Temperature conversions written in Java

Converter.java

package com.thomaskuenneth.temperatureconverter;

import java.text.DecimalFormat;
import java.text.NumberFormat;
import java.text.ParseException;

public class Converter {

    public static final String DEGREES_CELSIUS = "\u00B0C";
    public static final String DEGREES_FAHRENHEIT = "\u00B0F";
    public static final String KELVIN = "K";

    public static double stringToDouble(String s) throws ParseException {
        NumberFormat df = DecimalFormat.getInstance();
        Number n = df.parse(s);
        return n.doubleValue();
    }

    public static double celsiusToKelvin(double celsius) {
        return 273.15 + celsius;
    }

    public static double fahrenheitToKelvin(double fahrenheit) {
        return (fahrenheit + 459.67) * 5f / 9f;
    }

    public static double kelvinToCelsius(double kelvin) {
        return kelvin - 273.15;
    }

    public static double kelvinToFahrenheit(double kelvin) {
        return kelvin * 1.8 - 459.67;
    }

    public static double celsiusToFahrenheit(double celsius) {
        return (celsius * 1.8) + 32.0;
    }

    public static double fahrenheitToCelsius(double fahrenheit) {
        return (fahrenheit - 32) * (5.0 / 9.0);
    }

    public static String celsiusToString(double celsius) {
        return doubleToString(celsius, DEGREES_CELSIUS);
    }

    public static String fahrenheitToString(double fahrenheit) {
        return doubleToString(fahrenheit, DEGREES_FAHRENHEIT);
    }

    public static String kelvinToString(double kelvin) {
        return doubleToString(kelvin, KELVIN);
    }

    private static String doubleToString(double val, String suffix) {
        return String.format("%s %s",
                (val == (long) val)
                        ? String.format("%d", (long) val)
                        : String.format("%.2f", val), suffix);
    }
}