VersionControlMapSystem.java

import java.util.HashMap;
import java.util.Map;

public class VersionControlMapSystem {

    private static Integer currentVersion = null;
    private static VersionControlMapSystem versionControlMapSystem;
    private Map<Integer, Map<String, String>> controlMap = new HashMap<>();

    private VersionControlMapSystem(){
        System.out.println("Initializing Version Control Map System");
    }

    public static VersionControlMapSystem getInstance(){
        if(versionControlMapSystem==null) {
            versionControlMapSystem = new VersionControlMapSystem();
        }
        return versionControlMapSystem;
    }

    public void put(String key, String value){
        if(currentVersion==null) {
            currentVersion = 0;
            Map<String, String> valueMap = new HashMap<>();
            controlMap.put(currentVersion, valueMap);
        }
        controlMap.get(currentVersion).put(key, value);
    }

    public String get(String key){
      return controlMap.get(currentVersion).get(key);
    }

    public int getCurrentVersion(){
        return currentVersion;
    }

    public void snapshot(){
         int oldVersion = currentVersion;
         currentVersion++;

         //copy the valueMap to the new version
        Map<String, String> old_map = controlMap.get(oldVersion);
        Map<String, String> newMap = new HashMap<>();
        for(String key: old_map.keySet())
            newMap.put(key, old_map.get(key));
        controlMap.put(currentVersion, newMap);
    }

    public String getValVersion(int version, String key){
        if(controlMap.containsKey(version)){
            return controlMap.get(version).get(key);
        } else {
            System.out.println("Invalid version");
            return null;
        }
    }

    public static void main(String[] args) {
        VersionControlMapSystem vcm = VersionControlMapSystem.getInstance();
        vcm.put("memory", "16GB");
        vcm.put("os_version", "10");
        vcm.put("graphics", "intel_10_153");
        vcm.snapshot();
        vcm.put("os_version", "11");
        vcm.put("graphics", "intel_11_153");
        System.out.println("graphics in current version: " +  vcm.get("graphics"));
        System.out.println("Current Control Map version is : " +  vcm.getCurrentVersion());
        System.out.println("graphics in version 0 : " +  vcm.getValVersion(0, "graphics"));
        System.out.println("os_version in current version: " +  vcm.get("os_version"));
    }
}