public class NewClass {
    
    private static final Map<String, String> myMap
            = new HashMap<>();

    static {myMap.put("1", "One");
    }
    
    public static void main(String[] args) {
        myMap.put("2", "Two");
        System.out.println(myMap.get("1"));
        System.out.println(myMap.get("2"));
    }

}

java.util.Collections#unmodifiableMap