Error Handling With Try and Either

When programming in Java you can no longer run away from handling errors and exceptions in your code. Throwing and catching exceptions is a “glorified goto” and also hidden performance costs (why is creating a Throwable so expensive? It’s because usally it will call Throwable.fillInStackTrace()).

Let’s first have a look at this approach:

public class Customer {
    private final int age;
    public int getAge() {
        return age;
    }
}
public class Alcoholic {
    private final int alcoholConcentration;
    public int getAlcoholConcentration() {
        return alcoholConcentration;
    }
}
public class UnderageException extends Exception {
    public UnderageException(String message) {
        super(message);
    }
}
Alcoholic buyAlcoholic(Customer customer) throws UnderageException {
    if (customer.getAge() < 18) {
        throw new UnderageException("Age: " + customer.getAge());
    }
    return new Alcoholic(5);
}

try {
    Customer tooYoung = new Customer(16);
    Alcoholic alcoholic = buyAlcoholic(tooYoung);
} catch (UnderageException ex) {
    // Troubles
}

Nasty, isn’t it? And doesn’t really go well with functional programming (or other situation like concurrency).

The functional way

In Scala, there is a specific type that represents computations that may result in an exception: Try (there is also a similar type, called Either, but we’ll cover it later), is there a way to get it in Java also?

VAVR to the rescue!

Try is a monadic container type which represents a computation that may either result in an exception, or return a successfully computed value. Instances of Try, are either an instance of Success or Failure.

An instance of Success<A> simply wraps a value of type A, an instance of Failure<A> wraps a Throwable (i.e. an exception or other kind of error).

Try<Alcoholic> alcoholic = Try.of(() -> buyAlcoholic(new Customer(16)));

This beauty returns a value of type Try<Alcoholic>. If the given Customer has a legal age, this will be a Success<Alcoholic>, otherwise the buyAlcoholic method throws a UnderageException, however, it will be a Failure<Alcoholic>.

Working with Try instances is actually very similar to working with Option values: you can check if a Try is a success by calling isSuccess and then retrieve the wrapped value with get, but there aren’t many situations where you will want to do that.

Normally you want to return a default value (using orElse* or getOrElse* methods) or transform a failure into a success (using recover* or recoverWith* methods).

Chain operations

Like Option, Try supports all the higher-order methods you know from other monadic containers:

Try<Integer> concentration = alcoholic.map(Alcoholic::getAlcoholConcentration);

Mapping a Try<Alcoholic> that is a Success<Alcoholic> to a Try<Integer> results in a Success<Integer>. If it’s a Failure<Alcoholic>, the resulting Try<Integer> will be a Failure<Integer>, on the other hand, containing the same exception as the Failure<Alcoholic>.

Filter and foreach

It is also passible to filter a Try or call foreach on it.

The filter method returns a Failure if the Try on which it is called is already a Failure or if the predicate passed to it returns false (in which case the wrapped exception is a NoSuchElementException). The function passed to foreach is executed only if the Try is a Success, which allows you to execute a side-effect.

Pattern Matching

If you want to know whether a Try instance you have received as the result of some computation represents a success or not and execute different code branches depending on the result you make use of pattern matching:

String result = Match(concentration).of(
        Case($Success($()), c -> "drinked " + c),
        Case($Failure($()), t -> "troubles"));

Recovering

If recover is called on a Success instance, that instance is returned as is. Otherwise, if the partial function is defined for the given Failure instance, its result is returned as a Success.

Try<Integer> recovered = concentration
    .recover(th -> Match(th).of(
        Case($(instanceOf(UnderageException.class)), () -> 0),
        Case($(), () -> -1)));

Try conclusion

The Try type allows to encapsulate computations that result in errors in a container and to chain operations on the computed values in a very elegant way.

In the next part we are going to deal with Either, an alternative type for representing computations that may result in errors, but with a wider scope of application that goes beyond error handling.