2020-05-exceptions-25.java

import java.util.Optional;
import java.util.function.Consumer;
import java.util.function.Function;

public class Either<L, R> {

    public static <L, R> Either<L, R> left(L value) {
        return new Either<>(value, null);
    }

    public static <L, R> Either<L, R> right(R value) {
        return new Either<>(null, value);
    }

    private final Optional<L> left;
    private final Optional<R> right;

    private Either(L left, R right) {
        this.left = Optional.ofNullable(left);
        this.right = Optional.ofNullable(right);
    }

    private Either(Optional<L> left, Optional<R> right) {
        this.left = left;
        this.right = right;
    }

    public <T> T map(Function<? super L, ? extends T> mapLeft, //
                     Function<? super R, ? extends T> mapRight) {

        return this.left.<T> map(mapLeft) //
                        .orElseGet(() -> this.right.map(mapRight).get());
    }

    public <T> Either<T, R> mapLeft(Function<? super L, ? extends T> leftFn) {
        return new Either<>(this.left.map(leftFn), //
                            this.right);
    }

    public <T> Either<L, T> mapRight(Function<? super R, ? extends T> rightFn) {
        return new Either<>(this.left, //
                            this.right.map(rightFn));
    }

    public void apply(Consumer<? super L> leftFn, Consumer<? super R> rightFn) {
        this.left.ifPresent(leftFn);
        this.right.ifPresent(rightFn);
    }
}