Java 8 stream map catch exception

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Java 8 stream map catch exception

Java 8 gave us Optional, a mighty weapon against the most frequent Exception in Java: NullPointerException. Unfortunately, Java 8 also brought new headaches regarding exceptions, as the default functional interfaces in Java 8 don't declare throwing any checked exceptions. So every time you get a checked exception within a lambda, you have to fix that somehow. Converting Checked into Runtime Exceptions The default suggestion offered by most IDEs to auto-fix this issue will produce code like this: SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd"); List dateList = asList("2020-10-11", "2020-nov-12", "2020-12-01"); List dates = dateList.stream().map(s -> { try { return format.parse(s); } catch (ParseException e) { throw new RuntimeException(e); } }).collect(toList()); Horrible code. We could create a dedicated function doing just .parse and then cast a spell on it with Lombok's @SneakyThrows: List dates = dateList.stream() .map(s -> uglyParse(format, s)) .collect(toList()); ... } @SneakyThrows private static Date uglyParse(SimpleDateFormat format, String s) { return format.parse(s); } But creating this new method just to hack it with Lombok feels wrong. Plus, you might be part of a team that banned (!) Lombok. But indeed, we created it for a purely technical reason: to hide the annoying checked exception which doesn't fit with the java.util.Function interface, which doesn't declare to throw anything. Let's play a bit and create a ThrowingFunction interface declaring to throw any checked exception: interface ThrowingFunction { R apply(T t) throws Exception; } Then, our s->format.parse(s) expression could be target-typed to this new interface, so the following line compiles: ThrowingFunction p = s -> format.parse(s); // or ThrowingFunction p = format::parse; Unfortunately, the Stream.map() operation takes a java.util.Function, you can't change that. But let's imagine we had a function that would take a ThrowingFunction and return back a `classic' Function that doesn't throw any checked exception anymore. Function f = wrapAsRuntime(p); List dates = dateList.stream().map(f).collect(toList()); And here's the strange wrapAsRuntime function: private static Function wrapAsRuntime(ThrowingFunction p) { return t -> { try { return p.apply(t); } catch (Exception e) { throw new RuntimeException(e); } }; } If that's complete nonsense for you, then I would advise that you try to type it yourself. It helps a lot! Notice that we've used generics to make it highly reusable. That's quite a good idea, isn't it? It's so good that of course others had it many years ago... Introducing the Unchecked.function() from the jool library that does EXACTLY what we did above. Using it, the final code looks like: List dates = dateList.stream().map(Unchecked.function(format::parse)).collect(toList()); If you've been using Java 8 for many years, then this library is a must-have. Best-practice: Whenever checked exceptions are annoying you in lambdas -> or method references ::, use Unchecked.* to rethrow it as a RuntimeException This doesn't involve any hack in the bytecode (as @SneakyThrows does), but only plain java code. Passing a function as a parameter to another function is a very useful practice that I will be blogging about soon, but functions that both take and return functions ? those I don't like. It's one of the most complex, hard to read, and especially hard to debug in Java. But since it's a library doing it, and the purpose is obvious, I never hesitated to use it many of my projects. Now let's shift a bit the perspective. No matter how you twist it, the processing of the entire stream stops when the first exception is thrown. But what if we don't want to crash but instead collect all the errors. The Try Monad Let's change the requirements a bit: we now want to parse all the valid dates and return them IF at least half of them are parseable, otherwise we should throw an exception. This time we can't let an exception terminate the execution of our stream. Instead, we want to go through all of the items and collect both parsed dates and exceptions. For example, if we are given 3 correctlyformatted dates and 2 invalid ones, we should return the 3 ones that we were able to parse correctly. Whenever you want to collect the exceptions happening in items, consider using the vavr Try monad. The Try class from the vavr library is a specialization of the Either concept present in many functional programming languages. An instance can store either the result or the occurred exception (if any). List tries = dateList.stream() .map(s -> Try.of( () -> format.parse(s) // throwing code )) .collect(toList()); If the throwing code crashes with an exception, the surrounding Try.of function will catch that exception and return a failed Try. Therefore, in the tries list above, there can be items with isSuccess() either true or false. To count the success ratio, the shortest (geekest) form is: double successRatio = tries.stream() .mapToInt(t -> t.isSuccess() ? 1 : 0) .average() .orElse(0); Then, if (successRatio > .5) { return tries.stream() .filter(Try::isSuccess) .map(Try::get) .collect(toList()); } else { throw new IllegalArgumentException("Too many invalid dates"); } Problem solved. To better understand the code, we can extract a function from it, that returns a Try: private static Try tryParse(SimpleDateFormat format, String s) { return Try.of(() -> format.parse(s)); } This resembles the style of handling exceptions in other languages like Go and Haskell, which return the exception to their callers. By the way, if you think a bit, you could solve the problem without the Try, by sweeping the data twice: first to count the parseable dates, and then to actually parse them. Or even a single pass using a combination of a .map returning a null/Optional.empty for errors, followed by a .filter. That could work too, but the Try approach might be more readable. Tip: Consider *vavr.Try when you want to collect both results and exceptions in a single pass through data. By the way, if you keep thinking at the "Monad" word, here's a nice article to get you past that: Monads for Java developers Disclaimer: avoid streaming a large number of items in batch processing. Instead, stick with the industry default: process the data in chunks, and consider introducing Spring Batch for state-of-the-art batches. Conclusions Checked exceptions don't play nice with the Java Stream API. Use @SneakyThrows (Lombok) or Unchecked (jOOL) to get rid of checked exceptions with Streams Consider Try (vavr) whenever you want to collect the errors occurring for an element instead of terminating the Stream. Learn More In case you liked the article, then check out my website (victorrentea.ro) for recorded talks, live and recorded webinars, blog posts and more goodies. If you have any questions or remarks you can also reach out to me on LinkedIN, Twitter, or Facebook. Author: Victor Rentea Related In Java 8 Collection and Stream API, Iteratable is implemented, and in the for statement You can now use lambda expressions for the part you were calculating. However, Functional Interfaces such as Consumer are not defined to throw an exception. So the inability to throw a CheckedException ( is a drawback of the lambda expression ( . It is said to be 8461). Wrapping or squeezing checked exceptions into runtime exceptions that are not checked at compile time is not good from an exception handling perspective. .. Qiita and Stackoverflow Also i ku [tsu]( /yoshi389111/items/c6b7d373a00f8fd3d5f3) or A solution is presented, but [grabbing inspection exceptions](https://s //SeregaLBN/StreamUnthrower), the lambda is a poorly visible code, [heavy implementation per function interface]( lambdas / tree / master / src / main / java / com / github / fge / lambdas / consumers) or not. By Google teacher and everyone is worried / (? ;) \ Rethrow by supplementing Runtime Exception (I don't want to) Unwrap Runtime in CheckException. I don't want to do this because the code is out of sight ... try { Arrays.asList(1, 2, 3).forEach(e -> { try { //Some business logic that throws checked exception methodTrowingIOException(e.intValue()); } catch(IOException ex) { // throw new UncheckedIOException(ex); throw new RuntimeException(ex); } })); } catch (RuntimeException re) { Throwable e = re.getCause();//Throw check exception throw e; } You can use UncheckedIOException, but unwrapping is troublesome and RuntimeException is used. It's not much different from. Define a FunctionalInterface with an Exception in the throws clause @FunctionalInterface public interface CheckedFunction { R apply(T t) throws Exception; } void foo (CheckedFunction f) { ... } However, this is a [for Each](https: /) that assumes Consumer. /docs.javase/jp/8/docs/api/java/lang/Iterable.html#forEach-java.util.function.Consumer-) etc., so rejected. Sneaky throw Use idioms to handle checked exceptions Sneaky throw is a Java compiler type-checking secret that tricks a checked exception into a RuntimeException at compile time [Lombok]( This is the technique used at . This article is detailed. @SuppressWarnings("unchecked") @Nonnull public static void sneakyThrow(@Nonnull Throwable ex) throws E { throw (E) ex; } //I'm throwing an IOException, but I don't need throws or catch because the compiler tricks me into a RuntimeException, but it actually throws an IOException. @Test(expected = IOException.class) public void testSneakyThrow() { Throwing.sneakyThrow(new IOException()); } Applying this idiom, we define the following Consumer. import java.util.function.Consumer; @FunctionalInterface public interface ThrowingConsumer extends Consumer { @Override default void accept(final T e) { try { accept0(e); } catch (Throwable ex) { Throwing.sneakyThrow(ex); } } void accept0(T e) throws Throwable; } When using it, do as follows. To receive lambda, use the above Throwing Consumer that inherits the Consumer interface. This is an advanced version of StackOverflow Answer. @Test public void testThrowingConsumer() throws IOException { thrown.expect(IOException.class); thrown.expectMessage("i=3"); Arrays.asList(1, 2, 3).forEach((ThrowingConsumer) e -> { int i = e.intValue(); if (i == 3) { throw new IOException("i=" + i); } }); } More concise representation using the Rethrow static method With Throwing Consumer, the part that receives the lambda expression in For Each with Throwing Consumer was a little complicated to express. I want to make this more concise. public final class Throwing { private Throwing() {} @Nonnull public static Consumer rethrow(@Nonnull final ThrowingConsumer consumer) { return consumer; } @SuppressWarnings("unchecked") @Nonnull public static void sneakyThrow(@Nonnull Throwable ex) throws E { throw (E) ex; } } Just wrap your lambda expression with the rethrow method. This is my final answer so far. The compiler assumes that forEach will only throw a runtime exception, so either throws IOException at the caller or try / catch at the right time. import static hivemall.utils.lambda.Throwing.rethrow; @Test public void testRethrow() throws IOException { thrown.expect(IOException.class); thrown.expectMessage("i=3"); Arrays.asList(1, 2, 3).forEach(rethrow(e -> { int i = e.intValue(); if (i == 3) { throw new IOException("i=" + i); } })); } Please let me know if there is a better way.

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