Either

Either.java

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

public final class Either<Left, Right> {

    public static <L, R> Either<L, R> ofLeft(L left) {
        return new Either<>(Objects.requireNonNull(left), null);
    }

    public static <L, R> Either<L, R> ofRight(R right) {
        return new Either<>(null, Objects.requireNonNull(right));
    }

    private final Left left;
    private final Right right;

    private Either(Left left, Right right) {
        this.left = left;
        this.right = right;
    }

    public void accept(Consumer<? super Left> left, Consumer<? super Right> right) {
        if (isLeft()) {
            left.accept(this.left);
        } else {
            right.accept(this.right);
        }
    }

    public <Result> Result reduce(Function<? super Left, Result> left, Function<? super Right, Result> right) {
        return isLeft() ? left.apply(this.left) : right.apply(this.right);
    }

    public <X, Y> Either<X, Y> map(Function<? super Left, X> left, Function<? super Right, Y> right) {
        return isLeft() ? ofLeft(left.apply(this.left)) : ofRight(right.apply(this.right));
    }

    public Optional<Left> getLeft() {
        return isLeft() ? Optional.of(left) : Optional.empty();
    }

    public Optional<Right> getRight() {
        return isRight() ? Optional.of(right) : Optional.empty();
    }

    public boolean isLeft() {
        return left != null;
    }

    public boolean isRight() {
        return right != null;
    }

    @Override
    public String toString() {
        return isLeft() ? String.format("Left[%s]", left) : String.format("Right[%s]", right);
    }
}