scala try/catch not catching some exceptions

Question

Unless the try/catch doesn't work the why I think it should work, I'd expect the following exception be caught. Instead it just throws NPE.

try {
  scala.io.Source.fromInputStream(null)
} catch {
  case e:Throwable => println("oh cr*p!")
}

In contrast, the following code does work.

try {
  1/0
} catch {
  case e:Throwable => println("oh cr*p")
}

Answer 1

io.Source is lazy, thus does not evaluate its input, until it is needed. Therefore the exception is not thrown when it is initialized, but when it is used for the first time. This example shows that:

scala> class Foo(val x: io.Source)
defined class Foo

scala> new Foo(io.Source.fromInputStream(null))
res2: Foo = Foo@1c79f780

No exception here. But as soon, as you use it (in this case to print it to the console) it throws an exception:

scala> res2.x
java.lang.NullPointerException
    at java.io.Reader.<init>(Reader.java:78)
    at java.io.InputStreamReader.<init>(InputStreamReader.java:129)

And a little tip: don't catch throwables, because that will also catch things like StackOverflowError and OutOfMemoryError, which you don't want to be catched.