PrintAnimal.scala

scala> class Animal 
defined class Animal 
scala> class Dog extends Animal 
defined class Dog 
scala>import java.util.ArrayList 

scala> def printAnimals(animal: ArrayList[Animal]) = {
     | for(i <- 0 to animal.size - 1) {
     | println(animal.get(i))
     | }
     | }
printAnimals: (animal: java.util.ArrayList[Animal])Unit

scala> var dogsList = new ArrayList[Dog]
dogsList: java.util.ArrayList[Dog] = []

scala> printAnimals(dogsList)
<console>:15: error: type mismatch;
 found   : java.util.ArrayList[Dog]
 required: java.util.ArrayList[Animal]
Note: Dog <: Animal, but Java-defined class ArrayList is invariant in type E.
You may wish to investigate a wildcard type such as `_ <: Animal`. (SLS 3.2.10)
       printAnimals(dogsList)