Scala – Throw Custom Exception

exceptionscala

I'm trying to throw a custom exception.

The implementation of the custom exception class is:

case class customException(smth:String)  extends Exception

In my code I wrapped a piece of code that I'm sure throws throw an exception with try/catch to throw my customException.

try{
    val stateCapitals = Map(
      "Alabama" -> "Montgomery",
      "Alaska" -> "Juneau",
      "Wyoming" -> "Cheyenne")

    println("Alabama: " + stateCapitals.get("AlabamaA").get)
}
catch{
    case x:Exception=>throw classOf[CustomException]
}

I got a compilation error that says :

        found   : java.lang.Class[CustomException]
[INFO]  required:    java.lang.Throwable 
[INFO]       case x:Exception=>throw classOf[CustomException]

How could I throw my own custom exception on this case?
Later I'm checking if the thrown exception is of a type[x] to do something specific.

Best Answer

You're not throwing an exception, but the class of an exception (just read the compiler error message...). You have to throw an exception instance.

case x:Exception => throw new CustomException("whatever")

Related Solutions

When to throw an exception

My personal guideline is: an exception is thrown when a fundamental assumption of the current code block is found to be false.

Example 1: say I have a function which is supposed to examine an arbitrary class and return true if that class inherits from List<>. This function asks the question, "Is this object a descendant of List?" This function should never throw an exception, because there are no gray areas in its operation - every single class either does or does not inherit from List<>, so the answer is always "yes" or "no".

Example 2: say I have another function which examines a List<> and returns true if its length is more than 50, and false if the length is less. This function asks the question, "Does this list have more than 50 items?" But this question makes an assumption - it assumes that the object it is given is a list. If I hand it a NULL, then that assumption is false. In that case, if the function returns either true or false, then it is breaking its own rules. The function cannot return anything and claim that it answered the question correctly. So it doesn't return - it throws an exception.

This is comparable to the "loaded question" logical fallacy. Every function asks a question. If the input it is given makes that question a fallacy, then throw an exception. This line is harder to draw with functions that return void, but the bottom line is: if the function's assumptions about its inputs are violated, it should throw an exception instead of returning normally.

The other side of this equation is: if you find your functions throwing exceptions frequently, then you probably need to refine their assumptions.

Python – How to test that a Python function throws an exception

Use TestCase.assertRaises (or TestCase.failUnlessRaises) from the unittest module, for example:

import mymod

class MyTestCase(unittest.TestCase):
    def test1(self):
        self.assertRaises(SomeCoolException, mymod.myfunc)

Best Answer

Related Solutions

When to throw an exception

Python – How to test that a Python function throws an exception

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