Why java streams are not able to handle exception even after surrounding with a try-catch block?

Question

I have the following piece of code with java streams

You can see that I am having an error on the 4th line. Basically the error says Unhandled Exception: AddressException. But you can see that I am catching it within catch block. But still that is not working. Eventhough, if I use a try catch block within the map method it works as shown below

  public List<InternetAddress> getListOfInternetEmailAddresses(List<String> toEmails){
       List<InternetAddress> internetAddresses = new ArrayList<>();
            internetAddresses = toEmails.stream().map(a->{
                InternetAddress ia = null;
                try{
                   ia = new InternetAddress(a);
                } catch (AddressException e) {
                    
                }
                return ia;
            }).collect(Collectors.toList());
        return internetAddresses;
    }

Does anyone know why this behaviour and if knows please give some insights to that. One more quetion, does the anonymous inner class will also behave the same ?

The part in `map` is a function itself which is not expecting to throw a checked exception — Jens, Dec 21 '22 at 08:11
because neither of the **called** methods (`stram()`, `map()`, `collect()` or even `toList()`) throw that Exception - `InternetAddress::new` is not calling the method, it is a method reference that can be called *elsewhere* (`map()` will eventually call it, but it takes a `Function` as argument and that has no *mechanics* to intercept a checked Exception) — user16320675, Dec 21 '22 at 09:56

score 4 · Accepted Answer · answered Dec 21 '22 at 09:03

Error is shown because you provided method with different signature (additional thows clause). You have to provide implementation that is compatible with java.util.function.Function#apply signature

R apply(T var1);

There is several ways to deal with your problem:

anonymous function with try-catch

    public List<InternetAddress> getListOfInternetEmailAddresses(List<String> toEmails) {
        return toEmails.stream().map(new Function<String, InternetAddress>() {
            @Override
            public InternetAddress apply(String email) {
                try {
                    return new InternetAddress(email);
                } catch (AddressException e) {
                    e.printStackTrace();
                }
                return null;
            }
        }).collect(Collectors.toList());
    }

try-catch in lambda

    public List<InternetAddress> getListOfInternetEmailAddresses(List<String> toEmails) {
        return toEmails.stream().map(email -> {
            try {
                return new InternetAddress(email);
            } catch (AddressException e) {
                e.printStackTrace();
            }
            return null;
        }).collect(Collectors.toList());
    }

extracted handling method

    public List<InternetAddress> getListOfInternetEmailAddresses(List<String> toEmails) {
        return toEmails.stream().map(this::createInternetAddress).collect(Collectors.toList());
    }

    private InternetAddress createInternetAddress(String email) {
        try {
            return  new InternetAddress(email);
        } catch (AddressException e) {
            e.printStackTrace();
        }
        return null;
    }

generalized extracted handling method

@FunctionalInterface
public interface FunctionThrowing <T, R, E extends Exception> {
    R apply(T var1) throws E;

    static <T, R, E extends Exception> Function<T, R> handled(FunctionThrowing<T, R, E> ft) {
        return result -> {
            try {
                return ft.apply(result);
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        };
    }
}

    public List<InternetAddress> getListOfInternetEmailAddresses(List<String> toEmails) {
        List<InternetAddress> internetAddresses = new ArrayList<>();
        internetAddresses = toEmails.stream().map(FunctionThrowing.handled(InternetAddress::new)).collect(Collectors.toList());
        return internetAddresses;
    }

Why java streams are not able to handle exception even after surrounding with a try-catch block?

1 Answers1