Scala / java中的异常

Question

I'm a beginner to scala programming and jvm languages. I want to convert a string in yyyy-MM-dd to date format like this:

import java.text.SimpleDateFormat
import java.util.Date

val format = new SimpleDateFormat("yyyy-MM-dd")

def strTodate(stringDate: String): Date = {
    format.parse(stringDate)
  }

How can I can take care of exception in case strTodate is called on a wrongly formatted string like strTodate("18/03/03") ? I'll like to handle the exception and also print the string

Answer 1

scala has three ways to handle errors.

1. Option : has None or Some
2. Try : has Success or Failure
3. Either : has left or right. Right is always right result.

I prefer Either of all and here is how you can do it as Either[String, Date] where Left is String , Right is Date .

Example,

import java.text.SimpleDateFormat
import java.util.Date
import scala.util.Try
import scala.util.{Failure, Success}

val format = new SimpleDateFormat("yyyy-MM-dd")

def strTodate(stringDate: String): Either[String, Date] = {
  Try {
    format.parse(stringDate)
  } match {
    case Success(s) => Right(s)
    case Failure(e: ParseException) => Left(s"bad format: $stringDate")
    case Failure(e: Throwable) => Left(s"Unknown error formatting : $stringDate")
  }
}

val date1 = strTodate("2018-09-26")
println(date1) // Right(Wed Sep 26 00:00:00 PDT 2018)

val date2 = strTodate("2018/09/26")
println(date2) // Left(bad format: 2018/09/26)

Answer 2

Handling exceptions:

import java.io.IOException
import java.text.SimpleDateFormat
import java.util.Date

object app  {
  val format = new SimpleDateFormat("yyyy-MM-dd")

  def strTodate(stringDate: String): Either[Exception, Date] = {
    try {
      Right(format.parse(stringDate))
    } catch {
      case ioException : IOException =>
        Left(ioException)
      case e: Exception =>
        Left(e)
    }
  }

  def main(args: Array[String]) : Unit =
    strTodate("2018-02-02") match {
      case Right(date) => println(date)
      case Left(err) => println(err.getMessage)
    }
}