Result.java

import java.util.Optional;

public abstract class Result<T,E extends Throwable> {

    public static <T,E extends Throwable> Ok<T,E> ok(T a) { return new Ok(a); }
    public static <T,E extends Throwable> Err<T,E> err(E a) { return new Err(a); }

    public abstract T unwrap() throws E;
    public abstract boolean isOk();
    public abstract boolean isErr();
    public abstract Optional<T> getValue();
    public abstract Optional<E> getError();

    public static final class Ok<T,E extends Throwable> extends Result<T,E> {
        private final T val;
        public Ok(T val) { this.val = val; }
        public T unwrap() throws E { return this.val; }
        public boolean isOk() { return true; }
        public boolean isErr() { return false; }
        public Optional<T> getValue() { return Optional.ofNullable(this.val); }
        public Optional<E> getError() { return Optional.empty(); }
        @Override
        public String toString() { return getClass().getName() + "[" + val + "]"; }

    }

    public static final class Err<T,E extends Throwable> extends Result<T,E> {
        private final E err;
        public Err(E err) { this.err = err; }
        public T unwrap() throws E { throw this.err; }
        public boolean isOk() { return false; }
        public boolean isErr() { return true; }
        public Optional<T> getValue() { return Optional.empty(); }
        public Optional<E> getError() { return Optional.ofNullable(this.err); }
        @Override
        public String toString() { return getClass().getName() + "[" + err + "]"; }
    }
}