Java samaaegneHashMap

Selles õpetuses õpime näidete abil tundma Java ConcurrentHashMap klassi ja selle toiminguid.

ConcurrentHashMapKlassi Java kogud raames annab niidi ohutu kaardil. See tähendab, et mitu lõime saavad korraga kaardile juurde pääseda, ilma et see mõjutaks kaardi kirjete järjepidevust.

See rakendab liidest ConcurrentMap.

Looge ConcurrentHashMap

Samaaegse hashmi loomiseks peame java.util.concurrent.ConcurrentHashMapesmalt paketi importima . Kui pakett on imporditud, saate järgmiselt luua Java-s samaaegseid räsimappe.

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

Ülaltoodud koodis oleme loonud samaaegse hashmapi nimega numbrid.

Siin,

  • Võti - kordumatu identifikaator, mida kasutatakse iga elemendi (väärtuse) seostamiseks kaardil
  • Väärtus - kaardil klahvidega seotud elemendid

Märka osa new ConcurrentHashMap(8, 0.6). Siin on esimene parameeter võimsus ja teine ​​parameeter loadFactor .

  • maht - selle kaardi maht on 8. See tähendab, et see võib salvestada 8 kirjet.
  • loadFactor - selle kaardi koormustegur on 0,6. See tähendab, et alati, kui meie räsitabelit täidetakse 60%, teisaldatakse kanded uude räsitabelisse, mis on algse räsitabeli kahekordne.

Vaikimisi maht ja koormustegur

Samaaegset hashmi on võimalik luua ilma selle võimsust ja koormustegurit määramata. Näiteks,

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

Vaikimisi,

  • kaardi maht on 16
  • koormustegur on 0,75

ConcurrentHashMapi loomine teistest kaartidest

Nii saame luua samaaegse hashmi, mis sisaldab kõigi teiste kaartide elemente.

 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); ) )

Väljund

 HashMap: (neli = 4, kaks = 2) samaaegneHashMap: (neli = 4, kaks = 2, kolm = 3)

ConcurrentHashMap meetodid

ConcurrentHashMapKlassi ette meetodid, mis võimaldavad meil sooritada erinevaid toiminguid kaardil.

Sisestage elemendid ConcurrentHashMap

  • put() - sisestab kaardile määratud võtme / väärtuse kaardistamise
  • putAll() - lisab sellele kaardile kõik kirjed määratud kaardilt
  • putIfAbsent() - sisestab kaardile määratud võtme / väärtuse kaardistuse, kui määratud võtit kaardil pole

Näiteks,

 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); ) )

Väljund

 Paarisarvude samaaegneHashMap: (Kuus = 6, Neli = 4, Kaks = 2) SamaaegneHashMap numbrite kaart: (Kuus = 6, Üks = 1, Neli = -4, Kaks = 2)

Juurdepääs ConcurrentHashMapi elementidele

1. Kasutades kirjetSet (), keySet () ja väärtusi ()

  • entrySet() - tagastab kaardi kõigi võtmete / väärtuste kaardistamise komplekti
  • keySet() - tagastab kaardi kõigi võtmete komplekti
  • values() - tagastab kaardi kõigi väärtuste hulga

Näiteks,

 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()); ) )

Väljund

 ConcurrentHashMap: (üks = 1, kaks = 2, kolm = 3) võtme / väärtuse kaardistamine: (üks = 1, kaks = 2, kolm = 3) võtmed: (üks, kaks, kolm) väärtused: (1, 2, 3 )

2. get () ja getOrDefault () kasutamine

  • get()- tagastab määratud võtmega seotud väärtuse. Tagastab, nullkui võtit ei leitud.
  • getOrDefault()- tagastab määratud võtmega seotud väärtuse. Tagastab määratud vaikeväärtuse, kui võtit ei leita.

Näiteks,

 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); ) )

Väljund

 ConcurrentHashMap: (üks = 1, kaks = 2, kolm = 3) get () kasutamine: 3 getOrDefault () kasutamine: 5

Eemaldage ConcurrentHashMap elemendid

  • remove(key) - tagastab ja eemaldab kaardilt määratud võtmega seotud kirje
  • 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?

  • ConcurrentHashMapTasemega lahtrisse mitme lõime juurde selle andmeteks samaaegselt.
  • Vaikimisi on samaaegne hashmap jagatud 16 segmendiks . See on põhjus, miks 16 niidil lubatakse kaarti samaaegselt muuta. Kaardile pääseb korraga juurde suvaline arv niite.
  • putIfAbsent()Meetod ei alistada kanne kaardil kui kindlaksmääratud võti on juba olemas.
  • See tagab oma sünkroonimise.

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