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 as follows.

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. Try the following example program to handle exception.

Example

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

object Demo {
   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")
         }
      }
   }
}

Save the above program in Demo.scala. The following commands are used to compile and execute this program.

Command

\>scalac Demo.scala
\>scala Demo

Output

Missing file exception

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. Try the following program.

Example

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

object Demo {
   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...")
      }
   }
}

Save the above program in Demo.scala. The following commands are used to compile and execute this program.

Command

\>scalac Demo.scala
\>scala Demo

Output

Missing file exception
Exiting finally...