Scala provides a great functionality for you in order to extend and enrich your classes' behaviors. These traits are similar to the interface in which you define the function prototypes or signatures. So, with this, you can have mix-ins of functionality coming from different traits and, in this way, you enriched your classes' behavior. So, what's so good about traits in Scala? They enable the composition of classes from these traits, with traits being the building blocks. As always, let's look at in an example. This is how a conventional logging routine is set up in Java:
Note that, even though you can mix in any number of traits you want. Moreover, like Java, Scala does not have the support of multiple inheritances. However, in both Java and Scala, a subclass can only extend a single superclass. For example, in Java:
class SomeClass {
//First, to have to log for a class, you must initialize it
final static Logger log = LoggerFactory.getLogger(this.getClass());
...
//For logging to be efficient, you must always check, if logging level for current message is enabled
//BAD, you will waste execution time if the log level is an error, fatal, etc.
log.debug("Some debug message");
...
//GOOD, it saves execution time for something more useful
if (log.isDebugEnabled()) { log.debug("Some debug message"); }
//BUT looks clunky, and it's tiresome to write this construct every time you want to log something.
}
For a more detailed discussion, refer to this URL https://stackoverflow.com/questions/963492/in-log4j-does-checking-isdebugenabled-before-logging-improve-performance/963681#963681.
However, it's different with traits. It's very tiresome to always check for the log level being enabled. It would be good, if you could write this routine once and reuse it anywhere, in any class right away. Traits in Scala make this all possible. For example:
trait Logging {
lazy val log = LoggerFactory.getLogger(this.getClass.getName)
//Let's start with info level...
...
//Debug level here...
def debug() {
if (log.isDebugEnabled) log.info(s"${msg}")
}
def debug(msg: => Any, throwable: => Throwable) {
if (log.isDebugEnabled) log.info(s"${msg}", throwable)
}
...
//Repeat it for all log levels you want to use
}
If you look at the preceding code, you will see an example of using string starting with s. This way, Scala offers the mechanism to create strings from your data called String Interpolation.
String Interpolation, allows you to embed variable references directly in processed string literals. For example:
scala> val name = "John Breslin"
scala> println(s"Hello, $name") // Hello, John Breslin.
Now, we can get an efficient logging routine in a more conventional style as a reusable block. To enable logging for any class, we just mix in our Logging trait! Fantastic! Now that's all it takes to add a logging feature to your class:
class SomeClass extends Logging {
...
//With logging trait, no need for declaring a logger manually for every class
//And now, your logging routine is either efficient and doesn't litter the code!
log.debug("Some debug message")
...
}
It is even possible to mix-up multiple traits. For example, for the preceding trait (that is, Logging) you can keep extending in the following order:
trait Logging {
override def toString = "Logging "
}
class A extends Logging {
override def toString = "A->" + super.toString
}
trait B extends Logging {
override def toString = "B->" + super.toString
}
trait C extends Logging {
override def toString = "C->" + super.toString
}
class D extends A with B with C {
override def toString = "D->" + super.toString
}
However, it is noted that a Scala class can extend multiple traits at once, but JVM classes can extend only one parent class.
Now, to invoke the above traits and classes, use new D() from Scala REPL, as shown in the following figure:
Figure 10: Mixing multiple traits
Everything has gone smoothly so far in this chapter. Now, let's move to a new section where we will discuss some topics for the beginner who wants to drive themselves into the realm of Scala programming.
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