JavaでOption

JavaOption.java

interface Function1<T, U> {
    U apply(T t);
}

interface Optional<T> {
    T get();
    T getOrElse(T defaultValue);
    boolean isDefined();
    <U> Optional<U> map(Function1<T, U> f);
}

class Option<T> {

    public static <T> Optional<T> apply(T value) {
        if (value == null) {
            return new None<T>();
        } else {
            return new Some<T>(value);
        }
    }

}

class Some<T> implements Optional<T> {

    private T value;

    public Some(T value) {
        this.value = value;
    }

    public T get() {
        return value;
    }

    public T getOrElse(T defaultValue) {
        return value;
    }

    public boolean isDefined() {
        return true;
    }

    public <U> Optional<U> map(Function1<T, U> f) {
        return Option.apply(f.apply(value));
    }

}


class None<T> implements Optional<T> {

    public None() {
    }

    public T get() {
        throw new java.util.NoSuchElementException();
    }

    public T getOrElse(T defaultValue) {
        return defaultValue;
    }

    public boolean isDefined() {
        return false;
    }

    public <U> Optional<U> map(Function1<T, U> f) {
        return new None<U>();
    }

}


public class Main {
    public static void main(String[] args) {
        Optional<String> s1 = Option.apply("foo");
        Optional<String> s2 = Option.apply(null);

        System.out.println(s1.getOrElse("bar"));
        System.out.println(s2.getOrElse("bar"));

        Optional<String> s3 = Option.apply("3");
        Optional<Integer> i3 =
            s3.map(
                new Function1<String, Integer>() {
                    public Integer apply(String s) {
                        return Integer.parseInt(s);
                    }
                }
            ).map(
                new Function1<Integer, Integer>() {
                    public Integer apply(Integer i) {
                        return i * 100;
                    }
                }
            );

        System.out.println(i3.get());

    }
}