public class FileDetails
{

    String URL;
    String Port;
    String UserName;
    String Pwd;

    public String getUserName() {
        return UserName;
    }

    public String getPort() {
        return Port;
    }
    public String getPwd() {
        return Pwd;
    }
    public String getURL() {
        return URL;
    }
    public void setPort(String port) {
        this.Port = port;
    }
    public void setPwd(String pwd) {
        this.Pwd = pwd;
    }
    public void setURL(String URL) {
        this.URL = URL;
    }
    public void setUserName(String userName) {
        this.UserName = userName;
    }
}

public class FileService extends FileDetails
{
    public FileService(FileDetails fileDetails)
    {
        FileDetails fileDet = null;

        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.configure(DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        try
        {
            fileDet = objectMapper.readValue(new File("res.json"), fileDetails.class);
        }
        catch(IOException e)
        {
            e.printStackTrace();
        }
        if (fileDet != null) {
            fileDetails.setUserName(fileDet.getUserName());
            fileDetails.setPwd(fileDet.getPwd());
            fileDetails.setURL(fileDet.getURL());
            fileDetails.setPort(fileDet.getPort());
        } else {
            System.out.println("Didn't set details");
        }
    }
}

FileDetails fileDetails = new FileDetails();
FileService fileService = new FileService(fileDetails);

public class FileDetails {
    private String url;
    private String port;
    private String userName;
    private String password;

    public FileDetails(String url, String port, String userName, String password) {
         this.url = url;
         this.port = port;
         this.userName = userName;
         this.password = password;
    }

    public FileDetails(Path detailsFile) {
         try (InputStream in = Files.newInputStream(detailsFile, StandardOpenOption.READ)) {
             ObjectMapper mapper = new ObjectMapper();
             mapper.configure(DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);
             FileDetails details = mapper.readValue(in, FileDetails.class);

            this.url = details.url;
            this.port = details.port;
            this.userName = details.userName;
            this.password = details.password;
        } catch (IOException e) {
            e.printStackTrace(System.err);
        }
    }
    // getters and setters omitted
}