, k2) -> k2;
}

@Test
public void testConvertValue() {
    Collection<OrderItem> collection = coll;
    Set<OrderItem> set = toSet(collection);
    Map<Long, OrderItem> map = toMap(set, OrderItem::getOrderId);
    Map<Long, Double> orderId2Price = convertMapValue(map, item -> item.getActPrice());
    Map<Long, String> orderId2Token = convertMapValue(map, (id, item) -> id + item.getName());
}