Better Exception Handling in Java 8 Streams Using Javaslang
In this post, I will provide tips for better exception handling in Java 8 streams using Functional Java library Javaslang.
Problem
To illustrate with an example, let’s say we want to print day of the week for given stream date strings in format MM/dd/YYYY.
Initial solution.
Let’s start with an initial solution below and iteratively improve on it.
public class StreamExceptionHandling {
private static final DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MM/dd/yyyy");
/**
* Converts given date string in format "MM/dd/yyyy" to LocalDate.
*/
private static LocalDate parseDate(String dateString) {
return LocalDate.from(formatter.parse(dateString));
}
public static void main(String args[]) {
Stream.of("12/31/2014",
"01-01-2015",
"12/31/2015",
"not a date",
"01/01/2016")
.map(StreamExceptionHandling::parseDate) //parse string to LocalDate
.map(DayOfWeek::from) // Map LocalDate to Day of Week
.forEach(System.out::println); // Print
}
}
This will output
WEDNESDAY
Exception in thread "main" java.time.format.DateTimeParseException: Text '01-01-2015' could not be parsed at index 2
...
Huh, If a date string is invalid, this fails at the first DateTimeParseException without continuing with valid dates.
Java 8 Optional to rescue.
We can refactor parseDate to return Optional<LocalDate> to make it discard invalids and continue processing valid dates.
/**
* Converts given date string in format "MM/dd/yyyy" to LocalDate.
* Returns Optional of LocalDate if it is valid, otherwise Optional.empty
*/
private static Optional<LocalDate> parseDate(String dateString){
LocalDate localDate = null;
try {
localDate = LocalDate.from(formatter.parse(dateString));
}catch (DateTimeParseException e){
System.out.println(e.getMessage());
}
return Optional.ofNullable(localDate);
}
public static void main(String args[]) {
Stream.of("12/31/2014",
"01-01-2015",
"12/31/2015",
"not a date",
"01/01/2016")
.map(StreamExceptionHandling::parseDate)//Parse String to LocalDate
.filter(Optional::isPresent) //Filter valid ones
.map(Optional::get)//Get wrapped LocalDate
.map(DayOfWeek::from) //Map to day of week
.forEach(System.out::println); //Print
}
This will skip errors and converts all the valid dates.
WEDNESDAY
Text '01-01-2015' could not be parsed at index 2
THURSDAY
Text 'not a date' could not be parsed at index 0
FRIDAY
This is great improvement, but the exception has to be handled within parseDate method and can’t be passed back to main method to deal with it. We can’t make parseDate method throw checked exception as Streams API doesn’t play well with methods that throw exceptions.
Better solution with Javaslang’s Try Monad
Javaslang is a functional library for Java 8+. We will use Try object from Javaslang which can be either a instance of Success or Failure. Basically Try is a monadic container type which represents a computation that may either result in an exception, or return a successfully computed value. Here is the modified code using Try
/**
* Converts given date string in format "MM/dd/yyyy" to LocalDate.
* Returns Try of LocalDate representing valid LocalDate or Throwable in case of invalid value.
*/
private static Try<LocalDate> parseDate(String dateString){
return Try.of(() -> LocalDate.from(formatter.parse(dateString)));
}
public static void main(String args[]) {
Stream.of("12/31/2014",
"01-01-2015",
"12/31/2015",
"not a date",
"01/01/2016")
.map(StreamExceptionHandling::parseDate)//Parse String to LocalDate
.peek(v-> v.onFailure(t -> System.out.println("Failed due to " + t.getMessage())))//Print error on failure
.filter(Try::isSuccess)//Filter valids
.map(Try::get)//Get wrapped value
.map(DayOfWeek::from)//Map to day of week
.forEach(System.out::println);//Print
}
The Output is
WEDNESDAY
Failed due to Text '01-01-2015' could not be parsed at index 2
THURSDAY
Failed due to Text 'not a date' could not be parsed at index 0
FRIDAY
Now the exception is passed back to main to deal with it. Try also has API’s to implement a recovery logic or return default value in case of error. To demonstrate this, let’s say we also want to support MM-dd-YYYY as alternate string format for date. Below example shows how we can easily implement recovery logic.
public class StreamExceptionHandling {
private static final DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MM/dd/yyyy");
private static final DateTimeFormatter alternateFormatter = DateTimeFormatter.ofPattern("MM-dd-yyyy");
/**
* Converts given date string in format "MM/dd/yyyy" to LocalDate.
* Returns Try of LocalDate representing valid LocalDate or Throwable in case of invalid value.
*/
private static Try<LocalDate> parseDate(String dateString){
return Try.of(() -> LocalDate.from(formatter.parse(dateString)));
}
/**
* Converts given date string in format "MM/dd/yyyy" to LocalDate.
* Returns Try of LocalDate representing valid LocalDate or Throwable in case of invalid value.
*/
private static Try<LocalDate> parseDateAlternate(String dateString){
return Try.of(() -> LocalDate.from(alternateFormatter.parse(dateString)));
}
public static void main(String args[]) {
Stream.of("12/31/2014",
"01-01-2015",
"12/31/2015",
"not a date",
"01/01/2016")
.map(StreamExceptionHandling::parseDate)//Parse String to LocalDate
.map(v-> v.recoverWith( e -> parseDateAlternate(((DateTimeParseException)e).getParsedString())))//Try recovering with alternate formatter
.peek(v-> v.onFailure(t -> System.out.println("Failed due to " + t.getMessage())))//Print error on failure
.filter(Try::isSuccess)//Filter valids
.map(Try::get)//Get wrapped value
.map(DayOfWeek::from)//Map to day of week
.forEach(System.out::println);//Print
}
}
The output shows that now the date 01-01-2015 is also successfully converted.
WEDNESDAY
THURSDAY
THURSDAY
Failed due to Text 'not a date' could not be parsed at index 0
FRIDAY
So Try Monad can be used to elegantly deal with exceptions and fail fast on errors. In next article , I will post how Validation control applicative functor can be used to fail slow and accumulate errors.