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Scala Class Examples: Trait Keyword
Use a class and create it with the new-keyword. A trait is used to extend a class.Classes. A Game class has fields and methods. It has the current score. It has players. And it can take turns with a play method.
With traits, and the trait keyword, we can extend features of our classes. A Game might be extended to adjust the rules. It might add features or options.
First example. Here we introduce a Test class. On this class we find a method print(). This writes a greeting to the screen. We create a new instance of Test with the new-keyword.
Scala program that uses class
class Test {
def print() = {
// Print a greeting.
println("Hello")
}
}
// Create new instance of Test class.
val t = new Test()
t.print()
Output
Hello
Traits. A trait is used to extend a class. We can compose a class by adding those traits to its definition. This allows us to combine many capabilities in a class.
Here: We have a Page class, and an Image and Paragraphs traits. A trait is defined in a similar way as a class, but uses the trait keyword.
ComplexPage: This class is based on the Page class, but also adds the Image and Paragraphs traits. It contains all three methods.
Finally: We create a ComplexPage instance and call print, printImages and printParagraphs from the Page class and the two traits.
Scala program that uses traits
class Page {
def print() = {
println("Page")
}
}
trait Image {
def printImages() = {
// Print images.
println("Images")
}
}
trait Paragraphs {
def printParagraphs() = {
// Print paragraphs.
println("Paragraphs")
}
}
// Extend Page.
// ... Add Image and Paragraphs traits.
class ComplexPage extends Page
with Image
with Paragraphs {
}
// Create ComplexPage instance.
// ... Use Page, Image, Paragraphs methods on it.
val example = new ComplexPage()
example.print()
example.printImages()
example.printParagraphs()
Output
Page
Images
Paragraphs
A review. Scala provides both functional and object-oriented features. With classes and traits we compose complex models. This is streamlined but conceptually similar to Java.
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