java8将Map <X，Y> + Map <Y，Z>转换为Map <X，Z>

Question

Does Java8 could convenient to implement below requirement, that is

Map<X,Y> + Map<Y,Z> = Map<X,Z>

Now my code is

ArrayList<String> couponCodes = newArrayList("aaa", "bbb", "ccc");
// couponCode -- shopId
ArrayList<ShopCoupon> shopCoupons = newArrayList(new ShopCoupon("aaa", 1), new ShopCoupon("bbb", 2), new ShopCoupon("ccc", 3));
Map<String, Integer> couponCodeShopIdMap = shopCoupons.stream().collect(toMap(sc -> sc.getCouponCode(), sc -> sc.getShopId()));
// shopId -- name
ArrayList<Shop> shops = newArrayList(new Shop(1, "zhangsan"), new Shop(2, "lisi"), new Shop(3, "wangwu"));
Map<Integer, String> shopIdNameMap = shops.stream().collect(toMap(s -> s.getId(), s -> s.getName()));

//couponCode -- name
Map<String, String> couponCodeNameMap = couponCodes.stream().collect(toMap(c -> c, c -> shopIdNameMap.get(couponCodeShopIdMap.get(c))));
System.out.println(couponCodeNameMap);

I want to know is some more convenient manner to implement this requirement?

Answer 1

Another way would be to stream over the entry set of the first map directly, eg:

public static <X, Y, Z> Map<X, Z> join(Map<X, Y> left, Map<Y, Z> right) {
    return left.entrySet()
               .stream()
               .collect(toMap(Map.Entry::getKey, e -> right.get(e.getValue())));
}

The only thing you have to deal with is when a value is present in the left map and not the corresponding key in the right map, you'll have a NullPointerException , since the value mapper function is not allowed to return null . In this case you can either use getOrDefault to provide a non-null default value to be associated with using e -> right.getOrDefault(e.getValue(), defaultValue) , or if you don't want to have the mapping in the resulting map, mimick the toMap collector by filtering upfront the inexistant mappings.

public static <X, Y, Z> Map<X, Z> join(Map<X, Y> left, Map<Y, Z> right) {
    return left.entrySet().stream().collect(
        HashMap::new,
        (m, e) -> {
            //if you still want the mapping k -> null, 
            //you can just use put directly
            Z value = right.get(e.getValue());
            if(value != null) m.put(e.getKey(), value);
        },
        Map::putAll);
}

Answer 2

您可以简单地使用Java 8中的默认地图方法

shopIdNameMap.forEach((key,value)->shopIdNameMap.replace(key,couponCodeNameMap.get(value)));