How to force instantiation of static fields

Question

I was quite surprised of the output of the following code:

Country class

public class Country {

    private static Map<String, Country> countries = new HashMap<String, Country>();

    private final String name;

    @SuppressWarnings("LeakingThisInConstructor")
    protected Country(String name) {
        this.name = name;
        register(this);
    }

    /** Get country by name */
    public static Country getCountry(String name) {
        return countries.get(name);
    }

    /** Register country into map */
    public static void register(Country country) {
        countries.put(country.name, country);
    }

    @Override
    public String toString() {
        return name;
    }

    /** Countries in Europe */
    public static class EuropeCountry extends Country {

        public static final EuropeCountry SPAIN = new EuropeCountry("Spain");
        public static final EuropeCountry FRANCE = new EuropeCountry("France");

        protected EuropeCountry(String name) {
            super(name);
        }
    }

}

Main method

System.out.println(Country.getCountry("Spain"));

Output

null

Is there any clean way of forcing the class that extend Country to be loaded so the countries map contains all the Country instances?

Answer 1

Yes, use static initializer block:

public class Country {

    private static Map<String, Country> countries = new HashMap<String, Country>();

    static {
        countries.put("Spain", new EuroCountry("Spain"));

    }

...

Answer 2

Your class EuropeCountry was not loaded at the time you called Country.getCountry("Spain"). The correct solution would be

private static Map<String, Country> countries = new HashMap<String, Country>();

static {
    // Do something to load the subclass
    try {
        Class.forName(EuropeCountry.class.getName());
    } catch (Exception ignore) {}
}

This is just an example... There are other ways to achieve the same (see also Peter's answer)