How can I simulate Haskell's “Either a b” in Java

Question

How can I write a typesafe Java method that returns either something of class a or something of class b? For example:

public ... either(boolean b) {
  if (b)         


        

Answer 1

Since you've tagged Scala, I'll give a Scala answer. Just use the existing Either class. Here's an example usage:

def whatIsIt(flag: Boolean): Either[Int,String] = 
  if(flag) Left(123) else Right("hello")

//and then later on...

val x = whatIsIt(true)
x match {
  case Left(i) => println("It was an int: " + i)
  case Right(s) => println("It was a string: " + s)
}

This is completely type-safe; you won't have problems with erasure or anything like that... And if you simply can't use Scala, at least use this as an example of how you can implement your own Either class.