Scala Trait – Multiple Class and Examples with New Experiments

1. Objective

In our last Scala Tutorial, we studied Scala Annotations and in this Scala Trait tutorial, we will learn about what is a trait in Scala Programming Language. Moreover, we will discuss syntax and examples of Scala traits. Along with this, we will look at Scala traits with abstract and non-abstract methods. At last, we will cover override with a trait and multiple traits in a Class.

So, let’s start Scala Trait.

Scala Trait - Multiple Class and Examples with New Experiments

Scala Trait – Multiple Class and Examples with New Experiments

2. Introduction to Scala Trait

Traits in Scala are like partially implemented interfaces. It may contain abstract and non-abstract methods. It may be that all methods are abstract, but it should have at least one abstract method. Not only are they similar to Java interfaces, Scala compiles them into those with corresponding implementation classes holding any methods implemented in the traits.

You can say that using Scala trait, we can share interfaces and fields between classes. Within trait, we can declare variables and values. When we do not initialize them, they are abstract. In other cases, the implementing class for the trait internally implements them.

Finally, we can never instantiate Scala traits, and it has no parameters, but we can have classes and objects extend it.
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3. A Syntax of Scala Trait

To define trait, we use the ‘trait’ keyword:

scala> trait Footwear

defined trait Footwear

We can also use Scala traits with abstract methods and as generic types:

scala> trait Iterator[A]{
| def hasNext:Boolean
| def next():A
| }

defined trait Iterator
Now, let’s take an example of a simple trait.

4. Scala Trait Example

scala> trait Greeting{
| def greet()
| }
defined trait Greeting
scala> class Rendezvous extends Greeting{
| def greet(){
| println("Hello")
| }
| }
defined class Rendezvous
scala> var r=new Rendezvous()
r: Rendezvous = Rendezvous@4cf5d999
scala> r.greet()

Hello
This was a simple trait with one abstract method. Now, let’s take another example.
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5. With Abstract and Non-Abstract Methods

Here, we will see the example of trait with the abstract and non-abstract method.

scala> trait Greeting{
| def greet()
| def show(){ //non-abstract
| println("This is the out method")
| }
| }
defined trait Greeting
scala> class Rendezvous extends Greeting{
| def greet(){
| println("Hello")
| }
| }
defined class Rendezvous
scala> var r=new Rendezvous()
r: Rendezvous = Rendezvous@4c6fe482
scala> r.greet()

Hello

scala> r.show()

This is the out method
While we cannot extend multiple abstract classes, we can extend multiple traits in Scala.

6. Multiple Traits in a Class

For a class extending multiple traits in Scala, we use ‘extends’ for one trait and ‘with’ for the rest. Hence, using traits, we can achieve multiple inheritances.

scala> trait A{
| def showA()
| }
defined trait A
scala> trait B{
| def showB()
| }
defined trait B
scala> class MyClass extends A with B{
| def showA(){
| print("A")
| }
| def showB(){
| print("B")
| }
| }
defined class MyClass
scala> var m=new MyClass()
m: MyClass = MyClass@3a4181ba
scala> m.showA() 
A
Scala> m.showB()
B

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7. An Experiment with Trait in Scala

What happens when we do not implement all members of Scala trait in a class that extends it? Let’s find out.

scala> trait Greeting{Scala Trait Mixins
Scala Trait Mixins
| def greet()
| }
defined trait Greeting
scala> class Rendezvous extends Greeting{
| def show(){
| println("This is a rendezvous")
| }
| }
<console>:12: error: class Rendezvous needs to be abstract, since method greet in trait Greeting of type ()Unit is not defined
class Rendezvous extends Greeting{
^

Since we did not implement greet() from the trait Greeting, we must declare it abstract. This works:

scala> trait Greeting{
| def greet()
| }
defined trait Greeting
scala> abstract class Rendezvous extends Greeting{
| def show(){
| println("This is a rendezvous")
| }
| }
defined class Rendezvous

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8. Override with Scala Trait

To implement abstract members of a trait, we can use the ‘override’ keyword.

scala> trait Iterator[A]{
| def hasNext:Boolean
| def next():A
| }
defined trait Iterator
scala> class IntIterator(to:Int) extends Iterator[Int]{
| private var current=0
| override def hasNext:Boolean=current<to
| override def next():Int={
| if(hasNext){
| val t=current
| current+=1
| t
| } else 0
| }
| }
defined class IntIterator
scala> val iterator=new IntIterator(10)
iterator: IntIterator = IntIterator@5b2728db
scala> iterator.next()
res6: Int = 0
scala> iterator.next()
res7: Int = 1

So, this was all about Scala Trait Tutorial. Hope you like our explanation.
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9. Conclusion

Hence, in this article, we can say that Scala trait is a partially implemented interface. We can use it to share fields and interfaces between classes. In addition, we got know about example and syntax of Scala traits and experiment with Scala trait. In conclusion, we cover the multiple Scala traits in a class and use abstract and non-abstract methods with scala trait. Furthermore, if you have any query, feel free to ask in the comment section.
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For reference

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