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jbgi/Program.java

Created October 15, 2017 06:54
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Using existentials to implement abstract type alias in Java
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Program.java
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
interface Person {
int age();
}
interface People<T> {
T fromList(List<Person> ps);
List<Integer> getAges(T people);
People<?> module = new People<List<Person>>() {
public List<Person> fromList(List<Person> ps) {
return ps;
}
public List<Integer> getAges(List<Person> people) {
return people.stream().map(Person::age).collect(Collectors.toList());
}
};
}
// run with:
// javac Program.java
// java -cp . Program
public class Program<T> {
People<T> P;
Program(People<T> P) {this.P = P;}
void run() {
T people = P.fromList(Arrays.asList(() -> 21, () -> 37));
System.out.println(P.getAges(people)); // print "[21, 37]"
}
public static void main(String[] args) {
new Program<>(People.module).run();
}
}
@jbgi
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jbgi commented Oct 15, 2017

but the cast-free alternative is only one more method call:

public class Program {
  public static void main(String[] args) {
    run(People.module);
  }
  static <PeopleT> void run(People<PeopleT> module) {
    PeopleT people = module.fromList(Arrays.asList(() -> 21, () -> 37));
    System.out.println(module.getAges(people)); // print "[21, 37]"
  }
}

so I personally would not do it.

@yawaramin
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Ah, true. The run method is indeed more elegant.

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