Catch exception in Scala constructor - scala

Question

What is the Scala equivalent of this Java code, where someMethodThatMightThrowException is defined elsewhere?
class MyClass {
String a;
String b;
MyClass() {
try {
this.a = someMethodThatMightThrowException();
this.b = someMethodThatMightThrowException();
} finally {
System.out.println("Done");
}
}
}

Answer 1

class MyClass {
private val (a, b) =
try {
(someMethodThatMightThrowException(),
someMethodThatMightThrowException())
} finally {
println("Done")
}
}
try is an expression in Scala, so you can use it's value. With tuples and pattern matching you can use statement to get more than one value.
Alternatively you could use almost the same code as in Java:
class MyClass {
private var a: String = _
private var b: String = _
try {
a = someMethodThatMightThrowException()
b = someMethodThatMightThrowException()
} finally {
println("Done")
}
}

Answer 2

with companion object
case class MyClass(a: String, b: String)
object MyClass {
def apply() = try {
new MyClass(
a = someMethodThatMightThrowException(),
b = someMethodThatMightThrowException()
)
} finally {
println("Done")
}
}
with constructor overload a little bit harder, because we can't wrap this(...):
def tryIt[T](something: => T) = try{
something
} finally {
println("Done")
}
case class MyClass(a: String, b: String) {
def this() = this(
tryIt(someMethodThatMightThrowException),
tryIt(someMethodThatMightThrowException)
)
}

Answer 3

What is a or b assigned if an exception occurs? Wrap a and b in a Try to handle the exceptional cases. You can also pattern match on these to extract values.
scala> class MyClass(val a: Try[String], val b: Try[String])
defined class MyClass
scala> new MyClass(Try("foo"(0).toString), Try("foo"(3).toString))
res0: MyClass = MyClass#6bcc9c57
scala> res0.a
res1: scala.util.Try[String] = Success(f)
scala> res0.b
res2: scala.util.Try[String] = Failure(java.lang.StringIndexOutOfBoundsException: String index out of range: 3)
scala> res0.a.get
res3: String = f
scala> res0.b.get
java.lang.StringIndexOutOfBoundsException: String index out of range: 3
at java.lang.String.charAt(String.java:658)
...
Edits for comment. Uses default argumens for a and b.
null is bad but that's what you asked for. See Option
class MyClass(val a: Try[String] = null, val b: Try[String] = null)
scala> new MyClass(Success("a"))
res50: MyClass = MyClass#625aaaca
scala> res50.a
res51: scala.util.Try[String] = Success(a)
scala> res50.b
res52: scala.util.Try[String] = null
scala> new MyClass(b = Success("b"))
res53: MyClass = MyClass#68157e85
scala> res53.a
res54: scala.util.Try[String] = null
scala> res53.b
res55: scala.util.Try[String] = Success(b)

Answer 4

How about something closer to:
scala> def foo = ???
foo: Nothing
scala> :pa
// Entering paste mode (ctrl-D to finish)
case class Foo(a: String = Foo.afoo, b: String = Foo.bfoo)
object Foo {
import util._
def afoo = Try (foo) recover { case _ => "a" } get
def bfoo = Try (foo) recover { case _ => "b" } get
}
// Exiting paste mode, now interpreting.
warning: there were 2 feature warning(s); re-run with -feature for details
defined class Foo
defined object Foo
scala> Foo()
res0: Foo = Foo(a,b)