learn-co-curriculum / dsc-enterprise-hsbc-scala-classes

Classes

About class

• Classes are the templates or blueprints of a construct that encapsulates state and manages behavior.
• Classes have been around for a long time and in every object oriented language.
• Since Scala is a half object oriented language, half functional language, there are naturally classes.

Our first class

• A class is public by default, so no need for a public modifier
• The (firstName:String, lastName:String) is the primary constructor!
• In Scala the primary constructor is "top-heavy" with a constructor that contains all the information
• Other constructors are smaller constructors that feed the top constructor
• The reason for the top heavy constructor is immutability

class Employee(firstName:String, lastName:String)

defined class Employee

Instantiating the class

Instantiating the class is fairly straightforward

val emp = new Employee("Dennis", "Ritchie")

emp: Employee = Employee@1d79abe9

Can’t access or modify member of a class?

• As it stands in our class above, we can neither access or modify our class
• Most of the time we don’t want to modify our class for immutability purposes
• To be able to access the members, we will predicate each of the values with val
• To be able to mutate the member variables, we predicate each of the values with var (Not recommended)

As it stands, running javap -p Employee from the shows the following Java code:

public class Employee {
  public Employee(java.lang.String, java.lang.String);
}

NOTE: javap -p shows the translated Java code from Scala.

Accessing and Mutating

• val will create a Scala-style "getter"
• var will create a Scala-style "setter"

class Employee(val firstName:String, var lastName:String)
val emp = new Employee("Dennis", "Ritchie")

defined class Employee
emp: Employee = Employee@79034a3b

println(emp.firstName)           //Works because of val

Dennis

println(emp.lastName)            //Works because of var

Ritchie

emp.lastName = "Hopper" //Works because of var

emp.lastName: String = Hopper

println(emp.lastName)

Hopper

Viewing bytecode with val and var

• The bytecode generated from javap -p Employee
  • Using val for firstName
  • Using var for lastName

public class Employee {
  private final java.lang.String firstName;
  private java.lang.String lastName;
  public java.lang.String firstName();
  public java.lang.String lastName();
  public void lastName_$eq(java.lang.String);
  public Employee(java.lang.String, java.lang.String);
}

Classes Conclusion

• Classes are templates or blueprints
• val creates accessors, methods that will allow to access the inner state
• var create mutators and accessors, mutators allow us to change inner state.

IMPORTANT: We rarely use var, it would be best to avoid.