Java ConcurrentHashMap

U ovom uputstvu naučit ćemo o klasi Java ConcurrentHashMap i njezinim operacijama uz pomoć primjera.

ConcurrentHashMapKlasa Java zbirki okvir pruža kartu nit-sigurno. Odnosno, više niti može pristupiti karti odjednom bez utjecaja na dosljednost unosa na karti.

Provodi sučelje ConcurrentMap.

Stvorite ConcurrentHashMap

Da bismo stvorili istodobnu hashmapu, prvo moramo uvesti java.util.concurrent.ConcurrentHashMappaket. Jednom kada uvozimo paket, evo kako možemo stvoriti istodobne hash-mape u Javi.

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

U gornjem kodu stvorili smo istodobnu hashmapu nazvanu brojevi.

Ovdje,

  • Ključ - jedinstveni identifikator koji se koristi za povezivanje svakog elementa (vrijednosti) na karti
  • Vrijednost - elementi pridruženi ključevima na karti

Primijetite dio new ConcurrentHashMap(8, 0.6). Ovdje je prvi parametar kapacitet, a drugi parametar loadFactor .

  • kapacitet - Kapacitet ove karte je 8. Što znači da može pohraniti 8 unosa.
  • loadFactor - Faktor opterećenja ove karte je 0,6. To znači da, kad god se naša hash tablica popuni za 60%, unosi se premještaju u novu hash tablicu dvostruke veličine izvorne hash tablice.

Zadani kapacitet i faktor opterećenja

Moguće je stvoriti istodobnu hashmapu bez definiranja njenog kapaciteta i faktora opterećenja. Na primjer,

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

Prema zadanim postavkama,

  • kapacitet karte bit će 16
  • faktor opterećenja iznosit će 0,75

Stvaranje ConcurrentHashMap na drugim kartama

Evo kako možemo stvoriti istodobnu hashmapu koja sadrži sve elemente drugih karata.

 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); ) )

Izlaz

 HashMap: (Četiri = 4, Dvije = 2) IstodobnaHashMap: (Četiri = 4, Dvije = 2, Tri = 3)

Metode ConcurrentHashMap

ConcurrentHashMapKlasa pruža metode koje omogućuju nam da obavljaju različite operacije na karti.

Umetnite elemente u ConcurrentHashMap

  • put() - na kartu ubacuje određeno mapiranje ključa / vrijednosti
  • putAll() - na ovu kartu ubacuje sve unose s određene karte
  • putIfAbsent() - ubacuje navedeno mapiranje ključa / vrijednosti na kartu ako navedeni ključ nije prisutan na karti

Na primjer,

 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); ) )

Izlaz

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

Pristupite elementima ConcurrentHashMap

1. Korištenje entrySet (), keySet () i vrijednosti ()

  • entrySet() - vraća skup svih mapiranja ključa / vrijednosti karte
  • keySet() - vraća skup svih tipki karte
  • values() - vraća skup svih vrijednosti karte

Na primjer,

 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()); ) )

Izlaz

 ConcurrentHashMap: (One = 1, Two = 2, Three = 3) preslikavanja ključa / vrijednosti: (One = 1, Two = 2, Three = 3) Tipke: (One, Two, Three) Vrijednosti: (1, 2, 3 )

2. Korištenje get () i getOrDefault ()

  • get()- Vraća vrijednost povezanu s navedenim ključem. Vraća nullako ključ nije pronađen.
  • getOrDefault()- Vraća vrijednost povezanu s navedenim ključem. Vraća navedenu zadanu vrijednost ako ključ nije pronađen.

Na primjer,

 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); ) )

Izlaz

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

Uklonite ConcurrentHashMap elemente

  • remove(key) - vraća i uklanja unos povezan s navedenim ključem s karte
  • 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?

  • ConcurrentHashMapKlasa omogućuje više niti za pristup unose istovremeno.
  • Prema zadanim postavkama istodobna hashmapa podijeljena je u 16 segmenata . To je razlog zbog kojeg je 16 niti istodobno dopušteno istodobno mijenjati kartu. Međutim, istovremeno može pristupiti karti bilo koji broj niti.
  • putIfAbsent()Metoda neće nadjačati unos u karti ako Navedeni ključ već postoji.
  • Pruža vlastitu sinkronizaciju.

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