我可以结合使用Java8（流，lambda）和番石榴吗？

Question

When dealing with transformations(eg: transform List<People> people to List<Integer> ages , where People is a class that contains a property age ), normally there are 2 ways (for me) of doing this:

1. use java8:

   people.stream().map(p -> p.getAge()).collect(toList()) ;

2. or user guava2:

   Lists2.transform(people, People2AgeTransformer.INSTANCE); where the People2AgeTransformer is a transformer that implements the Function interface to return the age.

Recently, i accidentlly found that java8 and guava can be combined and therefore the code can be :

Lists2.transform(people, p->p.getAge());

this piece of code complies and runs OK without any error, which get me confused. The method Lists2.transform() requires the 2nd args to be the implementation of interface com.google.common.base.Function , while the java8 lambda is actually the implementation of interface java.util.function.Function .(Well, they both declares a method B apply(A input) to transform A to B.)

I don't understand why this would work since they are two different interface of different package.

Answer 1

the java8 lambda is actually the implementation of interface java.util.function.Function

No it's not. p -> p.getAge() does not have a predefined type - its type is inferred at compile time depending on the context in which it's called. Any functional interface that has a int/Integer getAge(Person p) method will work.

So both java.util.Function<Person, Integer> and com.google.common.base.Function<Person, Integer> are compatible.

Answer 2

If you have a function assigned to variable or passed via parameter, it's really easy to adapt them via Java-8 method references:

java.util.function.Function<Person, Integer> javaFunction = p -> p.getAge();
com.google.common.base.Function<Person, Integer> guavaFunction = javaFunction::apply;
java.util.function.Function<Person, Integer> javaFunction2 = guavaFunction::apply;

So if you already have Java 8 Function object in variable fn and need to pass it into some Guava code, just use fn::apply . If you have Java 8 Predicate in variable pred , use pred::test when passing it into Guava code. Similar method references could be used for other functional interfaces.