Ways to Initialize HashMap Collection in Java

The preferred way to initialize a HashMap in Java is by using the default constructor and instantiating it as a Map interface.

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

As Map is an interface in Java, map variable here can store an instance of any class that implements the Map interface, such as HashMap. This allows for flexibility and code maintainability, as you can switch to a different implementation of the Map interface without changing the rest of your code.

Initialize HashMap as an Immutable Map

Map<String, String> map = Collections.unmodifiableMap(map);

Using Java 8 Streams

import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class HashMapUsingStreams {

    public static void main(String[] args) {
        Map<String, String> countryCapitalMap = Stream.of(new String[][] {
                { "USA", "Washington, D.C." },
                { "Canada", "Ottawa" },
                { "France", "Paris" },
                { "Germany", "Berlin" },
                { "Japan", "Tokyo" }
        }).collect(Collectors.toMap(data -> data[0], data -> data[1]));

        System.out.println(countryCapitalMap);
    }
}

Java 9 or later: Using Initialization with Values

HashMap<KeyType, ValueType> hashMap = new HashMap<>(Map.of(
    key1, value1,
    key2, value2,
    key3, value3
));

another way!

HashMap<KeyType, ValueType> hashMap = new HashMap<>(Map.ofEntries(
    entry(key1, value1),
    entry(key2, value2),
    entry(key3, value3)
));