Code of "Why Java Collections Framework is a Cats & Dogs Library"

Test.java

//
// depends on javaslang-2.1.0.jar
//

import javaslang.collection.*;
import java.util.List;
import java.util.ArrayList;

import static javaslang.API.*;

public class Test {

    public static void main(String[] args) {
        new Test() {{
            immutableShowcase();
            mutableShowcase();
        }};
    }

    void immutableShowcase() {

        Seq<Dog> dogs = Vector(new Dog());

        // Type narrowing, pets = dogs does not compile!
        Seq<Pet> pets = Seq.narrow(dogs);

        // Vector(Dog, Cat)
        println(pets.append(new Cat()));

        // Vector(Dog)
        println(pets);

        // Vector(Dog)
        println(dogs);

    }

    void mutableShowcase() {

        List<Dog> dogs = new ArrayList<>();
        dogs.add(new Dog());

        // Type narrowing, pets = dogs does not compile!
        List<Pet> pets = narrow(dogs);

        // Heap pollution!
        pets.add(new Cat());

        // [Dog, Cat]
        println(pets);

        // Huh!? [Dog, Cat]
        println(dogs);

    }

    @SuppressWarnings("unchecked")
    static <T> java.util.List<T> narrow(java.util.List<? extends T> list) {
        return (java.util.List<T>) list;
    }

}

interface Pet {}

class Cat implements Pet {
    @Override public String toString() { return "Cat"; }
}

class Dog implements Pet {
    @Override public String toString() { return "Dog"; }
}