Multi mapping in java

Multi-map в Java 8

Мультикарта (multi-map) может пригодиться в реальной работе или на собеседовании. Почему-то в некоторых компаниях при приёме на работу любят давать алгоритмические задачки на её использование (лично я так не поступаю).
По сути это обычная карта (Map) в которой значением является коллекция (List или Set).
Сейчас в JCF (Java Collections Framework) нет готового класса для Multimap.

До выхода Java 8 приходилось логику работы писать вручную.
Например так.
Допустим, нам поступают такие данные:

Их нужно представить в виде:

Тогда логику можно реализовать, например, в таком виде:

import java.util.*; public class Main < public static void main(String[] args) < // Получили город и страну String city = "Moscow", country = "ru"; // Карта куда всё будем складывать Map> map = new HashMap<>(); List list = map.get(country); // Если это наш первый раз, // тогда создаем список и кладем его по ключу (название страны) if (list == null) < list = new ArrayList<>(); map.put(country, list); > list.add(city); > >

Это не удобно, поэтому многие используют Multimap из Apache Commons или Google Guava.

В восьмой Java, с помощью анонимных функций и нового API, логику можно сделать однострочником:

// В случае первого раза вызывать функцию: (key) -> value map.computeIfAbsent(country, (k)-> new ArrayList()).add(city);

N.B.: Совет использовать computeIfAbsent для создания мультикарты явно указан в документации.

Если же у нас на входе готовая карта, то её можно прокрутить foreach (хотя это не очень красиво):
Например так:

Map map = new HashMap<>(); map.put("Madrid", "es"); map.put("Moscow", "ru"); map.put("Omsk", "ru"); map.put("London", "uk"); Map> result = new HashMap<>(); map.forEach((city, country) -> < result.computeIfAbsent(country, (k) ->new ArrayList()).add(city); >);

Интересней было бы воспользоваться Stream API.
Например так:

import java.util.*; import static java.util.Map.Entry; import static java.util.stream.Collectors.*; public class Main < public static void main(String[] args) < Mapmap = new HashMap<>(); map.put("Madrid", "es"); map.put("Moscow", "ru"); map.put("Omsk", "ru"); map.put("London", "uk"); // Сгруппировать по стране (Entry::getValue), // а название города Entry::getKey записывать в список (toList) Map> m = map.entrySet().stream().collect( // Группировать по названию страны (es, ru, uk и т.д.) groupingBy(Entry::getValue, // Запихиваем город (Moscow, London и т.д.) в список (toList) mapping(Entry::getKey, toList()))); > >

Здесь используется функциональное программирование, восприятие которого требует определенного “привыкания”. Поскольку я периодически сталкиваюсь как со сторонниками “императивщины”, так и со сторонниками “функциональщины”, могу сказать, что у каждого своя правда. В каждом конкретном случае следует исходить из конечных целей.

В качестве причины почему действительно стоит изучать функциональный поход могу сказать, что игнорировать его сейчас уже нельзя, т.к. в противном случае программист просто перестанет понимать программы написанные другими программистами. Когда появилась 5-ая Java было очень много ненавистников дженериков, сейчас польза их очевидна, а умение их использовать просто необходимо.

Источник

What is a Multimap in Java?

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A Multimap is a new collection type that is found in Google’s Guava library for Java. A Multimap can store more than one value against a key. Both the keys and the values are stored in a collection, and considered to be alternates for Map> or Map> (standard JDK Collections Framework).

Creation

Multimap is an interface that is extended by its children interfaces: ListMultimap and SetMultimap . The standard way of creating a Multimap is to use the MultimapBuilder class and specify the data structures that are to be used for the keys and values during building:

// create a ListMultimap with Integer keys in a hash table // and String values in a LinkedList: ListMultimap m = MultimapBuilder.hashKeys().linkedListValues().build(); 

Another way to create a Multimap is to use the static method create() in the concrete classes that implement the interface (e.g., ArrayListMultimap , LinkedListMultimap , etc.):

ListMultimap m = ArrayListMultimap.create(); 

Multimap.get(key):

All of the values associated with a key are retrieved in the form of a collection using Multimap.get(key) ; if that key does not exist in the Multimap, an empty collection is returned. The reference to the collection returned by this method can be used to modify the values for that key; these modifications will be reflected​ in the original Multimap.

Adding and deleting key-value pairs:

A key-value pair can be added using Multimap.get(K).add(V) , but it is more direct to use Multimap.put(K, V) :

// Using returned collection to add values: map.get(1).add("name"); // Direct method: map.put(1, "name"); 

Similarly, a key-value pair can be removed from a Multimap indirectly or directly:

// Indirectly: map.get(1).remove("name"); // Directly: map.remove(1, "name"); 

Источник

Guide to Guava Multimap

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1. Overview

In this article, we will look at one of Map implementations from Google Guava library – Multimap. It is a collection that maps keys to values, similar to java.util.Map, but in which each key may be associated with multiple values.

2. Maven Dependency

First, let’s add a dependency:

 com.google.guava guava 31.0.1-jre 

The latest version can be found here.

3. Multimap Implementation

In the case of Guava Multimap, if we add two values for the same key, the second value will not override the first value. Instead, we will have two values in the resulting map. Let’s look at a test case:

String key = "a-key"; Multimap map = ArrayListMultimap.create(); map.put(key, "firstValue"); map.put(key, "secondValue"); assertEquals(2, map.size()); 

Printing the map‘s content will output:

When we will get values by key “a-key” we will get Collection that contains “firstValue” and “secondValue” as a result:

Collection values = map.get(key);

Printing values will output:

4. Compared to the Standard Map

Standard map from java.util package doesn’t give us the ability to assign multiple values to the same key. Let’s consider a simple case when we put() two values into a Map using the same key:

String key = "a-key"; Map map = new LinkedHashMap<>(); map.put(key, "firstValue"); map.put(key, "secondValue"); assertEquals(1, map.size()); 

The resulting map has only one element (“secondValue”), because of a second put() operation that overrides the first value. Should we want to achieve the same behavior as with Guava’s Multimap, we would need to create a Map that has a List as a value type:

String key = "a-key"; Map> map = new LinkedHashMap<>(); List values = map.get(key); if(values == null) < values = new LinkedList<>(); values.add("firstValue"); values.add("secondValue"); > map.put(key, values); assertEquals(1, map.size());