Traits are used to share interfaces and fields between classes. They are similar to Java 8’s interfaces. Classes and objects can extend traits, but traits cannot be instantiated and therefore have no parameters.
Defining a trait
A minimal trait is simply the keyword trait and an identifier:
traitHairColorTraits become especially useful as generic types and with abstract methods.
traitIterator[A]{defhasNext:Booleandefnext():A}traitIterator[A]:defhasNext:Booleandefnext():AExtending the trait Iterator[A] requires a type A and implementations of the methods hasNext and next.
Using traits
Use the extends keyword to extend a trait. Then implement any abstract members of the trait using the override keyword:
traitIterator[A]{defhasNext:Booleandefnext():A}classIntIterator(to:Int)extendsIterator[Int]{privatevarcurrent=0overridedefhasNext:Boolean=current<tooverridedefnext():Int={if(hasNext){valt=currentcurrent+=1t}else0}}valiterator=newIntIterator(10)iterator.next()// returns 0iterator.next()// returns 1traitIterator[A]:defhasNext:Booleandefnext():AclassIntIterator(to:Int)extendsIterator[Int]:privatevarcurrent=0overridedefhasNext:Boolean=current<tooverridedefnext():Int=ifhasNextthenvalt=currentcurrent+=1telse0endIntIteratorvaliterator=IntIterator(10)iterator.next()// returns 0iterator.next()// returns 1This IntIterator class takes a parameter to as an upper bound. It extends Iterator[Int] which means that the next method must return an Int.
Subtyping
Where a given trait is required, a subtype of the trait can be used instead.
importscala.collection.mutable.ArrayBuffertraitPet{valname:String}classCat(valname:String)extendsPetclassDog(valname:String)extendsPetvaldog=newDog("Harry")valcat=newCat("Sally")valanimals=ArrayBuffer.empty[Pet]animals.append(dog)animals.append(cat)animals.foreach(pet=>println(pet.name))// Prints Harry Sallyimportscala.collection.mutable.ArrayBuffertraitPet:valname:StringclassCat(valname:String)extendsPetclassDog(valname:String)extendsPetvaldog=Dog("Harry")valcat=Cat("Sally")valanimals=ArrayBuffer.empty[Pet]animals.append(dog)animals.append(cat)animals.foreach(pet=>println(pet.name))// Prints Harry SallyThe trait Pet has an abstract field name that gets implemented by Cat and Dog in their constructors. On the last line, we call pet.name, which must be implemented in any subtype of the trait Pet.
More resources
- Learn more about traits in the Scala Book
- Use traits to define Enum
