Keeping insertion order

import java.util.LinkedHashMap;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> missions = new LinkedHashMap<>();
        missions.put("Mercury", 1959);
        missions.put("Gemini", 1964);
        missions.put("Mercury", 1961);
        missions.putIfAbsent("Apollo", 1967);
        missions.remove("Gemini");

        for (var entry : missions.entrySet()) {
            System.out.println(entry.getKey() + " -> " + entry.getValue());
        }
    }
}