Try monad in Java 8

Question

Is there a built-in support for monad that deals with exception handling? Something similar to Scala's Try. I am asking because I don't like unchecked exceptions.

Answer 1

There are at least two generally available (e.g. on Maven Central) - Vavr and Cyclops both have Try implementations that take a slightly differing approach.

Vavr's Try follows Scala's Try very closely. It will catch all 'non-fatal' exceptions thrown during the execution of it's combinators.

Cyclops Try will only catch explicitly configured exceptions (of course you can, by default, also have it catch everything), and the default mode of operating is only to catch during the initial population method. The reasoning behind this is so that Try behaves in a somewhat similar way to Optional - Optional doesn't encapsulate unexpected Null values (i.e. bugs), just places where we reasonable expect to have no value.

Here is an example Try With Resources from Cyclops

 Try t2 = Try.catchExceptions(FileNotFoundException.class,IOException.class)
               .init(()->PowerTuples.tuple(new BufferedReader(new FileReader("file.txt")),new FileReader("hello")))
               .tryWithResources(this::read2);

And another example 'lifting' an existing method (that might divide by zero) to support error handling.

    import static org.hamcrest.Matchers.equalTo;
    import static org.junit.Assert.*;
    import static com.aol.cyclops.lambda.api.AsAnyM.anyM;
    import lombok.val;

    val divide = Monads.liftM2(this::divide);

    AnyM<Integer> result = divide.apply(anyM(Try.of(2, ArithmeticException.class)), anyM(Try.of(0)));

    assertThat(result.<Try<Integer,ArithmeticException>>unwrapMonad().isFailure(),equalTo(true));
 private Integer divide(Integer a, Integer b){
    return a/b;
 }

Answer 2

The "better-java-monads" project on GitHub has a Try monad for Java 8 here.