Define final members in Scala

To design an inheritance hierarchy at some point, you want to make sure that members are not quilt class overloaded. In Scala, as in Java, you do this by adding the final modifier to a member. For example, you could put a final modifier in front of the Arrayelement demo method, as shown in code 10.7.

Class Arrayelement extends Element {
Final Override

def demo () {
println ("Arrayelement ' s implementation invoked")
}
}

Code 10.7 declaring the final method

With this version of Arrayelement, an attempt at its subclass, Lineelement, overloaded demo method, will compile however:

Elem.scala:18:error:error Overriding method Demo
In class

arrayelement the type () unit;
Method Demo cannot override final
Override Def demo () {
ˆ

You may also want to make sure that the entire class has no subclasses. To do this simply declare the entire class as final by adding the final modifier to the declaration of the class. For example, code 10.8 shows how to declare arrayelement as final:

Final class Arrayelement extends Element

{
Override Def demo () {
println ("Arrayelement ' s implementation

invoked")
}
}

Code 10.8 declaring final class

With this version of Arrayelement, any attempt to define subclasses will fail:

Elem.scala:18:error:

illegal inheritance from final class
Arrayelement
Class Lineelement

extends Arrayelement {
ˆ

We will now remove the final modifier and demo method and return to the previously implemented element family. We will focus the rest of our chapter on completing the working version of the layout library.