In Java how to Initialize HashMap? 7 different ways

In Java How to initialize HashMap? How to directly initialize a HashMap (in a literal way).

There are 7 different ways you can initialize HashMap in Java.

Method-1: Simple Mutable map:

It’s a map which supports modification operations such as add, remove, and clear on it.

Method-2: Collections.singletonMap

In Java how to Initialize HashMap with Collections.unmodifiableMap

Method-3: Collections.singletonMap

In Java how to Initialize HashMap with Collections.singletonMap()

Method-4: Java 9 – Map.of()

In Java how to Initialize HashMap with Java 9 - Map.of()

Method-5: Java 9 – Map.ofEntries()

In Java how to Initialize HashMap with Java 9 - Map.ofEntries()

Method-6: Simple Custom Maps

In Java how to Initialize HashMap with Simple Custom Map

Method-7: Stream.of – AbstractMap.SimpleEntry

Let’s get started for Java program:

Create class: CrunchifyInitiateHashMap.java
Copy below code and put it into java file
Save file

Note: Take a look at comments in Java Program for detailed explanation.

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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 Result:

Just run above program in IntelliJ IDEA or Eclipse as a Java Application and you should see similar result as below.

/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