Object-Oriented Programming in Smalltalk: Concepts and Principles

July 16, 2024

Welcome back to our Smalltalk programming series! In this post, we will delve into the world of object-oriented programming in Smalltalk. Object-oriented programming (OOP) is a programming paradigm that revolves around the concept of objects, which encapsulate data and behavior. Smalltalk, being a pure object-oriented language, provides a rich environment for understanding and implementing OOP concepts.

Encapsulation in Smalltalk

Encapsulation is the concept of bundling data (attributes) and methods (behaviors) that operate on the data into a single unit known as an object. In Smalltalk, everything is an object, and encapsulation is a fundamental principle. Let’s take a look at a simple example:

Object subclass: #Person
    instanceVariableNames: 'name age'
    
    name: aName age: anAge
        name := aName.
        age := anAge.
    
    getName
        ^name.
    
    getAge
        ^age.

In the above Smalltalk code snippet, we define a Person class with name and age as instance variables. The methods name:, age:, getName, and getAge encapsulate the data and provide interfaces to access and manipulate it.

Inheritance in Smalltalk

Inheritance is a key concept in OOP that allows a class to inherit attributes and behaviors from another class. In Smalltalk, inheritance is achieved through subclassing. Let’s extend our Person class with a subclass Employee:

Person subclass: #Employee
    instanceVariableNames: 'salary'
    
    initializeWithName: aName age: anAge salary: aSalary
        super initializeWithName: aName age: anAge.
        salary := aSalary.
    
    getSalary
        ^salary.

The Employee class inherits the name and age attributes from the Person class and adds a new attribute salary. The method initializeWithName:age:salary: initializes the object with the provided values.

Polymorphism in Smalltalk

Polymorphism is the ability of objects of different classes to respond to the same message in different ways. In Smalltalk, polymorphism is achieved through message passing. Let’s demonstrate polymorphism with our Person and Employee classes:

| person employee |
person := Person new.
employee := Employee new.

person name: 'Alice' age: 30.
employee initializeWithName: 'Bob' age: 35 salary: 50000.

{person getAge. employee getSalary}.

In the above code snippet, both the person and employee objects respond to the messages getAge and getSalary respectively, showcasing polymorphic behavior.

Object-oriented programming in Smalltalk is a powerful paradigm that promotes code reusability, modularity, and extensibility. By understanding and applying concepts like encapsulation, inheritance, and polymorphism, developers can create robust and flexible software solutions. Stay tuned for more Smalltalk tutorials and happy coding!