Scala Abstract Classes: An In-Depth Guide to Polymorphism and Inheritance

What Are Abstract Classes?

Abstract classes are a form of class in Scala that cannot be instantiated directly. They are designed to serve as base classes for other classes, providing common functionality and defining a contract that derived classes must fulfill. Abstract classes can have both abstract and concrete members, allowing for a mix of defined behavior and required implementations in subclasses.

Defining Abstract Classes

To define an abstract class in Scala, use the abstract keyword, followed by the class keyword, the class name, and any constructor parameters:

codeabstract class Shape(val name: String) { 
    // ... 
} 

Abstract Members

Abstract members are methods or fields that have no implementation in the abstract class. They serve as placeholders for functionality that must be implemented by subclasses. To define an abstract member, simply declare its signature without providing an implementation:

codeabstract class Shape(val name: String) { 
    def area: Double 
} 

Concrete Members

Concrete members are methods or fields that have an implementation in the abstract class. These members provide default behavior for subclasses, which can be overridden if needed. To define a concrete member, provide its signature and implementation as you would in a regular class:

codeabstract class Shape(val name: String) { 
    def area: Double 
    
    def displayName: String = s"I am a $name" 
} 

Extending Abstract Classes

To create a class that extends an abstract class, use the extends keyword, followed by the name of the abstract class and any required constructor parameters:

codeclass Circle(val radius: Double) extends Shape("Circle") { 
    // ... 
} 

Overriding Abstract Members

When extending an abstract class, you must provide an implementation for each abstract member. This is done using the def keyword, along with the member's signature and implementation:

codeclass Circle(val radius: Double) extends Shape("Circle") {         
    override def area: Double = Math.PI * radius * radius 
} 

Overriding Concrete Members

You can also override concrete members in a subclass if you want to provide a different implementation. To do so, use the override keyword, followed by the member's signature and implementation:

codeclass Circle(val radius: Double) extends Shape("Circle") { 
override def area: Double = Math.PI * radius * radius 

override def displayName: String = s"I am a $name with radius $radius" 
} 

Abstract Classes vs. Traits

Scala also provides traits, which are similar to abstract classes but with some key differences:

• Multiple inheritance: Classes can extend multiple traits but only one abstract class.
• Constructor parameters: Abstract classes can have constructor parameters, but traits cannot.
• Initialization order: Traits are initialized in a linearized order, while abstract classes follow the standard class inheritance order.

When deciding between using an abstract class or a trait, consider the following factors:

• If you need to support multiple inheritance, use traits.
• If you need constructor parameters, use an abstract class.
• If you want to define a small, focused piece of functionality, consider using a trait.
• If you want to define a more complex base class with a specific inheritance hierarchy, consider using an abstract class.

Here's an example of using a trait alongside an abstract class:

codetrait Movable { 
    def move(x: Double, y: Double): Unit 
} 

abstract class Shape(val name: String) { 
    def area: Double 

    def displayName: String = s"I am a $name" 
} 

class Circle(val radius: Double) extends Shape("Circle") with Movable { 
    override def area: Double = Math.PI * radius * radius 

    override def move(x: Double, y: Double): Unit = { 
        // Implementation of moving the circle 
    } 
} 

Conclusion

In this blog post, we have explored Scala abstract classes in-depth, from their use cases and benefits to defining and extending them in your applications. We also compared abstract classes with traits and discussed when to use each in your Scala programs. By understanding abstract classes and how they fit into the Scala language's object-oriented programming features, you'll be better equipped to design modular, extensible, and maintainable code. With this comprehensive guide on abstract classes, you can effectively leverage Scala's powerful polymorphism and inheritance capabilities to enhance your programming skills and create high-quality applications.