Asked  6 Months ago    Answers:  5   Viewed   65 times

How can I throw CHECKED exceptions from inside Java 8 streams/lambdas?

In other words, I want to make code like this compile:

public List<Class> getClasses() throws ClassNotFoundException {     

    List<Class> classes = 
        Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
              .map(className -> Class.forName(className))
              .collect(Collectors.toList());                  
    return classes;
    }

This code does not compile, since the Class.forName() method above throws ClassNotFoundException, which is checked.

Please note I do NOT want to wrap the checked exception inside a runtime exception and throw the wrapped unchecked exception instead. I want to throw the checked exception itself, and without adding ugly try/catches to the stream.

 Answers

77

This LambdaExceptionUtil helper class lets you use any checked exceptions in Java streams, like this:

Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
      .map(rethrowFunction(Class::forName))
      .collect(Collectors.toList());

Note Class::forName throws ClassNotFoundException, which is checked. The stream itself also throws ClassNotFoundException, and NOT some wrapping unchecked exception.

public final class LambdaExceptionUtil {

@FunctionalInterface
public interface Consumer_WithExceptions<T, E extends Exception> {
    void accept(T t) throws E;
    }

@FunctionalInterface
public interface BiConsumer_WithExceptions<T, U, E extends Exception> {
    void accept(T t, U u) throws E;
    }

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

@FunctionalInterface
public interface Supplier_WithExceptions<T, E extends Exception> {
    T get() throws E;
    }

@FunctionalInterface
public interface Runnable_WithExceptions<E extends Exception> {
    void run() throws E;
    }

/** .forEach(rethrowConsumer(name -> System.out.println(Class.forName(name)))); or .forEach(rethrowConsumer(ClassNameUtil::println)); */
public static <T, E extends Exception> Consumer<T> rethrowConsumer(Consumer_WithExceptions<T, E> consumer) throws E {
    return t -> {
        try { consumer.accept(t); }
        catch (Exception exception) { throwAsUnchecked(exception); }
        };
    }

public static <T, U, E extends Exception> BiConsumer<T, U> rethrowBiConsumer(BiConsumer_WithExceptions<T, U, E> biConsumer) throws E {
    return (t, u) -> {
        try { biConsumer.accept(t, u); }
        catch (Exception exception) { throwAsUnchecked(exception); }
        };
    }

/** .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) { throwAsUnchecked(exception); return null; }
        };
    }

/** rethrowSupplier(() -> new StringJoiner(new String(new byte[]{77, 97, 114, 107}, "UTF-8"))), */
public static <T, E extends Exception> Supplier<T> rethrowSupplier(Supplier_WithExceptions<T, E> function) throws E {
    return () -> {
        try { return function.get(); }
        catch (Exception exception) { throwAsUnchecked(exception); return null; }
        };
    }

/** uncheck(() -> Class.forName("xxx")); */
public static void uncheck(Runnable_WithExceptions t)
    {
    try { t.run(); }
    catch (Exception exception) { throwAsUnchecked(exception); }
    }

/** uncheck(() -> Class.forName("xxx")); */
public static <R, E extends Exception> R uncheck(Supplier_WithExceptions<R, E> supplier)
    {
    try { return supplier.get(); }
    catch (Exception exception) { throwAsUnchecked(exception); return null; }
    }

/** uncheck(Class::forName, "xxx"); */
public static <T, R, E extends Exception> R uncheck(Function_WithExceptions<T, R, E> function, T t) {
    try { return function.apply(t); }
    catch (Exception exception) { throwAsUnchecked(exception); return null; }
    }

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

}

Many other examples on how to use it (after statically importing LambdaExceptionUtil):

@Test
public void test_Consumer_with_checked_exceptions() throws IllegalAccessException {
    Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
          .forEach(rethrowConsumer(className -> System.out.println(Class.forName(className))));

    Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
          .forEach(rethrowConsumer(System.out::println));
    }

@Test
public void test_Function_with_checked_exceptions() throws ClassNotFoundException {
    List<Class> classes1
          = Stream.of("Object", "Integer", "String")
                  .map(rethrowFunction(className -> Class.forName("java.lang." + className)))
                  .collect(Collectors.toList());

    List<Class> classes2
          = Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
                  .map(rethrowFunction(Class::forName))
                  .collect(Collectors.toList());
    }

@Test
public void test_Supplier_with_checked_exceptions() throws ClassNotFoundException {
    Collector.of(
          rethrowSupplier(() -> new StringJoiner(new String(new byte[]{77, 97, 114, 107}, "UTF-8"))),
          StringJoiner::add, StringJoiner::merge, StringJoiner::toString);
    }

@Test    
public void test_uncheck_exception_thrown_by_method() {
    Class clazz1 = uncheck(() -> Class.forName("java.lang.String"));

    Class clazz2 = uncheck(Class::forName, "java.lang.String");
    }

@Test (expected = ClassNotFoundException.class)
public void test_if_correct_exception_is_still_thrown_by_method() {
    Class clazz3 = uncheck(Class::forName, "INVALID");
    }    

NOTE 1: The rethrow methods of the LambdaExceptionUtil class above may be used without fear, and are OK to use in any situation. A big thanks to user @PaoloC who helped solve the last problem: Now the compiler will ask you to add throw clauses and everything's as if you could throw checked exceptions natively on Java 8 streams.


NOTE 2: The uncheck methods of the LambdaExceptionUtil class above are bonus methods, and may be safely removed them from the class if you don't want to use them. If you do used them, do it with care, and not before understanding the following use cases, advantages/disadvantages and limitations:

• You may use the uncheck methods if you are calling a method which literally can never throw the exception that it declares. For example: new String(byteArr, "UTF-8") throws UnsupportedEncodingException, but UTF-8 is guaranteed by the Java spec to always be present. Here, the throws declaration is a nuisance and any solution to silence it with minimal boilerplate is welcome: String text = uncheck(() -> new String(byteArr, "UTF-8"));

• You may use the uncheck methods if you are implementing a strict interface where you don't have the option for adding a throws declaration, and yet throwing an exception is entirely appropriate. Wrapping an exception just to gain the privilege of throwing it results in a stacktrace with spurious exceptions which contribute no information about what actually went wrong. A good example is Runnable.run(), which does not throw any checked exceptions.

• In any case, if you decide to use the uncheck methods, be aware of these 2 consequences of throwing CHECKED exceptions without a throws clause: 1) The calling-code won't be able to catch it by name (if you try, the compiler will say: Exception is never thrown in body of corresponding try statement). It will bubble and probably be caught in the main program loop by some "catch Exception" or "catch Throwable", which may be what you want anyway. 2) It violates the principle of least surprise: it will no longer be enough to catch RuntimeException to be able to guarantee catching all possible exceptions. For this reason, I believe this should not be done in framework code, but only in business code that you completely control.

  • References:
    • http://www.philandstuff.com/2012/04/28/sneakily-throwing-checked-exceptions.html
    • http://www.mail-archive.com/javaposse@googlegroups.com/msg05984.html
    • Project Lombok annotation: @SneakyThrows
    • Brian Goetz opinion (against) here: How can I throw CHECKED exceptions from inside Java 8 streams?
    • https://softwareengineering.stackexchange.com/questions/225931/workaround-for-java-checked-exceptions?newreg=ddf0dd15e8174af8ba52e091cf85688e *
Tuesday, June 1, 2021
 
Elxx
answered 6 Months ago
29

You could create your own Exception class:

public class InvalidSpeedException extends Exception {

  public InvalidSpeedException(String message){
     super(message);
  }

}

In your code:

throw new InvalidSpeedException("TOO HIGH");
Wednesday, June 16, 2021
 
Laimoncijus
answered 6 Months ago
98

This should do the trick:

objects.stream().map(MyObj::getId).collect(Collectors.toList());

that said, the method reference :: operator allows you to reference any method in your classpath and use it as a lambda for the operation that you need.

As mentioned in the comments, a stream preserves order.

Friday, July 23, 2021
 
shin
answered 5 Months ago
66

Note: See UPDATE below

I've never use LocalDateTime before, so I decided to do some testing. Here are my findings:

  • Jersy 2.13 and this provider (works out the box with no extra configuration)

    <dependency>
        <groupId>org.glassfish.jersey.media</groupId>
        <artifactId>jersey-media-moxy</artifactId>
        <version>${jersey.version}</version>
    </dependency>
    
  • Jersey 2.13 with this provider (has support for JAXB annotation - dependency on jackson-module-jaxb-annotations), with custom adapter

    <dependency>
        <groupId>org.glassfish.jersey.media</groupId>
        <artifactId>jersey-media-json-jackson</artifactId>
        <version>${jersey.version}</version>
    </dependency>
    
    public class LocalDateTimeAdapter extends XmlAdapter<String, LocalDateTime> {
        @Override
        public LocalDateTime unmarshal(String s) throws Exception {
            return LocalDateTime.parse(s);
        }
        @Override
        public String marshal(LocalDateTime dateTime) throws Exception {
            return dateTime.toString();
        }   
    }
    
    // Getter for model class
    @XmlJavaTypeAdapter(LocalDateTimeAdapter.class)
    public LocalDateTime getDateTime() {
        return dateTime;
    }
    
  • Resteasy 3.0.9 with this provider, (also has support for JAXB annotation - dependency on jackson-module-jaxb-annotations), with custom adapter (See above)

    <dependency>
        <groupId>org.jboss.resteasy</groupId>
        <artifactId>resteasy-jackson2-provider</artifactId>
        <version>${resteasy.version}</version>
    </dependency>
    
  • Both Resteasy and Jersey with this dependency (also did not work without custom config, same as last two - with adapter)

    <dependency>
        <groupId>com.fasterxml.jackson.jaxrs</groupId>
        <artifactId>jackson-jaxrs-json-provider</artifactId>
        <version>2.4.0</version>
    </dependency>
    

    We need to make sure to register the JacksonJaxbJsonProvider


So I guess it seems that any provider that uses Jackson, does not give you the deisred result, without some custom configuration, whether its through an adapter (as seen above) or some other custom configuration. The jersey-media-moxy provider doesn't use Jackson.


UPDATE

For the most part, the information above is incorrect.

  • MOXy does not work by default. It works for serialization by simply calling toString(), which may or may not be what you want, and it won't work when de-serializing. If you are using MOXy, until it supports Java8 time, you will need to use an XMLAdapter

  • Jackson you will need to configure its Java8 time support. This is the case with both Jersey and RESTEasy.

Thursday, August 12, 2021
 
Uours
answered 4 Months ago
44

Something like this if you're bound to use Stream.generate specifically :

IntStream inStream = Stream.generate(new AtomicInteger(1)::getAndIncrement)
        .limit(10)
        .mapToInt(t -> t);
inStream.forEach(System.out::println);

Edit: Using IntStream.generate, you can perform it as

IntStream.generate(new AtomicInteger(1)::getAndIncrement).limit(10);

Note: A better solution in terms of the API design would definitely be to make use of Stream.iterate for such a use case.

Monday, October 11, 2021
 
akohout
answered 2 Months ago
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