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copied!<p>Their role is a bit similar to the ones (i.e. multiple type parameter) in a class, since traits are, after all, classes (without any constructor) meant to be added to some other class as a mixin.</p> <p>The <a href="http://www.scala-lang.org/docu/files/ScalaReference.pdf" rel="nofollow">Scala spec</a> gives the following example for Trait with multiple parameters:</p> <blockquote> <p>Consider an abstract class Table that implements maps from a type of keys A to a type of values B.<br> The class has a method set to enter a new key/value pair into the table, and a method get that returns an optional value matching a given key.<br> Finally, there is a method apply which is like get, except that it returns a given default value if the table is undeﬁned for the given key. This class is implemented as follows.</p> </blockquote> <pre><code>abstract class Table[A, B](defaultValue: B) { def get(key: A): Option[B] def set(key: A, value: B) def apply(key: A) = get(key) match { case Some(value) => value case None => defaultValue } } </code></pre> <blockquote> <p>Here is a concrete implementation of the Table class.</p> </blockquote> <pre><code>class ListTable[A, B](defaultValue: B) extends Table[A, B](defaultValue) { private var elems: List[(A, B)] def get(key: A) = elems.find(._1.==(key)).map(._2) def set(key: A, value: B) = { elems = (key, value) :: elems } } </code></pre> <blockquote> <p>Here is a trait that prevents concurrent access to the get and set operations of its parent class</p> </blockquote> <pre><code>trait Synchronized Table[A, B] extends Table[A, B] { abstract override def get(key: A): B = synchronized { super.get(key) } abstract override def set((key: A, value: B) = synchronized { super.set(key, value) } } </code></pre> <blockquote> <p>Note that <code>SynchronizedTable</code> does not pass an argument to its superclass, <code>Table</code>, even though <code>Table</code> is deﬁned with a formal parameter.<br> Note also that the super calls in <code>SynchronizedTable</code>’s get and set methods statically refer to abstract methods in class <code>Table</code>. This is legal, as long as the calling method is labeled abstract override (§5.2).</p> <p>Finally, the following mixin composition creates a synchronized list table with strings as keys and integers as values and with a default value 0:</p> </blockquote> <pre><code>object MyTable extends ListTable[String, Int](0) with SynchronizedTable </code></pre> <blockquote> <p>The object <code>MyTable</code> inherits its get and set method from <code>SynchronizedTable</code>.<br> The super calls in these methods are re-bound to refer to the corresponding implementations in <code>ListTable</code>, which is the actual supertype of <code>SynchronizedTable</code> in <code>MyTable</code>.</p> </blockquote>

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