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How to directly initialize a HashMap in a literal way in Java

How to directly initialize a HashMap in a literal way in Java

Abdul D.

—

The Problem

How do I directly initialize a HashMap in a literal way in Java?

The Solution

Unlike some other programming languages, Java does not have built-in syntax for initializing a HashMap in a literal way.

For practical purposes, a HashMap is generally initialized with a constructor and then populated using put():

Click to Copy
Map<String, Integer> map = new HashMap<>();
map.put("Mercury", 1);
map.put("Venus", 2);
map.put("Earth", 3);
map.put("Mars", 4);

Another common way to initialize a HashMap is to use an anonymous class and a HashMap instance initializer block:

Click to Copy
import java.util.HashMap;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>() {{
            put("Mercury", 1);
            put("Venus", 2);
            put("Earth", 3);
            put("Mars", 4);
        }};
    }
}

Note: This approach uses an anonymous inner class, which can lead to subtle memory leaks due to the reference to the enclosing instance. It is generally not recommended for production code but is fine for testing.

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