Wie initialisiert man HashMap in Java? 7 verschiedene Wege

Veröffentlicht: 2021-11-17

In Java Wie initialisiert man HashMap? Wie man eine HashMap direkt initialisiert (wörtlich).

Es gibt 7 different ways , HashMap in Java zu initialisieren.

Methode-1: Einfache veränderliche Karte:

Es ist eine Karte, die Änderungsoperationen wie Hinzufügen, Entfernen und Löschen unterstützt.

Methode-2: Collections.singletonMap

In Java, wie man HashMap mit Collections.unmodifiableMap initialisiert

Methode-3: Collections.singletonMap

In Java, wie man HashMap mit Collections.singletonMap() initialisiert

Methode-4: Java 9 – Map.of()

In Java, wie man HashMap mit Java 9 initialisiert - Map.of()

Methode-5: Java 9 – Map.ofEntries()

In Java, wie man HashMap mit Java 9 initialisiert - Map.ofEntries()

Methode 6: Einfache benutzerdefinierte Karten

In Java, wie man HashMap mit Simple Custom Map initialisiert

Methode-7: Stream.of – AbstractMap.SimpleEntry

Beginnen wir mit dem Java-Programm:

Klasse erstellen: CrunchifyInitiateHashMap.java
Kopieren Sie den folgenden Code und fügen Sie ihn in die Java-Datei ein
Datei speichern

Hinweis: Sehen Sie sich die Kommentare im Java-Programm an, um eine detaillierte Erklärung zu erhalten.

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

203

204

205

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 ) ;

}

Ergebnis von IntelliJ IDEA:

Führen Sie einfach das obige Programm in IntelliJ IDEA oder Eclipse als Java-Anwendung aus und Sie sollten ein ähnliches Ergebnis wie unten sehen.

/ 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