在Java中如何初始化HashMap？ 7種不同的方式

已發表: 2021-11-17
在Java中如何初始化HashMap？ 7種不同的方式

在Java中如何初始化HashMap？ 如何直接初始化 HashMap（以字面方式）。

有7 different ways可以在 Java 中初始化 HashMap。

方法一：簡單可變映射：

這是一個支持修改操作的地圖，例如添加，刪除和清除。

方法 2：Collections.singletonMap

在 Java 中如何使用 Collections.unmodifiableMap 初始化 HashMap

方法 3：Collections.singletonMap

在 Java 中如何使用 Collections.singletonMap() 初始化 HashMap

方法 4：Java 9 – Map.of()

在 Java 中如何使用 Java 9 初始化 HashMap - Map.of()

方法 5：Java 9 – Map.ofEntries()

在 Java 中如何使用 Java 9 初始化 HashMap - Map.ofEntries()

方法 6：簡單的自定義地圖

在 Java 中如何使用簡單的自定義映射初始化 HashMap

方法 7：Stream.of – AbstractMap.SimpleEntry

讓我們開始使用 Java 程序：

  • 創建類：CrunchifyInitiateHashMap.java
  • 複製以下代碼並將其放入java文件中
  • 保存存檔

注意：查看Java程序中的註釋以獲得詳細解釋。

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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 結果：

只需在 IntelliJ IDEA 或 Eclipse 中將上述程序作為 Java 應用程序運行，您應該會看到與以下類似的結果。

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/ 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
Key : company2 , Value : Facebook
Key : company3 , Value : Google
======
Key : company1 , Value : Crunchify
Key : company2 , Value : Facebook
Key : company3 , Value : Google
======
Key : company1 , Value : Crunchify
Key : company2 , Value : Facebook
Key : company3 , Value : Google
Key : company4 , Value : Crunchify
======
 
================== Method - 3 ==================
Key : company1 , Value : Crunchify
Key : company2 , Value : Facebook
======
 
================== Method - 4 ==================
Key : company1 , Value : Crunchify
Key : company2 , Value : Facebook
Key : company3 , Value : Google
Key : company4 , Value : Amazon
======
 
================== Method - 5 ==================
Key : company1 , Value : Crunchify
Key : company2 , Value : Facebook
Key : company3 , Value : Google
Key : company4 , Value : Twitter
======
 
================== Method - 6 ==================
Key : company1 , Value : Crunchify
Key : company2 , Value : Facebook
Key : company3 , Value : Google
Key : company4 , Value : LinkedIn
======
 
================== Method - 7 ==================
Key : company1 , Value : Crunchify
Key : company2 , Value : Facebook
Key : company3 , Value : Google
Key : company4 , Value : Microsoft
======
 
Process finished with exit code 0
 

如果您在運行此程序時遇到任何問題，請告訴我。