Scala Exception Handling

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Scala's exceptions work like exceptions in many other languages like Java. Instead of returning a value in the normal way, a method can terminate by throwing an exception. However, Scala doesn't actually have checked exceptions.

When you want to handle exceptions, you use a try{...}catch{...} block like you would in Java except that the catch block uses matching to identify and handle the exceptions.

Throwing exceptions:

Throwing an exception looks the same as in Java. You create an exception object and then you throw it with the throw keyword:

throw new IllegalArgumentException

Catching exceptions:

Scala allows you to try/catch any exception in a single block and then perform pattern matching against it using case blocks as shown below:

import java.io.FileReader
import java.io.FileNotFoundException
import java.io.IOException

object Test {
   def main(args: Array[String]) {
      try {
         val f = new FileReader("input.txt")
      } catch {
         case ex: FileNotFoundException =>{
            println("Missing file exception")
         }
         case ex: IOException => {
            println("IO Exception")
         }
      }
   }
}

When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
Missing file exception

C:/>

The behavior of this try-catch expression is the same as in other languages with exceptions. The body is executed, and if it throws an exception, each catch clause is tried in turn.

The finally clause:

You can wrap an expression with a finally clause if you want to cause some code to execute no matter how the expression terminates.

import java.io.FileReader
import java.io.FileNotFoundException
import java.io.IOException

object Test {
   def main(args: Array[String]) {
      try {
         val f = new FileReader("input.txt")
      } catch {
         case ex: FileNotFoundException => {
            println("Missing file exception")
         }
         case ex: IOException => {
            println("IO Exception")
         }
      } finally {
         println("Exiting finally...")
      }
   }
}

When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
Missing file exception
Exiting finally...

C:/>


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