Java ConcurrentHashMap

Tässä opetusohjelmassa opitaan Java ConcurrentHashMap -luokasta ja sen toiminnoista esimerkkien avulla.

ConcurrentHashMapLuokka Java kokoelmien kehys tarjoaa säieturvallinen kartalla. Toisin sanoen useat ketjut voivat käyttää karttaa kerralla vaikuttamatta kartalla olevien merkintöjen yhdenmukaisuuteen.

Se toteuttaa ConcurrentMap-käyttöliittymän.

Luo ConcurrentHashMap

Jotta voimme luoda samanaikaisen hashmapin, meidän on java.util.concurrent.ConcurrentHashMapensin tuotava paketti. Kun paketti on tuotu, voimme näin luoda samanaikaisia ​​hashappeja Java-sovellukseen.

 // ConcurrentHashMap with capacity 8 and load factor 0.6 ConcurrentHashMap numbers = new ConcurrentHashMap(8, 0.6f);

Yllä olevassa koodissa olemme luoneet samanaikaiset hashmap-nimetyt numerot.

Tässä,

  • Avain - yksilöllinen tunniste, jota käytetään kunkin elementin (arvon) liittämiseen karttaan
  • Arvo - elementit, jotka on liitetty avaimiin kartalla

Huomaa osa new ConcurrentHashMap(8, 0.6). Ensimmäinen parametri on kapasiteetti ja toinen parametri loadFactor .

  • kapasiteetti - Tämän kartan kapasiteetti on 8. Tämä tarkoittaa, että se voi tallentaa 8 merkintää.
  • loadFactor - Tämän kartan kuormituskerroin on 0,6. Tämä tarkoittaa, että aina kun hash-taulukkoamme täytetään 60%, merkinnät siirretään uuteen hash-taulukkoon, joka on kaksinkertainen alkuperäisen hash-taulukon kokoon nähden.

Oletuskapasiteetti ja kuormituskerroin

On mahdollista luoda samanaikainen hashmap määrittelemättä sen kapasiteettia ja kuormituskerrointa. Esimerkiksi,

 // ConcurrentHashMap with default capacity and load factor ConcurrentHashMap numbers1 = new ConcurrentHashMap();

Oletuksena,

  • kartan kapasiteetti on 16
  • kuormituskerroin on 0,75

ConcurrentHashMapin luominen muista kartoista

Näin voimme luoda samanaikaisen hashapin, joka sisältää kaikki muiden karttojen elementit.

 import java.util.concurrent.ConcurrentHashMap; import java.util.HashMap; class Main ( public static void main(String() args) ( // Creating a hashmap of even numbers HashMap evenNumbers = new HashMap(); evenNumbers.put("Two", 2); evenNumbers.put("Four", 4); System.out.println("HashMap: " + evenNumbers); // Creating a concurrent hashmap from other map ConcurrentHashMap numbers = new ConcurrentHashMap(evenNumbers); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); ) )

Tuotos

 HashMap: (Neljä = 4, Kaksi = 2) SamanaikainenHashMap: (Neljä = 4, Kaksi = 2, Kolme = 3)

Menetelmät ConcurrentHashMap

ConcurrentHashMapLuokan tarjoaa menetelmiä, joiden avulla voimme suorittaa erilaisia toimenpiteitä kartalle.

Lisää elementit ConcurrentHashMapiin

  • put() - lisää määritetyn avaimen / arvon kartoituksen kartalle
  • putAll() - lisää kaikki määritetyn kartan merkinnät tähän karttaan
  • putIfAbsent() - lisää määritetyn avaimen / arvon kartoituksen, jos määritettyä avainta ei ole kartalla

Esimerkiksi,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( // Creating ConcurrentHashMap of even numbers ConcurrentHashMap evenNumbers = new ConcurrentHashMap(); // Using put() evenNumbers.put("Two", 2); evenNumbers.put("Four", 4); // Using putIfAbsent() evenNumbers.putIfAbsent("Six", 6); System.out.println("ConcurrentHashMap of even numbers: " + evenNumbers); //Creating ConcurrentHashMap of numbers ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); // Using putAll() numbers.putAll(evenNumbers); System.out.println("ConcurrentHashMap of numbers: " + numbers); ) )

Tuotos

 Parillisten numeroiden yhtäkertainen hash-kartta: (kuusi = 6, neljä = 4, kaksi = 2)

Käytä ConcurrentHashMap-elementtejä

1. Käyttämällä entrySet (), keySet () ja arvoja ()

  • entrySet() - returns a set of all the key/value mapping of the map
  • keySet() - returns a set of all the keys of the map
  • values() - returns a set of all the values of the map

For example,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // Using entrySet() System.out.println("Key/Value mappings: " + numbers.entrySet()); // Using keySet() System.out.println("Keys: " + numbers.keySet()); // Using values() System.out.println("Values: " + numbers.values()); ) )

Output

 ConcurrentHashMap: (One=1, Two=2, Three=3) Key/Value mappings: (One=1, Two=2, Three=3) Keys: (One, Two, Three) Values: (1, 2, 3)

2. Using get() and getOrDefault()

  • get() - Returns the value associated with the specified key. Returns null if the key is not found.
  • getOrDefault() - Returns the value associated with the specified key. Returns the specified default value if the key is not found.

For example,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // Using get() int value1 = numbers.get("Three"); System.out.println("Using get(): " + value1); // Using getOrDefault() int value2 = numbers.getOrDefault("Five", 5); System.out.println("Using getOrDefault(): " + value2); ) )

Output

 ConcurrentHashMap: (One=1, Two=2, Three=3) Using get(): 3 Using getOrDefault(): 5

Remove ConcurrentHashMap Elements

  • remove(key) - returns and removes the entry associated with the specified key from the map
  • remove(key, value) - removes the entry from the map only if the specified key mapped to the specified value and return a boolean value

For example,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // remove method with single parameter int value = numbers.remove("Two"); System.out.println("Removed value: " + value); // remove method with two parameters boolean result = numbers.remove("Three", 3); System.out.println("Is the entry (Three=3) removed? " + result); System.out.println("Updated ConcurrentHashMap: " + numbers); ) )

Output

 ConcurrentHashMap: (One=1, Two=2, Three=3) Removed value: 2 Is the entry (Three=3) removed? True Updated ConcurrentHashMap: (One=1)

Bulk ConcurrentHashMap Operations

The ConcurrentHashMap class provides different bulk operations that can be applied safely to concurrent maps.

1. forEach() Method

The forEach() method iterates over our entries and executes the specified function.

It includes two parameters.

  • parallelismThreshold - It specifies that after how many elements operations in a map are executed in parallel.
  • transformer - This will transform the data before the data is passed to the specified function.

For example,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // forEach() without transformer function numbers.forEach(4, (k, v) -> System.out.println("key: " + k + " value: " + v)); // forEach() with transformer function System.out.print("Values are "); numbers.forEach(4, (k, v) -> v, (v) -> System.out.print(v + ", ")); ) )

Output

 ConcurrentHashMap: (One = 1, Two = 2, Three = 3) key: One value: 1 key: Two value: 2 key: Three value: 3 Values are 1, 2, 3,

In the above program, we have used parallel threshold 4. This means if the map contains 4 entries, the operation will be executed in parallel.

Variation of forEach() Method

  • forEachEntry() - executes the specified function for each entry
  • forEachKey() - executes the specified function for each key
  • forEachValue() - executes the specified function for each value

2. search() Method

The search() method searches the map based on the specified function and returns the matched entry.

Here, the specified function determines what entry is to be searched.

It also includes an optional parameter parallelThreshold. The parallel threshold specifies that after how many elements in the map the operation is executed in parallel.

For example,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // Using search() String key = numbers.search(4, (k, v) -> (return v == 3 ? k: null;)); System.out.println("Searched value: " + key); ) )

Output

 ConcurrentHashMap: (One=1, Two=2, Three=3) Searched value: Three

Variants of search() Method

  • searchEntries() - search function is applied to key/value mappings
  • searchKeys() - search function is only applied to the keys
  • searchValues() - search function is only applied to the values

3. reduce() Method

The reduce() method accumulates (gather together) each entry in a map. This can be used when we need all the entries to perform a common task, like adding all the values of a map.

It includes two parameters.

  • parallelismThreshold - It specifies that after how many elements, operations in a map are executed in parallel.
  • transformer - This will transform the data before the data is passed to the specified function.

For example,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // Using search() int sum = numbers.reduce(4, (k, v) -> v, (v1, v2) -> v1 + v2); System.out.println("Sum of all values: " + sum); ) )

Output

 ConcurrentHashMap: (One=1, Two=2, Three=3) Sum of all values: 6

In the above program, notice the statement

 numbers.reduce(4, (k, v) -> v, (v1, v2) -> v1+v2);

Here,

  • 4 is a parallel threshold
  • (k, v) -> v is a transformer function. It transfers the key/value mappings into values only.
  • (v1, v2) -> v1+v2 is a reducer function. It gathers together all the values and adds all values.

Variants of reduce() Method

  • reduceEntries() - returns the result of gathering all the entries using the specified reducer function
  • reduceKeys() - returns the result of gathering all the keys using the specified reducer function
  • reduceValues() - returns the result of gathering all the values using the specified reducer function

ConcurrentHashMap vs HashMap

Here are some of the differences between ConcurrentHashMap and HashMap,

  • ConcurrentHashMap is a thread-safe collection. That is, multiple threads can access and modify it at the same time.
  • ConcurrentHashMap provides methods for bulk operations like forEach(), search() and reduce().

Why ConcurrentHashMap?

  • ConcurrentHashMapLuokan avulla useita säikeitä käyttää sen merkinnät samanaikaisesti.
  • Oletuksena samanaikainen hashmap on jaettu 16 segmenttiin . Tämä on syy, miksi 16 säiettä saa samanaikaisesti muokata karttaa samanaikaisesti. Kuitenkin mikä tahansa määrä ketjuja voi käyttää karttaa kerrallaan.
  • putIfAbsent()Menetelmä ei korvaa merkintä kartassa jos määritetty avain on jo olemassa.
  • Se tarjoaa oman synkronoinnin.

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