怎么做功能组合？

Question

虽然不耐烦地等待Java 8发布,在阅读Brian Goetz的精彩"Lambda状态"文章之后,我注意到功能组合根本没有被覆盖.

根据上面的文章,在Java 8中应该可以:

// having classes Address and Person
public class Address {

    private String country;

    public String getCountry() {
        return country;
    }
}

public class Person {

    private Address address;

    public Address getAddress() {
        return address;
    }
}

// we should be able to reference their methods like
Function<Person, Address> personToAddress = Person::getAddress;
Function<Address, String> addressToCountry = Address::getCountry;

现在,如果我想将这两个函数组合成一个函数映射Person到country,我怎样才能在Java 8中实现这一点？

Answer 1

有一个默认的接口函数Function::andThen和Function::compose:

Function<Person, String> toCountry = personToAddress.andThen(addressToCountry);

Answer 2

使用compose和存在一个缺陷andThen.你必须有显式变量,所以你不能使用这样的方法引用:

(Person::getAddress).andThen(Address::getCountry)

它不会被编译.太遗憾了!

但是你可以定义一个实用程序函数并愉快地使用它:

public static <A, B, C> Function<A, C> compose(Function<A, B> f1, Function<B, C> f2) {
        return f1.andThen(f2);
    }

compose(Person::getAddress, Address::getCountry)