Java 8 Function类addThen默认方法

Question

我正在学习 Java-8 Lambda，我试图了解 java.util.function.Function 接口(interface)中的 addThen 默认方法。根据我的理解，addthen 将首先执行第一个函数，然后执行第二个方法。 所以我创建了一个如下所示的程序:

//Pojo class 
  class Bike {

    public Bike(String bikeName, int price, String bikeType) {
        this.bikeName = bikeName;
        this.price = price;
        this.bikeType = bikeType;
    }

    private String bikeType;
    private String bikeName;
    private int price;

    public String getBikeType() {
        return bikeType;
    }

    @Override
    public String toString() {
        return "Bike [bikeType=" + bikeType + ", bikeName=" + bikeName + ", price=" + price + "]";
    }

    public void setBikeType(String bikeType) {
        this.bikeType = bikeType;
    }

    public String getBikeName() {
        return bikeName;
    }

    public void setBikeName(String bikeName) {
        this.bikeName = bikeName;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }

}

//Main class
public class FunctionInbuildDefaultMethodsExample {

    public static void main(String[] args) {
        learningAndThen();
    }

    static void learningAndThen() {

        Function<Bike, String> updateBikefunction = (Bike bike) -> {
            System.out.println("OldBike Name is::" + bike.getBikeName());
            bike.setBikeName("PULSOR-200CC");
            return bike.getBikeName();
        };

        Function<Bike, String> updateBikePriceFunction = (Bike bike) ->  {

            System.out.println("OldBike Price is::" + bike.getPrice());
            bike.setPrice(95000);
            return bike.getBikeName();

        };

        /*
         * First update Bike and then price
         * */
         /*Compilation error here*/
        Function<Bike,String> bikeFunction = updateBikefunction.andThen(updateBikePriceFunction);
        bikeFunction.apply( new Bike("PULSOR-125CC", 65000, "BAJAJ"));

    }
 }

我在该行遇到编译错误

 Function<Bike,String> bikeFunction =
updateBikefunction.andThen(updateBikePriceFunction);

作为

"The method andThen(Function) in the type Function is not applicable for the arguments (Function)"

, 看了 Function 接口(interface)的源码，明白了 addThen 默认方法正在寻找 Function<String,Book> 类型的实例.我的问题是，如果addThen默认方法应该执行第一个函数，然后执行作为参数传递的下一个函数，为什么 addThen函数接口(interface)的默认方法是这样写的，除了Function<String,Book>类型的实例.

Answer 1

andThen 应用于该 Function 的结果。所以这会起作用，例如:

Function<Bike, String> bikeFunction = updateBikefunction
                                       .andThen(s -> s.toUpperCase());

因为结果是 Function 的 String。