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

I use an overloaded utility function called unchecked() which handles multiple use-cases.

---

SOME EAMPLE USAGES

unchecked(() -> new File("hello.txt").createNewFile());

boolean fileWasCreated = unchecked(() -> new File("hello.txt").createNewFile());

myFiles.forEach(unchecked(file -> new File(file.path).createNewFile()));

SUPPORTING UTILITIES

public class UncheckedUtils {

    @FunctionalInterface
    public interface ThrowingConsumer<T> {
        void accept(T t) throws Exception;
    }

    @FunctionalInterface
    public interface ThrowingSupplier<T> {
        T get() throws Exception;
    }

    @FunctionalInterface
    public interface ThrowingRunnable {
        void run() throws Exception;
    }

    public static <T> Consumer<T> unchecked(
            ThrowingConsumer<T> throwingConsumer
    ) {
        return i -> {
            try {
                throwingConsumer.accept(i);
            } catch (Exception ex) {
                throw new RuntimeException(ex);
            }
        };
    }

    public static <T> T unchecked(
            ThrowingSupplier<T> throwingSupplier
    ) {
        try {
            return throwingSupplier.get();
        } catch (Exception ex) {
            throw new RuntimeException(ex);
        }
    }

    public static void unchecked(
            ThrowingRunnable throwing
    ) {
        try {
            throwing.run();
        } catch (Exception ex) {
            throw new RuntimeException(ex);
        }
    }
}

Answer 2

By default, Java 8 Function does not allow to throw exception and as suggested in multiple answers there are many ways to achieve it, one way is:

@FunctionalInterface
public interface FunctionWithException<T, R, E extends Exception> {
    R apply(T t) throws E;
}

Define as:

private FunctionWithException<String, Integer, IOException> myMethod = (str) -> {
    if ("abc".equals(str)) {
        throw new IOException();
    }
  return 1;
};

And add throws or try/catch the same exception in caller method.

Answer 3

You can use unthrow wrapper

Function<String, Integer> func1 = s -> Unthrow.wrap(() -> myMethod(s));

or

Function<String, Integer> func2 = s1 -> Unthrow.wrap((s2) -> myMethod(s2), s1);

Answer 4

You can.

Extending @marcg 's UtilException and adding generic <E extends Exception> 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<T, R, E extends Exception> {
        R apply(T t) throws E;
    }

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

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

}

public class LambdaExceptionUtilTest {

    @Test
    public void testFunction() throws MyTestException {
        List<Integer> sizes = Stream.of("ciao", "hello").<Integer>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 { }
}

Answer 5

I will do something generic:

public interface Lambda {

    @FunctionalInterface
    public interface CheckedFunction<T> {

        T get() throws Exception;
    }

    public static <T> T handle(CheckedFunction<T> supplier) {
        try {
            return supplier.get();
        } catch (Exception exception) {
            throw new RuntimeException(exception);

        }
    }
}

usage:

 Lambda.handle(() -> method());

Answer 6

public void frankTest() {
    int pageId= -1;

    List<Book> users= null;
    try {
        //Does Not Compile:  Object page=DatabaseConnection.getSpringConnection().queryForObject("SELECT * FROM bookmark_page", (rw, n) -> new Portal(rw.getInt("id"), "", users.parallelStream().filter(uu -> uu.getVbid() == rw.getString("user_id")).findFirst().get(), rw.getString("name")));

        //Compiles:
        Object page= DatabaseConnection.getSpringConnection().queryForObject("SELECT * FROM bookmark_page", (rw, n) -> { 
            try {
                final Book bk= users.stream().filter(bp -> { 
                    String name= null;
                    try {
                        name = rw.getString("name");
                    } catch (Exception e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    return bp.getTitle().equals(name); 
                }).limit(1).collect(Collectors.toList()).get(0);
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return new Portal(rw.getInt("id"), "", users.get(0), rw.getString("name")); 
        } );
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}