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    <p>Most of the answers here are proposing some variant of the "null object" pattern, by denoting an empty String to mean "undefined" (as in <code>val optId = if(id.isEmpty) None else Some(id)</code>)</p> <p>The catch here is that an empty string might be a valid value! This is true of <strong>any</strong> String, though you can mitigate the problem by using something really outrageous, possibly involving non-printable characters. e.g:</p> <pre><code>val UndefinedString = "THIS-IS-A-REALLY-UNLIKELY-VALID-VALUE" private def div( id: String = UndefinedString, cssClass: String = UndefinedString ): JQuery = { val optId = Option(id) filter (_ != UndefinedString ) val optCssClass = Option(cssClass) filter (_ != UndefinedString ) ... // deal with optId and optCssClass using the Scala-way™ ... } </code></pre> <p>Better still, you could use a different type to denote your null object. As you can't subclass <code>String</code> you'll have to bump your params up the type hierarchy and make them <code>CharSequence</code>s</p> <pre><code>object NullCharSeq extends CharSequence { def charAt(idx: Int): Char = ??? def length(): Int = 0 def subSequence(start: Int, end: Int): CharSequence = this def toString(): String = ??? } def charSeqToOptStr(cs: CharSequence): Option[String] = cs match { case NullCharSeq =&gt; None case x =&gt; Option(x) map (_.toString) } private def div( id: CharSequence = NullCharSeq, cssClass: CharSequence = NullCharSeq ): JQuery = { val optId = charSeqToOptStr(id) val optCssClass = charSeqToOptStr(cssClass) ... // deal with optId and optCssClass using the Scala-way™ ... } </code></pre> <p>It's a heavyweight pattern for one-shot usage, but the cost is quickly amortized if you use it a lot (<code>NullCharSeq</code> and <code>charSeqToOptStr</code> only need to be defined once in the codebase).</p> <p>There's also zero risk of mistakenly passing your "undefined" String as though it were a valid value. Plus, you gain the ability to directly accept CharBuffer/StringBuffer/StringBuilder as your arguments.</p>
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    1. POPattern for optional-parameters in Scala using null
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    1. USKevin Wright
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    1. COHow is this different / better than my approach of just using null and wrapping it with Option(param), other than the emotional satisfaction of not writing that dreaded keyword? :-D Other than that, very clever solution.
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      1. USSebastian N.
    2. COIt's definitely the emotional satisfaction :) More broadly though, Null objects like this are nicer if you're passing them between multiple methods, and it's always safe to call methods on them if correctly implemented - no risk of the infamous `NullPointerException`. That said, I've used nulls-as-defaults myself on occasion, typically when I'm creating a DSL
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      1. USKevin Wright
 

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