Java 8 Lambda function that throws exception?

Question

I know how to create a reference to a method that has a String parameter and returns an int, it\'s:

Function         


        

Answer 1

You can actually extend Consumer (and Function etc.) with a new interface that handles exceptions -- using Java 8's default methods!

Consider this interface (extends Consumer):

@FunctionalInterface
public interface ThrowingConsumer extends Consumer {

    @Override
    default void accept(final T elem) {
        try {
            acceptThrows(elem);
        } catch (final Exception e) {
            // Implement your own exception handling logic here..
            // For example:
            System.out.println("handling an exception...");
            // Or ...
            throw new RuntimeException(e);
        }
    }

    void acceptThrows(T elem) throws Exception;

}

Then, for example, if you have a list:

final List list = Arrays.asList("A", "B", "C");

If you want to consume it (eg. with forEach) with some code that throws exceptions, you would traditionally have set up a try/catch block:

final Consumer consumer = aps -> {
    try {
        // maybe some other code here...
        throw new Exception("asdas");
    } catch (final Exception ex) {
        System.out.println("handling an exception...");
    }
};
list.forEach(consumer);

But with this new interface, you can instantiate it with a lambda expression and the compiler will not complain:

final ThrowingConsumer throwingConsumer = aps -> {
    // maybe some other code here...
    throw new Exception("asdas");
};
list.forEach(throwingConsumer);

Or even just cast it to be more succinct!:

list.forEach((ThrowingConsumer) aps -> {
    // maybe some other code here...
    throw new Exception("asda");
});

Update: Looks like there's a very nice utility library part of Durian called Errors which can be used to solve this problem with a lot more flexibility. For example, in my implementation above I've explicitly defined the error handling policy (System.out... or throw RuntimeException), whereas Durian's Errors allow you to apply a policy on the fly via a large suite of utility methods. Thanks for sharing it, @NedTwigg!.

Sample usage:

list.forEach(Errors.rethrow().wrap(c -> somethingThatThrows(c)));