java - 如何强制实例化静态字段

Question

我对以下代码的输出感到非常惊讶:

国家级

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);
        }
    }

}

主要方法

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

输出

null

是否有任何干净的方法强制加载扩展 Country 的类，以便国家 map 包含所有 Country 实例？

Answer 1

是的，使用 static initializer block :

public class Country {

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

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

    }

...