Java 8 stream map error handling

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Java 8 stream map error handling

Photo by Maximilian Weisbecker on UnsplashOver the course of my time working with companies migrating from Java 7 to Java 8, I saw many people being excited using cool new features like streams and higher-order functions. Java 8 does away with a lot of boilerplate code and makes the code a lot less verbose.But

not doing exception handling properly can eventually end up with us having bloated lambdas which defeat the purpose of having clean code.Here, I¡¯d like to discuss a few of the things I¡¯ve learned over time in handling exceptions within lambdas. Keep in mind, there are always multiple approaches to things like this, so

take this with a pinch of salt. If you think there are better ways to do the things below, please do let me know.Let¡¯s take an example use of streams: You are given a list of strings and you want to convert them all to integers. To achieve that, we can do something simple like this:The above snippet will work perfectly, but

what happens if we modify the input to contain an illegal string, say "xyz". The method parseInt() will throw a NumberFormatException , which is a type of unchecked exception.A naive solution, one that is typically seen is to wrap the call in a try/catch block and handle it. That would look like this:While this works, this

defeats the purpose of writing small lambdas to make code readable and less verbose. The solution that comes to mind is to wrap the lambda around another lambda that does the exception handling for you, but that is basically just moving the exception handling code somewhere else:The above solution can be made

much, much better by using generics. Let¡¯s build a generic exception handled consumer that can handle all kinds of exceptions. We will, then, be able to use it for many different use cases within our application.We can make use of the above code to build out our generic implementation. I will not go into the details of

how generics work, but a good implementation would look like this:As you can see, this new consumer is not bound to any particular type of object it consumes and accepts the type of Exception your code might throw as a parameter. We can now, simply use the handledConsumer method to build our consumers. The

code for parsing our list of Strings to Integers will now be this:If you have a different block of code that may throw a different exception, you can just reuse the above method. For example, the code below takes care of ArithmeticException due to a divide by zero.To take a look at checked exceptions, we will use the

example of Thread.sleep() method, which throws an InterruptedException .Let¡¯s say we have a list of integers containing the time in milliseconds for which we want the thread to sleep. Trying to do this:will give a compilation error as the checked exception has not been handled. Adding throws to the method signature of

sleeper() will not solve the problem either :This is because lambdas are after all anonymous class implementations. We need to handle the exception within the implementation of the interface the lambda is implementing. The calling parent method has nothing to do with it.Once again, the naive solution is to add a trycatch block inside the lambda, which obviously works:Let us take inspiration from our generic implementation to handle unchecked exceptions. We can extend the Consumer interface to handle checked exceptions. Let¡¯s see how.Step 1: Build a new interface that will handle checked exceptions. Let¡¯s call it

HandlingConsumer :Step 2: Add a static method to the interface to build the new functional interface we just wrote (in case you forgot, you can write static methods inside interfaces since Java 8), and make sure you convert the checked exception to a RuntimeException . Why? Because that allows us to handle the

exception in the calling methods and frees up the lambda to do its actual job:Our code to perform the thread sleep can now simply be this:The handlingConsumerBuilder method can be reused for any lambdas that need to handle checked exceptions while maintaining the sanity of our lambdas and reducing overall code

size.I hope you find these techniques useful in making your code more readable and less verbose, and also allows you to handle your exceptions properly.A lot of the above content has been learned from reading Eugen Paraschiv¡¯s website. I recommend you all go take a look at what he has to offer.I hope you enjoyed

this guide. Should you have any questions, feel free to comment below. Every came across a situation when working with Java Streams where you have to write a lambda expression that can throw a checked exception? You wouldn¡¯t have been able to use that lambda expression (or the equivalent method reference as

is because the Functional Interfaces that Java streams have do not support throwing a checked exception. In this post, we will see 4 ways to throw checked exceptions in Java Streams.Lambda expression in Streams and checked exceptionsWe start with a list of file paths and read the content of each as a List and

collect them into another list.List paths = List.of("/home/file1", "/home/file2"); List fileContents = paths.stream() .map(path -> Paths.get(path)) .map(path -> Files.readAllLines(path)) .collect(Collectors.toList());In the above code, the function passed to the second map will fail to compile with the following error:Unhandled

exception: java.io.IOExceptionThis is because of the method signature or contract of the Function functional interface. Let us look at how a Function is defined. A Function has a single abstract method as shown below:@FunctionalInterface public interface Function { /** * Applies this function to the given argument. * *

@param t the function argument * @return the function result */ R apply(T t); }It accepts an argument of type T and returns a value of type R. We can write the lambda expression we passed to the map method as an anonymous class:Function readAllLines = new Function() { @Override public List apply(String path) {

return Files.readAllLines(path); } };Again, this doesn¡¯t compile for the same reason. The apply method does not support throwing a checked exception. But the call to Files.readAllLines throws an IOException. Hence, the statement Files.readAllLines does not adhere to the Function¡¯s apply method contract.Now let us see

a few ways on how we can handle this. The first two methods do not use any external (3rd party) libraries whereas the last two do.Approach 1 ¨C Move the checked exception throwing method call to a separate functionThe essence of this is to catch the checked exception and wrap it in a unchecked exception

(See Difference between a checked and an unchecked exception).The mechanics are as follows:Move the method call in the lambda expression that can throw a checked exception into a separate private method. Add a catch block to catch the checked exception(s).Wrap them in a runtime exception and throw it

back.List fileContents = paths.stream() .map(path -> Paths.get(path)) .map(this::readFromFile) .collect(Collectors.toList());private List readFromFile(Path path) { try { return Files.readAllLines(path); } catch (IOException e) { throw new RuntimeException(e); } }The method reference this::readFromFile passes the private

function readFromFile as a Function to the map method. The readFromFile method calls the readAllLines method and returns the result. It also catches the IOException and wraps it in a RuntimeException.So, if the call to the readAllLines method throws an IOException, we will get it back wrapped in a

RuntimeException.Pros and ConsThis approach did not really handle the problem as it just moved it to a different place. In fact, this when written inline would look like:List fileContents = paths.stream() .map(path -> Paths.get(path)) .map(path -> { try { return Files.readAllLines(path); } catch (IOException e) { throw new

RuntimeException(e); } }) .collect(Collectors.toList());The merit of this is that the method invocation and exception handling when moved to a private method would Keep the stream pipeline clean.Avoids long lambda expressions.Approach 2 ¨C Create a new corresponding Functional interface that can throw checked

exceptionsLet us look at a different example. We have a method that takes a function and a parameter as arguments, applies the function with the given parameter and returns the result.public static String process(Function function, String param) { String result = function.apply(param); System.out.println(result); return

result; }When we invoke it as:process(s -> s + "-1", "abcd");Would print and return the string abcd-1 as the function appends -1 to the string passed in to the function.However, if we pass a function that can throw a checked exception, this will not compile. Example: Let us pass a function that reads from a

file.process(path -> Files.readString(Paths.get(path)), "/home/file1");Note that Files.readString method is available from Java 11+. But you can substitute it with any function that throws an exception.We will get the same compilation error as before because of the same reason. The first parameter of the process method

needs a Function, but the lambda expression used here can throw a checked exception. Thus, it does not satisfy the Function contract.Define a new Functional interface with checked exceptionWe will create a new FunctionalInterface similar to that of a Function but that can throw an Exception. It is shown

below.@FunctionalInterface public interface ThrowingFunction { /** * Applies this function to the given argument. * * @param t the function argument * @return the function result */ R apply(T t) throws Exception; }Now, we can change the method parameter from accepting a Function to a ThrowingFunction.public String

processWithThrowingFunction(ThrowingFunction function, String param) throws Exception { String result = function.apply(param); System.out.println(result); return result; } //Call it as processWithThrowingFunction(path -> Files.readString(Paths.get(path)), "/home/file1");The same lambda expression path ->

Files.readString(Paths.get(path)) now satisfies the Functional Interface ThrowingFunction because the single abstract method in it (apply) can throw any exception. Thus we can pass the call to Files.readString as a ThrowingFunction.Approach 3 ¨C Creating a bridge from the throwing functional interfaceThe second

approach requires changing a Function to a ThrowingFunction. Unfortunately, we cannot do it in a Java stream because the map method accepts only a Function.We can create a bridge or conversion between these two types i.e., convert a ThrowingFunction into a Function using a simple utility method.public static

Function transform(ThrowingFunction throwingFunction) { return param -> { try { return throwingFunction.apply(param); } catch (Exception ex) { throw new RuntimeException(ex); } }; }The transform method takes a ThrowingFunction and returns a new Function (written as a lambda expression). The lambda expression

parameter param is the Function interface¡¯s argument to the apply method. When written as an anonymous class would look like:return new Function() { @Override public R apply(T t) { try { return throwingFunction.apply(t); } catch (Exception ex) { throw new RuntimeException(ex); } } };The call chain is:Write a lambda

expression (or method reference) that can throw a checked expression (which is of type ThrowingFunction).Call the transform method by passing it to get back a normal Function.We can use now use this where a regular Function is used.process(transform(path -> Files.readString(Paths.get(path))), "/home/file1");The call

to transform returns a Function. Thus we can even pass it to the process method. Any IOException would be thrown back, wrapped in a RuntimeException.Using the bridge in a stream pipelineLet us use the transform method to convert a ThrowingFunction into a Function and use it in the Stream¡¯s map method.List

fileContents = paths.stream() .map(path -> Paths.get(path)) .map(transform(Files::readAllLines)) //transform(path -> Files.readAllLines(path)) .collect(Collectors.toList());When we call transform, it returns a new function that invokes readAllLines, but catches any checked exception and throws back as a

RuntimeException.Advantage of creating new functional interface (approaches 2 and 3)So far, though, we have been seeing from the standpoint of a Function. But we can apply this to all functional interfaces in Java like Supplier, Predicate, Consumer, etc. Once we create one checked version of Functional Interface for

each of the Functional Interfaces and create a bridge function to transform it, we can use it from many places.Approach 3 ¨C Streams utility from Apache Commons LangThis approach uses an utility method from the Apache Commons Lang library to throw checked exceptions in Java Streams. Hence, you need to have a

dependency on this library for this.The Apache Commons Lang has an utility class called Streams which provides functions for working with Java streams. It helps us to convert a Java Stream into a FailableStream which supports all operations on a Stream. In addition to that, in a FailableStream, the map, filter methods

(and others) work with Failable version of the Functional Interfaces like FailablePredicate, FailableConsumer, and FailableFunction etc.,These Failable* interfaces are defined within the Apache Commons Lang library. I have shown a few of them below:public FailableStream filter(final FailablePredicate predicate) public

FailableStream map(final FailableFunction mapper) public void forEach(final FailableConsumer action) These functional interfaces are declared to throw exception (very similar to the ThrowingFunction we created) For example, FailableFunction is defined as@FunctionalInterface public interface FailableFunction { /** *

Apply the function. * @param input the input for the function * @return the result of the function * @throws T if the function fails */ O apply(I input) throws T; }where the exception it throws is a type parameter (T). When it is used in the stream, they have used a wildcard (?). Hence, we can throw any checked

exception.Apache Commons Lang ¨C Streams#stream methodThere are two Streams.stream methods which acceptA Java stream orA Java Collectionand return a FailableStream. We can pass lambda expressions that can throw a (checked) exception to the map, filter, forEach and other operations.List paths =

List.of("/home/file1", "/home/file2"); List fileContents = Streams.stream(paths.stream()) .map(path -> Paths.get(path)) .map(Files::readAllLines) //path -> Files.readAllLines(path) .collect(Collectors.toList()); fileContents = Streams.stream(paths) .map(path -> Paths.get(path)) .map(Files::readAllLines) //path ->

Files.readAllLines(path) .collect(Collectors.toList());Approach 4 ¨C Lombok¡¯s SneakyThrowsThe last approach uses the Lombok library. This approach is very hacky and not recommended and requires careful usage.It provides an annotation called @SneakyThrows. It is used to sneakily throw checked exception without

declaring throws clause in the method¡¯s signature. You can think of this as cheating the compiler. The documentation states:The code generated by lombok will not ignore, wrap, replace, or otherwise modify the thrown checked exception; it simply fakes out the compiler. On the JVM (class file) level, all exceptions,

checked or not, can be thrown regardless of the throws clause of your methods, which is why this works.We can pass the list of exception classes to the @SneakyThrows annotation that we wish to be thrown.@SneakyThrows(IOException.class) private List readFromFile(Path path) { return Files.readAllLines(path); }This

method calls Files.readAllLines which can throw an IOException. But we got away with neither catching the checked exception nor declaring it in the method signature in the throws clause.Note: If you are using an IDE like Eclipse or Intellij, the above code will produce a compilation error (with red squiggly line). But you

should be able to run the code.List paths = List.of("/home/file1", "/home/file2"); List fileContents = paths.stream() .map(path -> Paths.get(path)) .map(this::readFromFile) .collect(Collectors.toList());The method reference used in the map calls the above method to read from the file. And we wrote the method without having

to handle the checked exception (very sneaky). We could still get an IOException during runtime.ConclusionThis post, 4 ways to throw checked exceptions in Java streams, started by looking at the problem of having a lambda expression that can throw checked exception(s) when using it within Java streams. We looked

at four ways to handle them where the last two methods used external libraries.It is up to you to choose which method to use (except that I do not suggest the Lombok approach unless you know what you are doing and have a good reason to do so).References

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