Java ConcurrentHashMap

I denne vejledning lærer vi om Java ConcurrentHashMap-klassen og dens operationer ved hjælp af eksempler.

Den ConcurrentHashMapklasse af Java kollektioner rammer giver en tråd-safe kortet. Det vil sige, at flere tråde kan få adgang til kortet på en gang uden at påvirke konsistensen af ​​poster på et kort.

Det implementerer ConcurrentMap-grænsefladen.

Opret et ConcurrentHashMap

For at oprette en samtidig hashmap skal vi først importere java.util.concurrent.ConcurrentHashMappakken. Når vi først har importeret pakken, kan du oprette samtidige hashmaps i Java.

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

I ovenstående kode har vi oprettet et sideløbende hashmap med navn.

Her,

  • Nøgle - en unik identifikator, der bruges til at knytte hvert element (værdi) til et kort
  • Værdi - elementer tilknyttet nøgler på et kort

Læg mærke til delen new ConcurrentHashMap(8, 0.6). Her er den første parameter kapacitet, og den anden parameter er loadFactor .

  • Kapacitet - Kortets kapacitet er 8. Det betyder, at det kan gemme 8 poster.
  • loadFactor - Belastningsfaktoren på dette kort er 0,6. Dette betyder, at hver gang vores hash-tabel er fyldt med 60%, flyttes posterne til en ny hash-tabel med dobbelt størrelse på den oprindelige hash-tabel.

Standardkapacitet og belastningsfaktor

Det er muligt at oprette en samtidig hashmap uden at definere dens kapacitet og belastningsfaktor. For eksempel,

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

Som standard,

  • kortets kapacitet er 16
  • belastningsfaktoren er 0,75

Oprettelse af ConcurrentHashMap fra andre kort

Her er hvordan vi kan oprette en samtidig hashmap, der indeholder alle elementerne i andre kort.

 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); ) )

Produktion

 HashMap: (Fire = 4, To = 2) Samtidig HashMap: (Fire = 4, To = 2, Tre = 3)

Metoder til ConcurrentHashMap

Den ConcurrentHashMapklasse indeholder metoder, der tillader os at udføre forskellige operationer på kortet.

Indsæt elementer til ConcurrentHashMap

  • put() - indsætter den angivne nøgle- / værdikortlægning på kortet
  • putAll() - indsætter alle poster fra det angivne kort til dette kort
  • putIfAbsent() - indsætter den angivne nøgle- / værdikortlægning på kortet, hvis den angivne nøgle ikke er til stede på kortet

For eksempel,

 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); ) )

Produktion

 ConcurrentHashMap af lige tal: (Six = 6, Four = 4, Two = 2) ConcurrentHashMap of numbers: (Six = 6, One = 1, Four = -4, Two = 2)

Få adgang til ConcurrentHashMap Elements

1. Brug af entrySet (), keySet () og værdier ()

  • entrySet() - returnerer et sæt med alle nøgle- / værdikortlæggelser på kortet
  • keySet() - returnerer et sæt af alle tasterne på kortet
  • values() - returnerer et sæt af alle kortets værdier

For eksempel,

 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()); ) )

Produktion

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

2. Brug af get () og getOrDefault ()

  • get()- Returnerer den værdi, der er knyttet til den angivne nøgle. Returnerer, nullhvis nøglen ikke findes.
  • getOrDefault()- Returnerer den værdi, der er knyttet til den angivne nøgle. Returnerer den angivne standardværdi, hvis nøglen ikke findes.

For eksempel,

 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); ) )

Produktion

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

Fjern ConcurrentHashMap Elements

  • remove(key) - returnerer og fjerner posten tilknyttet den angivne nøgle fra kortet
  • 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?

  • Den ConcurrentHashMapklasse tillader flere tråde til at få adgang til dens indgange samtidigt.
  • Som standard er den samtidige hashmap opdelt i 16 segmenter . Dette er grunden til, at 16 tråde får lov til samtidig at ændre kortet på samme tid. Et hvilket som helst antal tråde kan dog få adgang til kortet ad gangen.
  • Den putIfAbsent()metode vil ikke tilsidesætte posten i kortet, hvis den angivne nøgle findes allerede.
  • Det giver sin egen synkronisering.

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