Effective Java--第1条静态工厂方法代替构造方法

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
public class Services {
    private Services(){
    }
    // Map Service Name to Services 
    public static final Map<String,Provider> providers =  new ConcurrentHashMap<String, Provider>();
    public static final String DEFAULT_PROVIDER_NAME = "<def>";
    // Provider Register API 
    public static void registerDefaultProvider(Provider p){
        registerProvider(DEFAULT_PROVIDER_NAME,p);
    }
    public static void registerProvider(String defaultProviderName, Provider p) {
        providers.put(defaultProviderName, p);
    }
    public static Service newInstance(){
        return newInstance(DEFAULT_PROVIDER_NAME);
    }
    public static Service newInstance(String name){
        Provider p = providers.get(name);
        if(p == null){
            throw new IllegalArgumentException("No provider registered with name: " + name);
        }
        return p.newService();
    }
}

public class Test {
  /**
   * 静态工厂方法主要缺点，类如果不含公有的或者受保护的构造器，就不能子类话。
   * 静态工厂的第二个缺点在于，他们与其他静态方法实际上没有其他区别。
   * 
   * @param args
   */
    public static void main(String[] args) {
        // Providers would execute these lines
        Services.registerDefaultProvider(DEFAULT_PROVIDER);
        Services.registerProvider("comp", COMP_PROVIDER);
        Services.registerProvider("armed", ARMED_PROVIDER);
        // Clients would execute these lines
        Service s1 = Services.newInstance();
        Service s2 = Services.newInstance("comp");
        Service s3 = Services.newInstance("armed");
        System.out.printf("%s, %s, %s%n", s1, s2, s3);
    }
    private static Provider DEFAULT_PROVIDER = new Provider() {
        public Service newService() {
            return new Service() {
                @Override
                public String toString() {
                    return "Default service";
                }
            };
        }
    };
    private static Provider COMP_PROVIDER = new Provider() {
        public Service newService() {
            return new Service() {
                @Override
                public String toString() {
                    return "Complementary service";
                }
            };
        }
    };
    private static Provider ARMED_PROVIDER = new Provider() {
        public Service newService() {
            return new Service() {
                @Override
                public String toString() {
                    return "Armed service";
                }
            };
        }
    };
}