public class ServerStorage extends HashMap<ServerInfo, ServerType> {
    private final List<ServerInfo> a = new ArrayList<>();
    private final List<ServerInfo> b = new ArrayList<>();

    @Override
    public ServerType put(ServerInfo key, ServerType value) {
        switch (value) {
            case A: {
                a.add(key);
                break;
            }
            case B:
                b.add(key);
                break;
        }
        return super.put(key, value);
    }

    public List<ServerInfo> getAServers() {
        return a;
    }

    public List<ServerInfo> getBServers() {
        return b;
    }