Reference Immutatibility vs object immutability

It's important to understand the difference between immutability of reference and immutability of objects.

scala> class Car(brand:String, year:Int) {
     |      var fuelPercentage = 20
     |      def getBrand() = brand
     |      def getYear() = year
     |      def fillTank(): Unit = fuelPercentage = 100
     |      def getFuelPercentage() = fuelPercentage
     |      override def toString = getBrand() + " from " + year
     | }
defined class Car

scala>

scala> val mycar = new Car("Citroen", 2011)
mycar: Car = Citroen from 2011

scala> mycar.getFuelPercentage()
res2: Int = 20

scala> mycar.fillTank()

scala> mycar.getFuelPercentage()
res4: Int = 100

scala> mycar = new Car("Audi", 2012)
:9: error: reassignment to val
       mycar = new Car("Audi", 2012)
             ^

scala> var hiscar = new Car("Audi", 2012)
hiscar: Car = Audi from 2012

scala> hiscar = new Car("Porsche", 2012)
hiscar: Car = Porsche from 2012

scala>

As you can see mycar is an immutable reference because we used the 'val' keyword. hiscar on the other hand is a mutable reference due to using the keyword 'var'.  Both car objects are mutable. We can fill the tank up and the fuelpercentage will change.