Initialize HashMap in Java

Last Updated : 17 Jun, 2024

HashMap in Java is a part of the java.util package and allows storing key-value pairs. Initializing a HashMap can be done in multiple ways, including static blocks, utility methods from the Collections class, and modern approaches provided by Java 8 and Java 9. This article will guide you through these methods with clear examples and explanations.

Different Methods to Initialize a HashMap in Java

Using Static Block
Using Collections
Using Java 8 Methods (Streams)
Using Java 9 Methods (Map.of() and Map.ofEntries())

1. Initialize HashMap Using Static Block

Using a static block is an effective way to initialize a HashMap at class loading time, especially when we need a constant, unmodifiable map.

Example:

Java

// Java Program to Initialize a HashMap Using Static Block
import java.util.HashMap;
import java.util.Map;

public class StaticBlockInitialization {
    private static final Map<Integer, String> map;

    static {
        map = new HashMap<>();
        map.put(1, "C");
        map.put(2, "C++");
        map.put(3, "Java");
    }

    public static void main(String args[]) 
    {
        System.out.println("HashMap initialized using static block: " + map);
    }
}

Output

HashMap initialized using static block: {1=C, 2=C++, 3=Java}

Explanation:

The static block runs when the class is loaded.
HashMap map is initialized and populated with key-value pairs.
This approach is suitable for creating immutable maps that remain constant throughout the program’s execution.

2. Initialize HashMap Using Collections

Java Collections framework provides utility methods to initialize a HashMap. One such method is Collections.singletonMap, which creates a map with a single entry. This map can be used to initialize a HashMap.

Example:

Java

// Java Program to Initialize a HashMap Using Collections
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class CollectionsInitialization {
    public static void main(String args[]) 
     {
        Map<Integer, String> singletonMap = Collections.singletonMap(1, "C");
        
        // Creating a mutable map
        Map<Integer, String> map = new HashMap<>(singletonMap);
        map.put(2, "C++");
        map.put(3, "Java");

        System.out.println("HashMap initialized using Collections: " + map);
    }
}

Output

HashMap initialized using Collections: {1=C, 2=C++, 3=Java}

Explanation:

Collections.singletonMap creates an immutable map with a single entry.
This map is used to initialize a mutable HashMap which can be further modified.

3. Initialize HashMap Using Java 8 Methods (Using Stream)

Java 8 introduced Streams, which can be used to initialize a HashMap using arrays of key-value pairs.

Example:

Java

//Java Program to Initialize a HashMap Using Stream
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class StreamInitialization {
    public static void main(String args[]) 
    {
        Map<Integer, String> map = Stream.of(new Object[][] {
            { 1, "C" },
            { 2, "C++" },
            { 3, "Java" },
        }).collect(Collectors.toMap(data -> (Integer) data[0], data -> (String) data[1]));

        System.out.println("HashMap initialized using Java 8 Stream: " + map);
    }
}

Output

HashMap initialized using Java 8 Stream: {1=C, 2=C++, 3=Java}

Explanation:

Stream.of creates a stream of object arrays, each containing a key-value pair.
Collectors.toMap collects the stream elements into a HashMap.

4. Initialize HashMap Using Java 9 Methods

Java 9 introduced convenient factory methods like Map.of and Map.ofEntries for initializing HashMaps.

Example Using Map.of():

Java

//Java Program to Initialize a HashMap Using Map.of()
import java.util.Map;

public class Java9Initialization {
    public static void main(String[] args) {
        // Using Map.of for up to 10 key-value pairs
        Map<Integer, String> map1 = Map.of(1, "C", 2, "C++", 3, "Java");
        System.out.println("HashMap initialized using Java 9 Map.of: " + map1);
    }
}

Output

HashMap initialized using Java 9 Map.of: {1=C, 2=C++, 3=Java}

Explanation:

Map.of creates an immutable map with up to 10 key-value pairs.

Example Using Map.ofEntries():

Java

//Java Program to Initialize a HashMap using Map.ofEntries()
import java.util.Map;

public class Java9Initialization {
    public static void main(String[] args) {
        // Using Map.ofEntries for more than 10 key-value pairs
        Map<Integer, String> map2 = Map.ofEntries(
            Map.entry(1, "C"),
            Map.entry(2, "C++"),
            Map.entry(3, "Java")
        );
        System.out.println("HashMap initialized using Java 9 Map.ofEntries: " + map2);
    }
}

Output

HashMap initialized using Java 9 Map.ofEntries: {1=C, 3=Java, 2=C++}

Explanation:

Map.ofEntries allows creating an immutable map with more than 10 entries using Map.entry for each key-value pair.

Understanding "static" in "public static void main" in Java

Nidhi Sinha

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Initialize HashMap in Java

Different Methods to Initialize a HashMap in Java

1. Initialize HashMap Using Static Block

Example:

Explanation:

2. Initialize HashMap Using Collections

Example:

Explanation:

3. Initialize HashMap Using Java 8 Methods (Using Stream)

Example:

Explanation:

4. Initialize HashMap Using Java 9 Methods

Example Using Map.of():

Explanation:

Example Using Map.ofEntries():

Explanation:

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