Map с несколькими ключами java

How to Store Duplicate Keys in a Map in Java?

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1. Overview

In this tutorial, we’re going to explore the available options for handling a Map with duplicate keys or, in other words, a Map that allows storing multiple values for a single key.

2. Standard Maps

Java has several implementations of the interface Map, each one with its own particularities.

However, none of the existing Java core Map implementations allow a Map to handle multiple values for a single key.

As we can see, if we try to insert two values for the same key, the second value will be stored, while the first one will be dropped.

It will also be returned (by every proper implementation of the put(K key, V value) method):

Map map = new HashMap<>(); assertThat(map.put("key1", "value1")).isEqualTo(null); assertThat(map.put("key1", "value2")).isEqualTo("value1"); assertThat(map.get("key1")).isEqualTo("value2"); 

How can we achieve the desired behavior, then?

3. Collection as Value

Obviously, using a Collection for every value of our Map would do the job:

Map> map = new HashMap<>(); List list = new ArrayList<>(); map.put("key1", list); map.get("key1").add("value1"); map.get("key1").add("value2"); assertThat(map.get("key1").get(0)).isEqualTo("value1"); assertThat(map.get("key1").get(1)).isEqualTo("value2"); 

However, this verbose solution has multiple drawbacks and is prone to errors. It implies that we need to instantiate a Collection for every value, check for its presence before adding or removing a value, delete it manually when no values are left, etc.

From Java 8, we could exploit the compute() methods and improve it:

Map> map = new HashMap<>(); map.computeIfAbsent("key1", k -> new ArrayList<>()).add("value1"); map.computeIfAbsent("key1", k -> new ArrayList<>()).add("value2"); assertThat(map.get("key1").get(0)).isEqualTo("value1"); assertThat(map.get("key1").get(1)).isEqualTo("value2"); 

However, we should avoid it unless having a very good reason not to, such as restrictive company policies preventing us from using third-party libraries.

Otherwise, before writing our own custom Map implementation and reinventing the wheel, we should choose among the several options available out-of-the-box.

4. Apache Commons Collections

As usual, Apache has a solution for our problem.

Let’s start by importing the latest release of Common Collections (CC from now on):

 org.apache.commons commons-collections4 4.1 

4.1. MultiMap

The org.apache.commons.collections4.MultiMap interface defines a Map that holds a collection of values against each key.

It’s implemented by the org.apache.commons.collections4.map.MultiValueMap class, which automatically handles most of the boilerplate under the hood:

MultiMap map = new MultiValueMap<>(); map.put("key1", "value1"); map.put("key1", "value2"); assertThat((Collection) map.get("key1")) .contains("value1", "value2"); 

While this class is available since CC 3.2, it’s not thread-safe, and it’s been deprecated in CC 4.1. We should use it only when we can’t upgrade to the newer version.

4.2. MultiValuedMap

The successor of MultiMap is the org.apache.commons.collections4.MultiValuedMap interface. It has multiple implementations ready to be used.

Let’s see how to store our multiple values into an ArrayList, which retains duplicates:

MultiValuedMap map = new ArrayListValuedHashMap<>(); map.put("key1", "value1"); map.put("key1", "value2"); map.put("key1", "value2"); assertThat((Collection) map.get("key1")) .containsExactly("value1", "value2", "value2"); 

Alternatively, we could use a HashSet, which drops duplicates:

MultiValuedMap map = new HashSetValuedHashMap<>(); map.put("key1", "value1"); map.put("key1", "value1"); assertThat((Collection) map.get("key1")) .containsExactly("value1"); 

Both of the above implementations are not thread-safe.

Let’s see how we can use the UnmodifiableMultiValuedMap decorator to make them immutable:

@Test(expected = UnsupportedOperationException.class) public void givenUnmodifiableMultiValuedMap_whenInserting_thenThrowingException() < MultiValuedMapmap = new ArrayListValuedHashMap<>(); map.put("key1", "value1"); map.put("key1", "value2"); MultiValuedMap immutableMap = MultiMapUtils.unmodifiableMultiValuedMap(map); immutableMap.put("key1", "value3"); > 

5. Guava Multimap

Guava is the Google Core Libraries for Java API.

Let’s start by importing Guava on our project:

 com.google.guava guava 31.0.1-jre 

Guava followed the path of multiple implementations since the beginning.

The most common one is the com.google.common.collect.ArrayListMultimap, which uses a HashMap backed by an ArrayList for every value:

Multimap map = ArrayListMultimap.create(); map.put("key1", "value2"); map.put("key1", "value1"); assertThat((Collection) map.get("key1")) .containsExactly("value2", "value1"); 

5.1. Common Map Implementations

When we need a specific Map implementation, the first thing to do is check if it exists because Guava has probably already implemented it.

For example, we can use the com.google.common.collect.LinkedHashMultimap, which preserves the insertion order of keys and values:

Multimap map = LinkedHashMultimap.create(); map.put("key1", "value3"); map.put("key1", "value1"); map.put("key1", "value2"); assertThat((Collection) map.get("key1")) .containsExactly("value3", "value1", "value2"); 

Alternatively, we can use a com.google.common.collect.TreeMultimap, which iterates keys and values in their natural order:

Multimap map = TreeMultimap.create(); map.put("key1", "value3"); map.put("key1", "value1"); map.put("key1", "value2"); assertThat((Collection) map.get("key1")) .containsExactly("value1", "value2", "value3"); 

5.2. Forging Our Custom MultiMap

Many other implementations are available.

However, we may want to decorate a Map and/or a List that’s not yet implemented.

Luckily, Guava has a factory method allowing us to do it — the Multimap.newMultimap().

6. Conclusion

We’ve seen how to store multiple values for a key in a Map in all the main existing ways.

We’ve explored the most popular implementations of Apache Commons Collections and Guava, which should be preferred to custom solutions when possible.

As always, the full source code is available over on GitHub.

Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.

The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.

Critically, it has very minimal impact on your server’s performance, with most of the profiling work done separately — so it needs no server changes, agents or separate services.

Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes:

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Map с несколькими ключами java

“Анна Ивановна Решетникова, 4211 717171” И как они собирались хранить такой номер паспорта в типе Integer? Тем самым показав, что номер паспорта хранить в базе данных как число — это не лучшая идея. Так как номера паспорта могут содержать дефисы, пробелы, буквы и т.д. Поправьте меня, если я не прав. Иначе у меня вопросы к господину Милану.

Скажите, почему хронология вывода на экран поменялась?

Получение списка всех ключей и значений Еще одна удобная особенность HashMap — можно по-отдельности получить список всех ключей и всех значений. Для этого используются методы keySet() и values() Для чего в примере создаются Set и ArrayList? Если в sout можно прямо вызвать методы keySet() и values()?

Возможно ли в HashMap переопределить метод toString() и если да, то каким образом? P/S: Спросил нынче модный ChatGPT, так он какую-то чушь городит: то пишет можно и даёт нерабочие коды, каждый раз разные, то пишет что нельзя потому что HashMap происходит от абстрактного класса. Хотя я уже понял что верить ему нельзя во всем, на вопрос можно ли реализовать в Java множественное наследование (не через интерфейсы) он бодро рапортовал, что конечно можно и вывалил код типа public сlass A extends B, C, который естественно не работает. Извините за возможный оффтоп. Так что роботы пока не завоюют мир, поэтому, вопрос по toString() все еще актуален. )

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