Java ConcurrentHashMap

V tej vadnici bomo s pomočjo primerov spoznali razred Java ConcurrentHashMap in njegovo delovanje.

ConcurrentHashMapRazred okviru Java zbirk zagotavlja niti varna zemljevid. To pomeni, da lahko več niti hkrati dostopa do zemljevida, ne da bi to vplivalo na doslednost vnosov na zemljevidu.

Izvaja vmesnik ConcurrentMap.

Ustvarite ConcurrentHashMap

Če želite ustvariti sočasno hashmapo, moramo najprej uvoziti java.util.concurrent.ConcurrentHashMappaket. Ko uvozimo paket, lahko v Javi ustvarimo sočasne hashmape.

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

V zgornji kodi smo ustvarili sočasno hashmap z imenom številke.

Tukaj,

  • Ključ - enolični identifikator, ki se uporablja za povezovanje vsakega elementa (vrednosti) na zemljevidu
  • Vrednost - elementi, povezani s ključi na zemljevidu

Upoštevajte del new ConcurrentHashMap(8, 0.6). Tu je prvi parameter zmogljivost, drugi pa loadFactor .

  • zmogljivost - Kapaciteta te karte je 8. Kar pomeni, da lahko shrani 8 vnosov.
  • loadFactor - Faktor obremenitve tega zemljevida je 0,6. To pomeni, da se vsakič, ko je naša hash tabela zapolnjena za 60%, vnosi premaknejo v novo hash tabelo dvojne velikosti prvotne hash tabele.

Privzeta zmogljivost in faktor obremenitve

Možno je ustvariti sočasno hashmap, ne da bi opredelili njegovo zmogljivost in faktor obremenitve. Na primer

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

Privzeto,

  • zmogljivost zemljevida bo 16
  • faktor obremenitve bo 0,75

Ustvarjanje ConcurrentHashMap iz drugih zemljevidov

Tukaj je opisano, kako lahko ustvarimo sočasno hashmapo, ki vsebuje vse elemente drugih zemljevidov.

 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); ) )

Izhod

 HashMap: (Four = 4, Two = 2) ConcurrentHashMap: (Four = 4, Two = 2, Three = 3)

Metode ConcurrentHashMap

ConcurrentHashMapRazred določa metode, ki nam omogočajo, da opravljajo različne dejavnosti na zemljevidu.

Vstavite elemente v ConcurrentHashMap

  • put() - na zemljevid vstavi določeno preslikavo ključa / vrednosti
  • putAll() - na ta zemljevid vstavi vse vnose z določenega zemljevida
  • putIfAbsent() - na zemljevid vstavi določeno preslikavo ključa / vrednosti, če določeni ključ ni prisoten na zemljevidu

Na primer

 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); ) )

Izhod

 ConcurrentHashMap parnih števil: (Six = 6, Four = 4, Two = 2) ConcurrentHashMap števil:: Six = 6, One = 1, Four = -4, Two = 2)

Dostopajte do elementov ConcurrentHashMap

1. Uporaba entrySet (), keySet () in vrednosti ()

  • entrySet() - vrne nabor vseh preslikav ključ / vrednost zemljevida
  • keySet() - vrne nabor vseh tipk zemljevida
  • values() - vrne niz vseh vrednosti zemljevida

Na primer

 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()); ) )

Izhod

 ConcurrentHashMap: (One = 1, Two = 2, Three = 3) Preslikave ključa / vrednosti: (One = 1, Two = 2, Three = 3) Tipke: (One, Two, Three) Vrednosti: (1, 2, 3 )

2. Uporaba get () in getOrDefault ()

  • get()- Vrne vrednost, povezano z navedenim ključem. Vrne, nullče ključa ni mogoče najti.
  • getOrDefault()- Vrne vrednost, povezano z navedenim ključem. Vrne določeno privzeto vrednost, če ključa ni mogoče najti.

Na primer

 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); ) )

Izhod

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

Odstranite elemente ConcurrentHashMap

  • remove(key) - vrne in z zemljevida odstrani vnos, povezan z določenim ključem
  • 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?

  • ConcurrentHashMapRazred omogoča več niti za dostop do svojih vnosov hkrati.
  • Privzeto je sočasni hashmap razdeljen na 16 segmentov . To je razlog, zakaj lahko 16 niti istočasno spreminja zemljevid. Do zemljevida pa lahko hkrati dostopa poljubno število niti.
  • putIfAbsent()Metoda ne bo razveljavila vpis na zemljevidu, če je navedeni ključ že obstaja.
  • Omogoča lastno sinhronizacijo.

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