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Chapter 2.2: Inside an Object

About This Page

Questions Answered: How do I implement an (individual) object in Scala? What goes on inside an object when its method is called?

Topics: The program code of a singleton object: an object’s variables; implementing methods; using this to refer to oneself.

What Will I Do? A small programming assignment follows a shortish read.

Rough Estimate of Workload:? One hour or a bit more.

Points Available: A50.

Related Projects: IntroOOP.



In this chapter, we’ll use Scala to define the behavior of individual objects. As it turns out, that endeavor calls for skills that you already have. This is because an object definition has two main parts:

  • An object’s methods are functions attached to the object. To define these functions, we can use techniques that are largely familiar from earlier chapters.
  • An object needs to keep track of its attributes. For this purpose, we can use another familiar construct: variables.

The new thing here is how we connect functions and variables to an object.

Our Goal: An Employee Object

Let’s create a singleton object that represents an individual employee in an otherwise imaginary accounting system. But first, let’s see an example of how we intend to use the object.

An employee has attributes such as name and monthly salary; the code that defines the object defines the values of these attributes. We can request those values from the object:

employee.nameres0: String = Edelweiss Fume
employee.monthlySalaryres1: Double = 5000.0

The effectful method raiseSalary expects a number as a parameter. This number is a multiplier that is applied to the old salary. The following command, for instance, gives the employee a ten-percent raise:


That command changed the object’s state, as we can verify:

employee.monthlySalaryres2: Double = 5500.0

Given a year as a parameter, the effect-free method ageInYear sends back a return value to let the caller know how old the employee will be in that year. The employee object knows its own year of birth and uses it to compute its age:

employee.ageInYear(2019)res3: Int = 54

The object has a working time, which is defined as a decimal number. The value 1.0 indicates a full-time worker (100-percent working time):

employee.workingTimeres4: Double = 1.0

We can adjust the working time simply by assigning a new value to the attribute (as in the radio example of Chapter 2.1), thereby sending the object a message: “Set your working time to 0.6.” The value 0.6 indicates part-time work; we’ll make our example employee work sixty percent of a full week:

employee.workingTime = 0.6employee.workingTime: Double = 0.6

The effectful monthlyCost method determines the employee’s monthly cost to their employer. Our object computes this cost as the product of the monthly salary (now 5500 euros per month), the working time (now 60%), and a multiplier to cover additional costs (a parameter; 1.3 in the example below):

employee.monthlyCost(1.3)res5: Double = 4290.0

Finally, the description method returns a string that details the employee’s main characteristics in English. This effect-free method takes no parameters, so we don’t need any brackets as we call it (just as we didn’t when we asked the object for its name and salary above). Here, we ask the object for its description and then print what we get:

println(employee.description)Edelweiss Fume (b. 1965), salary 0.6 * 5500.0 e/month

Defining an Object and Methods in Code

The employee object: how it looks on the inside

Here’s the code that defines our employee object. You can also find the code in singletons.scala within the IntroOOP project.

Let’s begin with an overview before we look at each method in detail.

object employee {

  var name = "Edelweiss Fume"
  val yearOfBirth = 1965
  var monthlySalary = 5000.0
  var workingTime = 1.0

  def ageInYear(year: Int) = year - this.yearOfBirth


  def raiseSalary(multiplier: Double) = {
    this.monthlySalary = this.monthlySalary * multiplier


The definition of a singleton object begins with the word object. The next word is a programmer-chosen name for referring to the object.
The body of the definition goes inside curly brackets. The brackets are required by the Scala language.
Within the brackets, we have the object’s variables; their definitions look very familiar. This is where we assign (initial) values to the variables. The value of an object’s val variable can only be examined (as in employee.yearOfBirth), but a var variable can be assigned a new value, as we did with the employee’s working time in the REPL.
Below, we have the object’s methods, whose definitions start with the word def. There is no rule in Scala that forces us to write the variables above the methods, but many programmers prefer to do so. More on the methods below.
Notice the indentations that increase in size to highlight the program’s structure.

The employee object: how it works on the inside

The employee’s methods detailed

def ageInYear(year: Int) = year - this.yearOfBirth


def raiseSalary(multiplier: Double) = {
  this.monthlySalary = this.monthlySalary * multiplier

Scala’s keyword this means: “the object whose method is being executed”. Or, from the object’s perspective: “I myself”. Follow the word with a dot, and you can refer to different members of the object. As shown in the preceding animation, this is effectively a parameter variable that works just like other parameter variables do. It receives its value “from the left of the dot” in each method call.
The code instructs the object: “When your method ageInYear is called, respond with a number that you get by subtracting the value of your own yearOfBirth variable from the given year.”
“Multiply your own monthly salary, your own working time, and the given multiplier (the last of which is stored in a local parameter variable during the method call).”
“Multiply your own monthly salary with the given multiplier and assign the result to be your monthly salary (replacing the old value).”
Chapter 1.7 introduced some punctuation rules and conventions for defining functions. They apply to methods, too. It’s common to omit the curly brackets from an effect-free method if the method body is just a single expression.
description takes no parameters and doesn’t even have a parameter list.

On this

In O1, we’ll use the word this pretty much every time we wish to refer to the members (variables and methods) of the object that is executing a method. In a technical sense, however, there is often no strict requirement to use the word as we do.

Sometimes it is necessary to use this. As an example, below is a different version of the employee object that differs from our earlier version in just the name of a single variable:

object employee {

  var name = "Edelweiss Fume"
  val year = 1965
  var monthlySalary = 5000.0
  var workingTime = 1.0

  def ageInYear(year: Int) = year - this.year
  // etc.
In this version, the year of birth is stored in a variable called simply year, not yearOfBirth, as in the original.
Therefore, the object’s variable has the same name as ageInYear’s parameter variable.
The name year without a preceding this means the year given as a parameter.
The word this specifies that we mean the object’s variable, not the parameter. If we were to omit it from this version, the method would subtract its parameter value from that very same value, and would therefore invariably return zero.

In our original employee object, it would have been possible to omit each this prefix, because none of the local variables had the same name as the object’s variables.

Even when it’s not required, the word this emphasizes where the programmer uses an object’s variables, as opposed to where they use local variables. We recommend that you always use this when referring to an object’s own variables, because it makes the code easier to understand.

To an extent, this is a matter of taste. If you wish — and if you know what you’re doing — go ahead and leave out any this keywords that don’t need to be there. Outside of O1, such omissions are common.

Create an Object

Next, you’ll get to write an object that represents a bank account. The object should work as shown in the following example.

The account object: an example scenario

An account has a balance (in euro cents) and an account number. We can ask the object to report this information:

import o1._import o1._
account.balanceres6: Int = 0
account.numberres7: String = 15903000000776FI00

We can deposit money in the account. Here, we choose to represent sums of money as integers that correspond to euro cents. Let’s deposit 200 euros and review the balance:

account.deposit(20000)account.balanceres8: Int = 20000

Depositing a negative sum doesn’t affect the balance:

account.deposit(-1000)account.balanceres9: Int = 20000

The withdraw method takes money from the account and returns the amount that was successfully withdrawn:

account.withdraw(5000)res10: Int = 5000

Withdrawals that would result in a negative balance aren’t allowed. Below, we get only 150.00 euros as we empty the account:

account.withdraw(50000)res11: Int = 15000
account.balanceres12: Int = 0

Your assignment

Write an object account that works precisely as described above. To summarize the main points:

  • It has a number "15903000000776FI00".
  • It has a balance that is initially zero but may change.
  • It has a method deposit that adds a given amount of money to the account (as long as the given parameter value is positive) and doesn’t return anything.
  • It has a method withdraw that reduces the account’s balance by a given amount, or empties the account in case the given parameter is greater than the current balance. The method returns the amount that was actually withdrawn.
    • In this assignment, you’re not required to consider the possibility that a negative number is passed to withdraw. But if you want, you can write the method to leave the account untouched in such cases.

Write your code within the file o1/singletons.scala in project IntroOOP. There’s a comment in the file that shows where exactly.

How to go about it

We suggest the following approach:

  1. Study the example scenario in detail. Note the names and types of each variable!
  2. Find the indicated spot in singletons.scala. The first line of the object definition and the required curly brackets are already there.
  3. Define the object’s variables inside the curly brackets, assigning values to them. Indent your code with two space characters per line.
  4. See if it works. Fire up your REPL, choose the IntroOOP project, enter an import, and try the expressions account.balance and account.number.
  5. Write the methods deposit and withdraw.
    • Use the min and max functions known to you from earlier chapters.
    • The algorithm you need for withdraw should be already familiar from a program in Chapter 1.8.
  6. Reset the REPL session and test your methods on different parameter values. (Reminder: you may find it convenient to use the Relaunch Interpreter and Replay History button in the corner of the REPL.)
  7. Submit your code only when you’re convinced that your object works as specified.

A+ presents the exercise submission form here.

Something wrong?

Once you’re done with the assignment, think about the object you just defined.

Did you define a var variable named balance for your account object? If you did, that’s all good. But there’s something a bit questionable about this variable that may have already occurred to you.

We took it upon ourselves to create an account object whose balance is modified through deposit and withdraw, which apply due diligence and prevent the balance from becoming negative. However, there is presently nothing that prevents us from commanding the object to set its balance to a negative value like this: account.balance = -100000

Obviously, this is no problem at all if no-one ever issues such a command. We could say that assigning a negative balance counts as an error on the part of whoever uses the account object. However, there are many programmers who agree that programs should be written so that they minimize the risk of such mistakes. Soon enough, in Chapter 3.2, you’ll learn how to restrict the operations on an object’s variables. Right now, you don’t need to worry about the matter.

There are styles of programming where it wouldn’t be considered problematic to make an inappropriate command available. Scala programmers, however, tend to be among those whose who recommend minimizing such opportunities for error so as to be more efficient and build more reliable software.

Summary of Key Points

  • Some of the objects in Scala programs are so-called singleton objects: objects defined on an individual basis.
  • The definition of a singleton object comprises variable definitions and method definitions.
    • You can define a variable to be part of an object. Such a variable can be assigned values just as we’ve done with other variables.
    • You can define methods for an object just as we’ve defined functions before.
  • It’s extremely common that a method needs to use one or more of the object’s own variables or other methods. You can use Scala’s keyword this to make an object refer to itself and its own components.
  • Links to the glossary: object, singleton object; this.

An updated concept map:
















Additional credits appear at the ends of some chapters.

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