Table of Contents
Item 4: Scala 3 Best Practices - Enforce noninstantiability with a private constructor
Introduction to Enforcing Noninstantiability in [[Scala 3]]
In Scala 3, there are scenarios where a class is designed to provide utility methods or constants and should not be instantiated. Enforcing noninstantiability ensures that such classes are used only in the intended way, preventing unnecessary object creation and reducing the risk of misuse. This is typically achieved by making the constructor private, which prevents direct instantiation of the class.
Advantages of Enforcing Noninstantiability in [[Scala 3]]
Enforcing noninstantiability in Scala 3 offers several advantages: 1. **Prevents Misuse**: By preventing the instantiation of a class that is not intended to be instantiated, you avoid unintended behaviors or logical errors in your code. 2. **Clarifies Intent**: A private constructor clearly communicates that the class is intended to be used as a collection of static methods or constants, not as an instantiable object. 3. **Simplifies Maintenance**: Enforcing noninstantiability simplifies maintenance by ensuring that the class is used correctly, reducing the risk of errors in future code changes. 4. **Encourages Proper Design**: This approach encourages a design where only meaningful objects are instantiated, leading to a more structured and logical codebase.
Example 1: Enforcing Noninstantiability with a Private Constructor
In Scala 3, you can enforce noninstantiability by making the constructor private, ensuring that the class cannot be instantiated:
```scala class UtilityClass private () {
throw new UnsupportedOperationException("This class cannot be instantiated")
// Static-like methods in the companion object
companion object {
def utilityMethod(): Unit = {
println("This is a utility method.")
}
}
}
// Usage @main def run() = {
UtilityClass.utilityMethod() // Works fine
try {
val obj = new UtilityClass() // Throws an error
} catch {
case e: UnsupportedOperationException => println(e.getMessage) // Outputs: "This class cannot be instantiated"
}
}
```
In this example, the `UtilityClass` has a private constructor that throws an `UnsupportedOperationException` if an attempt is made to instantiate the class. This ensures that the class can only be used for its static-like methods, which are defined in the `companion object`.
Example 2: Enforcing Noninstantiability with an Object Declaration
In cases where the class is intended to be a singleton and should not be instantiated, Scala 3 provides the `object` declaration, which automatically enforces noninstantiability:
```scala object MathUtils {
def add(a: Int, b: Int): Int = a + b
def subtract(a: Int, b: Int): Int = a - b}
// Usage @main def run() = {
println(MathUtils.add(10, 5)) // Outputs: 15 println(MathUtils.subtract(10, 5)) // Outputs: 5} ```
In this example, the `MathUtils` object is declared as an `object`, which means it cannot be instantiated and all its members are accessed in a singleton fashion. This is the preferred approach in Scala 3 for creating utility classes.
Example 3: Enforcing Noninstantiability in a Companion Object
In Scala 3, you can also enforce noninstantiability within a companion object if the class itself needs to be instantiated but certain utilities within it should not:
```scala class ExampleClass {
def instanceMethod(): Unit = {
println("Instance method in ExampleClass")
}
companion object {
// Private constructor to enforce noninstantiability of the companion object
private def apply() = new ExampleClass()
def utilityMethod(): Unit = {
println("This is a companion utility method.")
}
}
}
// Usage @main def run() = {
ExampleClass.utilityMethod() // Works fine
try {
val obj = ExampleClass.apply() // This will not compile because apply is private
} catch {
case e: Exception => println(e.getMessage) // This will not be reached
}
}
```
In this example, the `ExampleClass` has a `companion object` that includes a private `apply` method. The constructor is marked private to prevent any instantiation of the companion object, enforcing that it only serves as a utility provider.
Example 4: Enforcing Noninstantiability in Abstract Classes
If the class is abstract and should not be instantiated but may be extended, a private constructor can still enforce noninstantiability for the base class:
```scala abstract class AbstractUtilityClass private () {
companion object {
def utilityMethod(): Unit = {
println("This is an abstract utility method.")
}
}
}
// Usage @main def run() = {
AbstractUtilityClass.utilityMethod() // Works fine
try {
val obj = new AbstractUtilityClass() {} // This will not compile because AbstractUtilityClass is abstract and its constructor is private
} catch {
case e: Exception => println(e.getMessage) // This will not be reached
}
}
```
In this example, the `AbstractUtilityClass` is abstract with a private constructor, ensuring that it cannot be instantiated directly. This approach is useful when you want to create utility classes that can be extended but not instantiated on their own.
When to Enforce Noninstantiability in [[Scala 3]]
Enforcing noninstantiability is particularly useful in the following scenarios: - **Utility Classes**: When creating a class that contains only static-like methods or constants and is not meant to be instantiated. - **Singleton Objects**: When you want to ensure that a class is never instantiated directly but can still be accessed in a controlled manner through a singleton object or a similar mechanism. - **Abstract Utility Classes**: When creating abstract utility classes that should not be instantiated but may be extended by other classes. - **API Design**: When designing an API or library where certain classes should not be instantiated by users, enforcing noninstantiability can prevent misuse and clarify the intended usage.
Conclusion
In Scala 3, enforcing noninstantiability with a private constructor is a best practice when you want to prevent a class from being instantiated. This technique is particularly useful for utility classes, singleton patterns, and situations where instantiating the class would lead to logical errors or misuse. By enforcing noninstantiability, you can write more maintainable, clear, and reliable code, especially in scenarios where class instances are not needed or should be tightly controlled.