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：

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

空

是否有任何干净的方法可以强制加载扩展 Country 的类，以便国家地图包含所有 Country 实例？

Answer 1

是的，使用静态初始化块：

public class Country {

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

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

    }

...

Answer 2

在您调用 Country.getCountry("Spain") 时，您的类 EuropeCountry 尚未加载。正确的解决方案是

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) {}
}

这只是一个例子...还有其他方法可以实现相同的目的（另请参阅彼得的回答）

Answer 3

您需要加载 EuropeCountry 类。在致电国家之前提及它就足够了。