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.

Extending @marcg 's UtilException and adding generic where necessary: this way, the compiler will force you again to add throw clauses and everything's as if you could throw checked exceptions natively on java 8's streams.

public final class LambdaExceptionUtil {

    @FunctionalInterface
    public interface Function_WithExceptions {
        R apply(T t) throws E;
    }

    /**
     * .map(rethrowFunction(name -> Class.forName(name))) or .map(rethrowFunction(Class::forName))
     */
    public static  Function rethrowFunction(Function_WithExceptions function) throws E  {
        return t -> {
            try {
                return function.apply(t);
            } catch (Exception exception) {
                throwActualException(exception);
                return null;
            }
        };
    }

    @SuppressWarnings("unchecked")
    private static  void throwActualException(Exception exception) throws E {
        throw (E) exception;
    }

}

public class LambdaExceptionUtilTest {

    @Test
    public void testFunction() throws MyTestException {
        List sizes = Stream.of("ciao", "hello").map(rethrowFunction(s -> transform(s))).collect(toList());
        assertEquals(2, sizes.size());
        assertEquals(4, sizes.get(0).intValue());
        assertEquals(5, sizes.get(1).intValue());
    }

    private Integer transform(String value) throws MyTestException {
        if(value==null) {
            throw new MyTestException();
        }
        return value.length();
    }

    private static class MyTestException extends Exception { }
}