Exception handling

In Scala exceptions are not checked so effectively all exceptions are runtime 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. This creates a very powerful but light-weight way to handle exceptions:

1. scala> def handle( f: => Unit ) = {
2.      | try { f } catch {
3.      | case _:AssertionError => println ("Whoops an assertion error")
4.      | case r:RuntimeException => println ("Runtime Exception: "+ r.getStackTraceString)
5.      | case e if (e.getMessage == null) => println ("Unknown exception with no message")
6.      | case e => println ("An unknown error has been caught" + e.getMessage)
7.      | }
8.      | }
9. handle: (=> Unit)Unit
11. scala> handle { throw new AssertionError("big bad error") }
12. Whoops an assertion error
14. scala> handle { throw new IllegalArgumentException("Sooooo illegal") }
15. Runtime Exception: line9$object$$iw$$iw$$iw$$anonfun$1.apply(:8)
16. line9$object$$iw$$iw$$iw$$anonfun$1.apply(:8)
17. line7$object$$iw$$iw$$iw$.handle(:7)
18. line9$object$$iw$$iw$$iw$.(:8)
19. line9$object$$iw$$iw$$iw$.()
20. RequestResult$line9$object$.(:3)
21. RequestResult$line9$object$.()
22. RequestResult$line9$object.result()
23. sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
24. sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
25. sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
26. java.lang.reflect.Method.invoke(Method.java:597)
27. scala.tools.nsc.Interpreter$Request.loadAndRun(Interpreter.scala:889)
28. scala.tools.nsc.Interpreter.interpret(Interpreter.scala:508)
29. scala.tools.nsc.Interpreter.interpret(Interpreter.scala:494)
30. scala.tools.nsc.InterpreterLoop.interpretStartingWith(InterpreterLoop.scala:242)
31. scala.tools.nsc.InterpreterLoop.command(InterpreterLoop.scala:230)
32. scala.tools.nsc.InterpreterLoop.repl(InterpreterLoop.scala:142)
33. scala.tools.nsc.InterpreterLoop.main(InterpreterLoop.scala:298)
34. scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:141)
35. scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)
37. scala> handle{ throw new java.io.IOException("cant read something") }
38. An unknown error has been caughtcant read something

Return values

As with most functional languages, most control structures ( if, for, try ) return values. The common java idiom:

1. String name=null;
2. if( xxx ) name="yyy";
3. else name="zzz";

can be replaced by

1. val name = if( xxx ) "yyy"; else "zzz";

The benefit (other than less boiler plate code) is that name can now be a val instead of a var.

Another other point about returns: The return keyword is not required when returning a value from methods or control structures. The last value is always the return value. This is why you will get an error if the last line in a method or control structure is an assignment.

Examples:

1. scala> val name = if( 1==2 ) "Jesse" else "Mauricio"
2. name: java.lang.String = Mauricio
4. scala> println(name)
5. Mauricio
7. scala> val collection = for( i <- 1 to 100; if(i%20 == 3) ) yield i
8. collection: Seq.Projection[Int] = RangeFM(3, 23, 43, 63, 83)
10. scala> collection.foreach( i => print( i +" ") )
11. 3 23 43 63 83
13. scala> val someObj:AnyRef = "Hello"
14. someObj: AnyRef = Hello
16. scala> val choice = someObj match {
17.      | case _:java.io.File => "File"
18.      | case _:String => "String"
19.      | case _ => "Dunno"
20.      | }
21. choice: java.lang.String = String
23. scala> val result = try {
24.      | "two".toInt
25.      | }catch{
26.      | case e:NumberFormatException => -1
27.      | case _ => 0
28.      | }
29. result: Int = -1
30. scala> var i=0
31. i: Int = 0
33. // while and do-while do not have return values
34. scala> while( i<4 ){
35.      | "22"
36.      | i += 2
37.      | }
39. scala> println( if(i>0) "great" else "less" )
40. great
42. // code blocks return the last statement
43. scala> val m = {
44.      | val x = 1
45.      | x + 2
46.      | }
47. m: Int = 3