Java map проверить наличие ключа

Interface Map

An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface.

The Map interface provides three collection views, which allow a map’s contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map’s collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not.

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map.

All general-purpose map implementation classes should provide two «standard» constructors: a void (no arguments) constructor which creates an empty map, and a constructor with a single argument of type Map , which creates a new map with the same key-value mappings as its argument. In effect, the latter constructor allows the user to copy any map, producing an equivalent map of the desired class. There is no way to enforce this recommendation (as interfaces cannot contain constructors) but all of the general-purpose map implementations in the JDK comply.

The «destructive» methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw UnsupportedOperationException if this map does not support the operation. If this is the case, these methods may, but are not required to, throw an UnsupportedOperationException if the invocation would have no effect on the map. For example, invoking the putAll(Map) method on an unmodifiable map may, but is not required to, throw the exception if the map whose mappings are to be «superimposed» is empty.

Some map implementations have restrictions on the keys and values they may contain. For example, some implementations prohibit null keys and values, and some have restrictions on the types of their keys. Attempting to insert an ineligible key or value throws an unchecked exception, typically NullPointerException or ClassCastException . Attempting to query the presence of an ineligible key or value may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible key or value whose completion would not result in the insertion of an ineligible element into the map may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as «optional» in the specification for this interface.

Many methods in Collections Framework interfaces are defined in terms of the equals method. For example, the specification for the containsKey(Object key) method says: «returns true if and only if this map contains a mapping for a key k such that (key==null ? k==null : key.equals(k)) .» This specification should not be construed to imply that invoking Map.containsKey with a non-null argument key will cause key.equals(k) to be invoked for any key k . Implementations are free to implement optimizations whereby the equals invocation is avoided, for example, by first comparing the hash codes of the two keys. (The Object.hashCode() specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlying Object methods wherever the implementor deems it appropriate.

Some map operations which perform recursive traversal of the map may fail with an exception for self-referential instances where the map directly or indirectly contains itself. This includes the clone() , equals() , hashCode() and toString() methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so.

Unmodifiable Maps

• They are unmodifiable. Keys and values cannot be added, removed, or updated. Calling any mutator method on the Map will always cause UnsupportedOperationException to be thrown. However, if the contained keys or values are themselves mutable, this may cause the Map to behave inconsistently or its contents to appear to change.
• They disallow null keys and values. Attempts to create them with null keys or values result in NullPointerException .
• They are serializable if all keys and values are serializable.
• They reject duplicate keys at creation time. Duplicate keys passed to a static factory method result in IllegalArgumentException .
• The iteration order of mappings is unspecified and is subject to change.
• They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
• They are serialized as specified on the Serialized Form page.

This interface is a member of the Java Collections Framework.

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How to Check If a Key Exists in a Map

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1. Overview

In this brief tutorial, we’ll look at ways to check if a key exists in a Map.

Specifically, we’ll focus on containsKey and get.

2. containsKey

Returns true if this map contains a mapping for the specified key

We can see that this method is a pretty good candidate for doing what we want.

Let’s create a very simple map and verify its contents with containsKey:

@Test public void whenKeyIsPresent_thenContainsKeyReturnsTrue() < Mapmap = Collections.singletonMap("key", "value"); assertTrue(map.containsKey("key")); assertFalse(map.containsKey("missing")); >

Simply put, containsKey tells us whether the map contains that key.

3. get

Now, get can sometimes work, too, but it comes with some baggage, depending on whether or not the Map implementation supports null values.

Again, taking a look at Map‘s JavaDoc, this time for Map#put, we see that it will only throw a NullPointerException:

if the specified key or value is null and this map does not permit null keys or values

Since some implementations of Map can have null values (like HashMap), it’s possible for get to return null even though the key is present.

So, if our goal is to see whether or not a key has a value, then get will work:

@Test public void whenKeyHasNullValue_thenGetStillWorks() < Mapmap = Collections.singletonMap("nothing", null); assertTrue(map.containsKey("nothing")); assertNull(map.get("nothing")); >

But, if we are just trying to check that the key exists, then we should stick with containsKey.

4. Conclusion

In this article, we looked at containsKey. We also took a closer look at why it’s risky to use get for verifying a key’s existence.

As always, check out the code examples over on Github.

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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.

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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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Как проверить, существует ли ключ на карте

Узнайте, как проверить, существует ли ключ в карте на Java, и об общей ловушке.

1. Обзор

В этом кратком руководстве мы рассмотрим способы проверки наличия ключа в Карте .

В частности, мы сосредоточимся на containsKey и get.

2. containsKey

Возвращает true , если эта карта содержит отображение для указанного ключа

Мы видим, что этот метод является довольно хорошим кандидатом для того, чтобы делать то, что мы хотим.

Давайте создадим очень простую карту и проверим ее содержимое с помощью containsKey :

@Test public void whenKeyIsPresent_thenContainsKeyReturnsTrue() < Mapmap = Collections.singletonMap("key", "value"); assertTrue(map.containsKey("key")); assertFalse(map.containsKey("missing")); >

Проще говоря, Содержит говорит нам, содержит ли карта этот ключ.

3. получить

Теперь get иногда тоже может работать, но он поставляется с некоторым багажом, в зависимости от того, поддерживает ли реализация Map нулевые значения.

Опять же, взглянув на JavaDoc Map , на этот раз для Map#put , мы видим, что он только вызовет исключение NullPointerException :

если указанный ключ или значение равно null и эта карта не допускает нулевые ключи или значения

Поскольку некоторые реализации Map могут иметь нулевые значения (например, HashMap ), для get возможно вернуть null , даже если ключ присутствует.

Итак, если наша цель состоит в том, чтобы увидеть, имеет ли ключ значение, то получить будет работать:

@Test public void whenKeyHasNullValue_thenGetStillWorks() < Mapmap = Collections.singletonMap("nothing", null); assertTrue(map.containsKey("nothing")); assertNull(map.get("nothing")); >

Но если мы просто пытаемся проверить, существует ли ключ, то нам следует придерживаться Содержит .

4. Заключение

В этой статье мы рассмотрели containsKey . Мы также более подробно рассмотрели, почему рискованно использовать get для проверки существования ключа.

Как всегда, ознакомьтесь с примерами кода на Github .

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