How to do function composition?
While rather impatiently waiting for Java 8 release and after reading brilliant 'State of the Lambda' article from Brian Goetz I noticed that function composition was not covered at all.
As per above article, in Java 8 the following should be possible:
// 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;
Now if I would like to compose these two functions to have a function mapping Person
to country, how can I achieve this in Java 8?
Solution 1:
There are default
interface functions Function::andThen
and Function::compose
:
Function<Person, String> toCountry = personToAddress.andThen(addressToCountry);
Solution 2:
There is one flaw in using compose
and andThen
. You have to have explicit variables, so you can't use method references like this:
(Person::getAddress).andThen(Address::getCountry)
It won't be compiled. What a pity!
But you can define an utility function and use it happily:
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)