retrofit/src/main/java/retrofit2/Retrofit.java
public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
    return nextCallAdapter(null, returnType, annotations);
  }
public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
    return nextCallAdapter(null, returnType, annotations);
  }
Returns the {@link CallAdapter} for {@code returnType} from the available {@linkplain
#callAdapterFactories() factories}.

@throws IllegalArgumentException if no call adapter available for {@code type}.