Highlights

• extends and final keywords are similar to those in Java.
• Like Java, Scala supports only single-class inheritance.
• You must use the override keyword when you override a method.
  • This is similar to the @Override annotation in Java.
• You can override fields.

Extending a Class

class Employee extends Person {
    var salary = 0.0
    // ...
}

Overriding a Method

class Person(val name: String) {
    override def toString = s"${getClass.getName}[name=$name]"
}

val p = new Person("Juan")
println(p)

Person[name=Juan]

Protected Fields and Methods

• Like C++ and Java, you can declare a member to be protected.
• The member is accessible from subclasses only.
  • Unlike Java, it is not “package protected.”
  • Use package objects for that.

Overriding Fields

Recall that a field in Scala consists of:

1. A private instance field.
2. Accessors/mutators for the field.

You can override a val (or parameterless def) with another val field of the same name.

Then, the subclass will have a private field and a public accessor, which overrides the one from the super class.

class Person(val name: String) {
    override def toString = s"${getClass.getName}[name=$name]"
}

class SecretAgent(codename: String) extends Person(codename) {
    override val name = "secret"
    override def toString = "shh"
}

val s = new SecretAgent("1234")
println(s)

shh

Abstract Classes & Methods

Can also override an abstract def with a val:

abstract class Person {
    def id: Int // No implementataion, specific to each kind of person.
    // ...
}

/**
 * A concrete implementation of Person.
 */
class Student(override val id: Int) extends Person {
    // The student ID is supplied by the ctor.
    // ...
    override def toString = s"Student with ID: ${id}"
}

val s = new Student("54321")
println(s)

Student with ID: 54321

Abstract Fields

Can also have abstract fields, i.e., fields without an initial value:

abstract class Person {
    val id: Int // No initializer---abstract field and accessor.
    var name: String // No initializer---abstract field, accessor, and mutator.
    // ...
}

• Class in Java bytecode has no fields.
• Concrete subclasses must “fill” these in:

class Employee(val id: Int) extends Person { // Has a concrete id property.
    var name = "John Doe"  // and a concrete name property that can be concretized.
    override def toString = s"Employee ${name} with ID: ${id}"
}

val e = new Employee(5)
println(e)

Employee John Doe with ID: 5

Scala Inheritance Hierarchy

• Primitive type classes and Unit, i.e., (), extend AnyVal.
  • These are value classes.
  • AnyVal has no methods; it’s just “marker.”
• Reference types extend AnyRef.
  • In the bytecode, this is java.lang.Object.
  • Defines methods useful for concurrency (e.g., locking).
• Any is the hierarchy root. It defines:
  • isInstanceOf[]
  • asInstanceOf[]
  • Equality and hash code methods.
• All Scala classes implement the marker interface ScalaObject.
• Null is a singleton type with the value null.
  • You can assign null to any reference.
• Nothing has no instances.

Scala inheritance hierarchy diagram

Nothing

• Useful for generic constructs.

  • The empty list Nil has the type List[Nothing].
  • It’s a subtype of List[T] for any T.

• The method ??? has return type Nothing.

  • It actually never returns but throws NotImplementedErr.
  • Useful for methods that have not yet been implemented:

  class Person(val name: String) {
      def speak() {println(s"My name is ${name}.")}
      def description = ???
  }
  
  val p = new Person("Sofia")
  p.speak() // OK!
  println(p.description)

  Outputs:

  My name is Sofia.
  scala.NotImplementedError: an implementation is missing ...

Object Equality

Value Classes

• For “wrapper” classes that have a single field. Can you think of any?
• Inefficient to allocate a new object that holds only one value.
• Value classes allow you to “inline” field access so it is accessed directly.

A value class:

1. Extends AnyVal.
2. Has a primary ctor with only one (val) parameter, i.e., the “wrapped” value, and no body.
3. Has no other fields or ctors.
4. Automatically provides equals and hashCode methods that simply delegate to the wrapped value.

Example

class MilTime(val time: Int) extends AnyVal {
    def minutes = time % 100
    def hours = time / 100
    override def toString = f"$time%04d"
}