Android hashmap get key

Метод keySet() возвращает контейнер Set, содержащий все ключи из personMap, который используется в цикле для перебора элементов Map.

Sparse arrays — аналог в Android

Разработчик Android посчитали, что HashMap не слишком оптимизирован для мобильных устройств и предложили свой вариант в виде специальных массивов. Данные классы являются родными для Android, но не являются частью Java. Очень рекомендуют использовать именно Android-классы. Не все программисты знают об этих аналогах, а также классический код может встретиться в различных Java-библиотеках. Если вы увидите такой код, то заменить его на нужный. Ниже представлена таблица для замены.

Существует ещё класс HashTable, который очень похож в использовании как и HashMap.

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Kotlin/Android HashMap tutorial with examples

In this tutorial, we’ll show you many Kotlin HashMap methods and functions that also work on Android. You will see many examples to:

• Create, initialize, add item, get value, update, remove entries in a HashMap
• Iterate over a HashMap
• Filter a HashMap
• Convert HashMap to List, transform a HashMap keys or values

Key points to note about Kotlin HashMap

• A HashMap is a collection of key-value pairs that maps each key to exactly one value.
• HashMap is unordered. The key-value pair coming later could be ordered first.
• We can add an entry with null value or null key to Kotlin HashMap

Create a new HashMap

We can create a new HashMap using constructor.
If you want to create a new HashMap that allows null values and null keys, just use the question mark ? right after the types.

Initialize a HashMap with values

The example show you way to initialize a HashMap using hashMapOf() function:

Add item to HashMap

We can add an item (key-value pair) to a Kotlin HashMap using:

You can also add all key/value pairs of another Map to current Map using putAll() method.

Kotlin HashMap get

The examples show you way to get value by key using:

• square brackets [] operator
• get() method
• getOrElse() method that returns default value if there is no value for the key
• getOrPut() method that returns default value if there is no value for the key, and put the entry with key and default value to the HashMap

We can get all keys or values with keys & values property.

Kotlin HashMap get key by value

We can find all keys that are paired with a given value using filterValues() method and Map’s keys property.
For example, there are 2 entries with value = 9999 in the HashMap:

Kotlin HashMap update values

We can update values in HashMap by given existing keys using replace() , put() or square brackets [] .

Check HashMap empty and size

• find the number of key/value pairs using .size
• check if a Map is empty or not using isEmpty() or isNotEmpty() method

Check HashMap contains key/value

We can check existence of:

Kotlin HashMap remove

The examples show you how to:

• remove element (entry) by key using remove() method or .keys.remove() method
• removes the first entry with the given value using .values.remove() method
• remove all entries from a HashMap using clear() method

Iterate over a HashMap in Kotlin

The examples show you how to iterate over a Kotlin HashMap using:

Kotlin HashMap filter

The examples show you how to filter a HashMap using:

• filterKeys() for new Map with keys matching the predicate
• filterValues() for new Map with values matching the
• filter() for both key and value in the predicate

Kotlin HashMap to List

We can easily transform a Kotlin Map to List using map() method. It will return a new List with elements transformed from entries in original Map.

Sometimes the transformation could produce null, so we filter out the nulls by using the mapNotNull() function instead of map()

You can also use mapTo() and mapNotNullTo() with a destination as input pamameter.

Kotlin transform HashMap keys

The example shows you how to transform HashMap keys in Kotlin using:

Kotlin transform HashMap values

The example shows you how to transform HashMap values in Kotlin using:

Kotlin Hashmap with different value types

To create a HashMap that can contain different value types, we use Any which is the supertype of all the other types.

Conclusion

In this Kotlin tutorial, we’ve learned some important points of HashMap. Now you can create, initialize a HashMap, add, update and remove items from a HashMap. You also know way to iterate over a HashMap, filter or transform a HashMap.

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Android hashmap get key

This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the «capacity» of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it’s very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.

As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.

If many mappings are to be stored in a HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table. Note that using many keys with the same hashCode() is a sure way to slow down performance of any hash table. To ameliorate impact, when keys are Comparable , this class may use comparison order among keys to help break ties.

Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be «wrapped» using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:

The iterators returned by all of this class’s «collection view methods» are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the iterator will throw a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.

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