Item 1----------考虑用静态工厂方法代替构造器

读书，有时候，我感觉总是有点绕和不具体。我阅读了代码，理解代码后，才有一种理解和把握的感觉。

优点三、

把某个对象的构建放给客户端来实现。

比如下面的实现，客户端Test，获取Service的实例具体类型，是由服务提供者Provider来确定的。

Services暴露服务提供者的注册方法，不同的服务提供者，会提供不同的实例。如下文，实现了DEFAULT_PROVIDER, COMP_PROVIDER, ARMED_PROVIDER三个服务提供者。

将服务提供者注册到Services中，用名称区别不同的服务提供者。

于是，客户端Test，在获取实例的时候，传递对应的服务提供者名称给Services，这样就可以获取指定类型的实例。

比如，Service s2 = Services.nexInstance("comp").获取的实例类型，是由COMP_PROVIDER来决定的。

// Simple test program for service provider framework
package org.effectivejava.examples.chapter02.item01;

public class Test {
        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" ;
                           }
                     };
              }
       };
}

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import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class Services {
        private Services() {
       } // Prevents instantiation (Item 4)

        // Maps service names to services
        private static final Map<String, Provider> providers = new ConcurrentHashMap<String, Provider>();
        public static final String DEFAULT_PROVIDER_NAME = "<def>";

        // Provider registration API
        public static void registerDefaultProvider(Provider p) {
               registerProvider(DEFAULT_PROVIDER_NAME, p);
       }

        public static void registerProvider(String name, Provider p) {
               providers.put(name, p);
       }

        // Service access API
        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();
       }
}

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//在接口中，可以定义要客户端实现的方法
public interface Service {
        // Service-specific methods go here
}

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// Service provider framework sketch - Service provider interface - Page 12
package org.effectivejava.examples.chapter02.item01;

public interface Provider {
       Service newService();
}