public final class Retrofit {
      Retrofit(
      	okhttp3.Call.Factory callFactory,
     	HttpUrl baseUrl,
      	List<Converter.Factory> converterFactories,
      	int defaultConverterFactoriesSize,
      	List<CallAdapter.Factory> callAdapterFactories,
      	int defaultCallAdapterFactoriesSize,
      	@Nullable Executor callbackExecutor,
      	boolean validateEagerly) {
    		this.callFactory = callFactory;
   		 this.baseUrl = baseUrl;
   		 this.converterFactories = converterFactories;  
    	 this.defaultConverterFactoriesSize = defaultConverterFactoriesSize;
   		 this.callAdapterFactories = callAdapterFactories; 
    	 this.defaultCallAdapterFactoriesSize = defaultCallAdapterFactoriesSize;
         this.callbackExecutor = callbackExecutor;
         this.validateEagerly = validateEagerly;
 	 }
      public static final class Builder {
      		 public Builder() {}
             public Builder baseUrl(URL baseUrl) {
      			Objects.requireNonNull(baseUrl, "baseUrl == null");
     		 	return baseUrl(HttpUrl.get(baseUrl.toString()));
    		}
          public Builder addCallAdapterFactory(CallAdapter.Factory factory) {
      			callAdapterFactories.add(Objects.requireNonNull(factory, "factory 					== null"));
      			return this;
    		}
             public Builder addConverterFactory(Converter.Factory factory) 	{
      				converterFactories.add(Objects.requireNonNull(factory, "factory == 						null"));
      				return this;
   			 }
    		public Retrofit build() {
    			if (baseUrl == null) {
       		 		throw new IllegalStateException("Base URL required.");
      			}
      			Platform platform = Platform.get();
     			okhttp3.Call.Factory callFactory = this.callFactory;
      			if (callFactory == null) {
        			callFactory = new OkHttpClient();
     		 	}
      			Executor callbackExecutor = this.callbackExecutor;
      			if (callbackExecutor == null) {
        			callbackExecutor = platform.defaultCallbackExecutor();
      			}
      			converterFactories.add(new BuiltInConverters());
      			converterFactories.addAll(this.converterFactories);
      			converterFactories.addAll(defaultConverterFactories);
     		 return new Retrofit(
          		baseUrl,
          		unmodifiableList(converterFactories),
          		defaultConverterFactoriesSize,
          		unmodifiableList(callAdapterFactories),
          		defaultCallAdapterFactories.size(),
          		callbackExecutor,
          		validateEagerly);
    		}
  		}
     }
}
//Retrofit 對象的創(chuàng)建
 Retrofit retrofit =
        new Retrofit.Builder()
            .baseUrl(API_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build();