@GET("hello/world")
Call<News> getNews(@Query("num") String num,@Query("page")String page);

@GET("book/search")
 Observable<Book> getSearchBook(@Query("q") String name,
                  @Query("tag") String tag, @Query("start") int start,
                  @Query("count") int count);

return serviceMethod.adapt(okHttpCall);

T adapt(Call<R> call) {
  return callAdapter.adapt(call);
 }

 ServiceMethod(Builder<R, T> builder) {
   //………………
  this.callAdapter = builder.callAdapter;
   //………………
  }

 public ServiceMethod build() {
   callAdapter = createCallAdapter();
   //…………
   return new ServiceMethod<>(this);
   }

 private CallAdapter<T, R> createCallAdapter() {
   Type returnType = method.getGenericReturnType();
   if (Utils.hasUnresolvableType(returnType)) {
    throw methodError(
      "Method return type must not include a type variable or wildcard: %s", returnType);
   }
   if (returnType == void.class) {
    throw methodError("Service methods cannot return void.");
   }
   Annotation[] annotations = method.getAnnotations();
   try {
    //noinspection unchecked
    return (CallAdapter<T, R>) retrofit.callAdapter(returnType, annotations);
   } catch (RuntimeException e) { // Wide exception range because factories are user code.
    throw methodError(e, "Unable to create call adapter for %s", returnType);
   }
  }

 public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
  return nextCallAdapter(null, returnType, annotations);
 }

public CallAdapter<?, ?> nextCallAdapter(@Nullable CallAdapter.Factory skipPast, Type returnType,
   Annotation[] annotations) {
  checkNotNull(returnType, "returnType == null");
  checkNotNull(annotations, "annotations == null");

  int start = callAdapterFactories.indexOf(skipPast) + 1;
  for (int i = start, count = callAdapterFactories.size(); i < count; i++) {
   CallAdapter<?, ?> adapter = callAdapterFactories.get(i).get(returnType, annotations, this);
   if (adapter != null) {
    return adapter;
   }
  }
  //省略一些不重要代碼
 }

 public Retrofit build() {
   //省略部分代碼
   Executor callbackExecutor = this.callbackExecutor;
   if (callbackExecutor == null) {
    callbackExecutor = platform.defaultCallbackExecutor();
   }

   // Make a defensive copy of the adapters and add the default Call adapter.
   List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);   
   callAdapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
   //省略部分代碼
   return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),
     unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);
 }

@Nullable Executor defaultCallbackExecutor() {
  return null;
 }

 static class Android extends Platform {
  @Override public Executor defaultCallbackExecutor() {
   return new MainThreadExecutor();
  }

  @Override CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
   if (callbackExecutor == null) throw new AssertionError();
   return new ExecutorCallAdapterFactory(callbackExecutor);
  }

  static class MainThreadExecutor implements Executor {
   private final Handler handler = new Handler(Looper.getMainLooper());

   @Override public void execute(Runnable r) {
    handler.post(r);
   }
  }
 }

 CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
  if (callbackExecutor != null) {
   return new ExecutorCallAdapterFactory(callbackExecutor);
  }
  return DefaultCallAdapterFactory.INSTANCE;
 }

 @Override
 public CallAdapter<?, ?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
  if (getRawType(returnType) != Call.class) {
   return null;
  }
  final Type responseType = Utils.getCallResponseType(returnType);
  return new CallAdapter<Object, Call<?>>() {
   @Override public Type responseType() {
    return responseType;
   }

   @Override public Call<Object> adapt(Call<Object> call) {
    return new ExecutorCallbackCall<>(callbackExecutor, call);
   }
  };
 }

 static final class ExecutorCallbackCall<T> implements Call<T> {
  final Executor callbackExecutor;
  final Call<T> delegate;

  ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
   this.callbackExecutor = callbackExecutor;
   this.delegate = delegate;
  }

  @Override public void enqueue(final Callback<T> callback) {
   checkNotNull(callback, "callback == null");

   delegate.enqueue(new Callback<T>() {
    @Override public void onResponse(Call<T> call, final Response<T> response) {
     callbackExecutor.execute(new Runnable() {
      @Override public void run() {
       if (delegate.isCanceled()) {
        // Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.
        callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
       } else {
        callback.onResponse(ExecutorCallbackCall.this, response);
       }
      }
     });
    }

    @Override public void onFailure(Call<T> call, final Throwable t) {
     callbackExecutor.execute(new Runnable() {
      @Override public void run() {
       callback.onFailure(ExecutorCallbackCall.this, t);
      }
     });
    }
   });
  }

  @Override public boolean isExecuted() {
   return delegate.isExecuted();
  }

  @Override public Response<T> execute() throws IOException {
   return delegate.execute();
  }

  @Override public void cancel() {
   delegate.cancel();
  }

  @Override public boolean isCanceled() {
   return delegate.isCanceled();
  }

  @SuppressWarnings("CloneDoesntCallSuperClone") // Performing deep clone.
  @Override public Call<T> clone() {
   return new ExecutorCallbackCall<>(callbackExecutor, delegate.clone());
  }

  @Override public Request request() {
   return delegate.request();
  }
 }