Scala for the Impatients---(10)Traits

Traits as Interfaces

Let’s start with something familiar. A Scala trait can work exactly like a Java interface. For example:

trait Logger {
    def log(msg: String) // An abstract method
}

A subclass can provide an implementation:

class ConsoleLogger extends Logger { // Use extends, not implements
    def log(msg: String) { println(msg) } // No override needed
}

If you need more than one trait, add the others using the with keyword: All Java interfaces can be used as Scala traits.

class ConsoleLogger extends Logger with Cloneable with Serializable

Traits with def, val and var

Never use val in a trait for abstract members and use def instead. See http://stackoverflow.com/questions/19642053/when-to-use-val-or-def-in-scala-traits. 

Traits with Concrete Implementations

In Scala, the methods of a trait need not be abstract.

trait ConsoleLogger {
def log(msg: String) { println(msg) }
}

Here is an example of using this trait:

class SavingsAccount extends Account with ConsoleLogger {
def withdraw(amount: Double) {
if (amount > balance) log("Insufficient funds")
else balance -= amount
}
...
}

Note the difference between Scala and Java. The SavingsAccount picks up a concrete implementation from the ConsoleLogger trait. This would not be possible with a Java interface.

Objects with Traits

You can add a trait to an individual object when you construct it.

trait Logged {
def log(msg: String) { }//do nothing implementation
}