In Java come inizializzare HashMap? 7 modi diversi

Pubblicato: 2021-11-17

In Java Come inizializzare HashMap? Come inizializzare direttamente una HashMap (in modo letterale).

Esistono 7 different ways inizializzare HashMap in Java.

Metodo-1: semplice mappa mutabile:

È una mappa che supporta operazioni di modifica come aggiungere, rimuovere e cancellare su di essa.

Metodo-2: Collections.singletonMap

In Java come inizializzare HashMap con Collections.unmodifiableMap

Metodo-3: Collections.singletonMap

In Java come inizializzare HashMap con Collections.singletonMap()

Metodo-4: Java 9 – Map.of()

In Java come inizializzare HashMap con Java 9 - Map.of()

Metodo-5: Java 9 – Map.ofEntries()

In Java come inizializzare HashMap con Java 9 - Map.ofEntries()

Metodo-6: semplici mappe personalizzate

In Java come inizializzare HashMap con Simple Custom Map

Metodo-7: Stream.of – AbstractMap.SimpleEntry

Iniziamo per il programma Java:

Crea classe: CrunchifyInitiateHashMap.java
Copia sotto il codice e inseriscilo nel file java
Salvare il file

Nota: dai un'occhiata ai commenti nel programma Java per una spiegazione dettagliata.

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package crunchify . com . java . tutorials ;

import java . util . AbstractMap ;

import java . util . Collections ;

import java . util . HashMap ;

import java . util . Map ;

import java . util . stream . Stream ;

import static java . util . stream . Collectors . toMap ;

/**

* @author Crunchify.com

* Program: In Java how to Initialize HashMap? 7 different ways.

public class CrunchifyInitiateHashMap {

// Method-1

// This is Mutable map: It's a map which supports modification operations such as add, remove, and clear on it.

public static Map < String , String > crunchifyMap ;

static {

// Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).

crunchifyMap = new HashMap < > ( ) ;

crunchifyMap . put ( "company1" , "Crunchify" ) ;

crunchifyMap . put ( "company2" , "Facebook" ) ;

crunchifyMap . put ( "company3" , "Google" ) ;

}

public static void main ( String [ ] args ) {

// Method-2

collectionsUnmodifiableMap ( ) ;

// Method-3

collectionsSingletonMap ( ) ;

// Method-4

java9Mapof ( ) ;

// Method-5

java9MapOfEntries ( ) ;

// Method-6

crunchifySimpleustomMap ( ) ;

// Method-7

streamOfSimpleEntry ( ) ;

}

// Method-2

private static void collectionsUnmodifiableMap ( ) {

System . out . println ( "\n================== Method-2 ==================\n" ) ;

Map < String , String > crunchifyMap = new HashMap < > ( ) ;

// put(): Associates the specified value with the specified key in this map (optional operation).

// If the map previously contained a mapping for the key, the old value is replaced by the specified value.

// (A map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true.)

crunchifyMap . put ( "company1" , "Crunchify" ) ;

crunchifyMap . put ( "company2" , "Facebook" ) ;

crunchifyMap . put ( "company3" , "Google" ) ;

// getClass(): Returns the runtime class of this Object.

// The returned Class object is the object that is locked by static synchronized methods of the represented class.

mapIteratorCrunchifyUtils ( crunchifyMap ) ;

// unmodifiableMap(): Returns an unmodifiable view of the specified map.

// Query operations on the returned map "read through" to the specified map, and attempts

// to modify the returned map, whether direct or via its collection views, result in an UnsupportedOperationException.

// The returned map will be serializable if the specified map is serializable.

Map < String , String > crunchifyUnModifiableMap = new

HashMap < > ( Collections . unmodifiableMap ( crunchifyMap ) ) ;

mapIteratorCrunchifyUtils ( crunchifyUnModifiableMap ) ;

crunchifyUnModifiableMap . put ( "company4" , "Crunchify" ) ;

mapIteratorCrunchifyUtils ( crunchifyUnModifiableMap ) ;

}

// Simple Map Iterator Utility

private static void mapIteratorCrunchifyUtils ( Map < String , String > crunchifyMap ) {

// forEach(): Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.

// Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.)

// Exceptions thrown by the action are relayed to the caller.

crunchifyMap . forEach ( ( key , value ) - > crunchifyPrintUtils ( "\tKey: " + key + ", Value: " + value ) ) ;

System . out . println ( "\t\t\t\t======" ) ;

}

// Simple log utility

private static void crunchifyPrintUtils ( Object crunchifyObject ) {

// println(): Prints a String and then terminate the line.

// This method behaves as though it invokes print(String) and then println().

System . out . println ( crunchifyObject ) ;

}

// Method-3

private static void collectionsSingletonMap ( ) {

System . out . println ( "\n================== Method-3 ==================" ) ;

// singletonMap(): Returns an immutable map, mapping only the specified key to the specified value.

// The returned map is serializable.

Map < String , String > crunchifyMap = new HashMap < > ( Collections . singletonMap ( "company1" , "Crunchify" ) ) ;

crunchifyMap . put ( "company2" , "Facebook" ) ;

mapIteratorCrunchifyUtils ( crunchifyMap ) ;

}

// Method-4

private static void java9Mapof ( ) {

System . out . println ( "\n================== Method-4 ==================" ) ;

// Map.of(): Returns an unmodifiable map containing three mappings. See Unmodifiable Maps for details.

Map < String , String > crunchifyMap = new HashMap < > ( Map . of (

"company1" , "Crunchify" ,

"company2" , "Facebook" ,

"company3" , "Google"

) ) ;

crunchifyMap . put ( "company4" , "Amazon" ) ;

mapIteratorCrunchifyUtils ( crunchifyMap ) ;

}

// Method-5

private static void java9MapOfEntries ( ) {

System . out . println ( "\n================== Method-5 ==================" ) ;

// Map.ofEntries(): Returns an unmodifiable map containing keys and values extracted from the given entries.

// The entries themselves are not stored in the map. See Unmodifiable Maps for details.

// Since: Java 9

// Throws: NullPointerException – if the key or value is null

Map < String , String > crunchifyMap = new HashMap < > ( Map . ofEntries (

Map . entry ( "company1" , "Crunchify" ) ,

Map . entry ( "company2" , "Facebook" ) ,

Map . entry ( "company3" , "Google" )

) ) ;

crunchifyMap . put ( "company4" , "Twitter" ) ;

mapIteratorCrunchifyUtils ( crunchifyMap ) ;

}

// Method-6

private static void crunchifySimpleustomMap ( ) {

System . out . println ( "\n================== Method-6 ==================" ) ;

// It works for all Java versions, mutable map.

Map < String , String > crunchifyMap = myCrunchifyMap ( ) ;

crunchifyMap . put ( "company4" , "LinkedIn" ) ;

mapIteratorCrunchifyUtils ( crunchifyMap ) ;

}

private static Map < String , String > myCrunchifyMap ( ) {

Map < String , String > crunchifyMap = new HashMap < > ( ) ;

crunchifyMap . put ( "company1" , "Crunchify" ) ;

crunchifyMap . put ( "company2" , "Facebook" ) ;

crunchifyMap . put ( "company3" , "Google" ) ;

return crunchifyMap ;

}

// Method-7

private static void streamOfSimpleEntry ( ) {

System . out . println ( "\n================== Method-7 ==================" ) ;

// Stream(): A sequence of elements supporting sequential and parallel aggregate operations.

// The following example illustrates an aggregate operation using Stream and IntStream.

// Stream.of(): Returns a sequential ordered stream whose elements are the specified values.

Map < String , String > crunchifyMap = Stream . of (

// AbstractMap: Sole constructor. (For invocation by subclass constructors, typically implicit.)

// SimpleEntry(): Creates an entry representing a mapping from the specified key to the specified value.

new AbstractMap . SimpleEntry < > ( "company1" , "Crunchify" ) ,

new AbstractMap . SimpleEntry < > ( "company2" , "Facebook" ) ,

new AbstractMap . SimpleEntry < > ( "company3" , "Google" ) )

// collect(): Performs a mutable reduction operation on the elements of this stream using a Collector.

// A Collector encapsulates the functions used as arguments to collect(Supplier, BiConsumer, BiConsumer),

// allowing for reuse of collection strategies and composition of collect operations such as multiple-level grouping or partitioning.

. collect (

// getKey: Returns the key corresponding to this entry.

// getValue: Returns the value corresponding to this entry.

toMap ( AbstractMap . SimpleEntry : : getKey ,

AbstractMap . SimpleEntry : : getValue )

) ;

crunchifyMap . put ( "company4" , "Microsoft" ) ;

mapIteratorCrunchifyUtils ( crunchifyMap ) ;

}

IntelliJ IDEA Risultato:

Basta eseguire il programma sopra in IntelliJ IDEA o Eclipse come applicazione Java e dovresti vedere un risultato simile come di seguito.

/ Users / app / Library / Java / JavaVirtualMachines / openjdk - 17.0.1 / Contents / Home / bin / java - javaagent : / Applications / IntelliJ IDEA . app / Contents / lib / idea_rt . jar = 53352 : / Applications / IntelliJ IDEA . app / Contents / bin - Dfile . encoding = UTF - 8 - classpath / Users / app / Documents / crunchify / github / CrunchifyTutorials / target / classes : / Users / app / Documents / crunchify / github / CrunchifyTutorials / WebContent / WEB - INF / lib / zxing - 2.1.jar : crunchify . com . java . tutorials . CrunchifyInitiateHashMap

================== Method - 2 ==================

Key : company1 , Value : Crunchify