example

public class DataHandler {

	private static final Path PATH = Paths.get(System.getProperty("user.home"), "Desktop", "data.txt");

	public static void main(String... args) {
		Person person = new Person("Male", "Sneaky", "Mr", "100");

		try {
			storeData(person);
		} catch (IllegalArgumentException | IllegalAccessException | IOException e) {
			// TODO something relative to the exception at hand.
			e.printStackTrace();
		}
	}

    public static void storeData(Person user) throws IOException, IllegalArgumentException, IllegalAccessException {
    	File file = PATH.toFile();

    	if (!file.exists()) {
        	file.createNewFile();
        }
    	List<String> appendedLines = Arrays.asList(user.serialize());

        Files.write(PATH, appendedLines, StandardOpenOption.APPEND);
    }

    private static class Person {

    	String gender, name, surname, age;

    	private Person(String gender, String name, String surname, String age) {
    		this.gender = gender;
    		this.name = name;
    		this.surname = surname;
    		this.age = age;
    	}

    	public String serialize() throws IllegalArgumentException, IllegalAccessException {
    		Field[] fields = getClass().getDeclaredFields();

    		StringBuilder output = new StringBuilder();

    		for (Field field : fields) {
    			Object value = field.get(this);

    			output.append(field.getName()).append("=").append(value).append("\n");
    		}
    		return output.toString();
    	}

    }

}