What happens if a sealed class and its permitted subclasses are spread across different packages within the same module?

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Key Points:

1. Module Boundaries:
   • Sealed classes and their permitted subclasses must be declared in the same module. This ensures that the class hierarchy is well-defined and controlled within the module.
   • The permits clause in the sealed class declaration specifies which classes are allowed to extend or implement it. These permitted subclasses must be declared in the same module as the sealed class.
2. Package Visibility:
   • While sealed classes and their permitted subclasses can be spread across different packages within the same module, they must still adhere to the visibility rules of Java. This means that the subclasses must have appropriate access modifiers to be visible and usable within the module.
   • The subclasses can be declared as public, protected, or package-private (default access), but they must be accessible within the module where the sealed class is defined.
3. Compilation and Runtime:
   • During compilation, the Java compiler ensures that all permitted subclasses are known and declared within the same module. This allows for compile-time checking of the class hierarchy, ensuring that no unauthorized subclasses can extend the sealed class.
   • At runtime, the classloader mechanism ensures that the sealed class and its permitted subclasses are loaded from the same module. The classloader follows the delegation model, where classes are loaded from parent to child classloaders, ensuring that classes are loaded only once and maintaining their uniqueness.
4. Classloader Hierarchy:
   • The classloader hierarchy in Java includes the Bootstrap ClassLoader, Platform ClassLoader, and Application ClassLoader. Each classloader searches for classes in its defined locations and delegates the search if necessary.
   • When a sealed class is encountered, the classloader ensures that its permitted subclasses are loaded from the same module. This is enforced by the visibility and uniqueness principles of the classloader mechanism.
5. Benefits of Sealed Classes:
   • Enhanced Security: Sealed classes provide an additional layer of security by restricting the inheritance and implementation of its subclasses. This prevents unauthorized modifications and ensures that only trusted subclasses can access and modify the sealed class.
   • Better Control over Class Hierarchies: By explicitly defining the allowed subclasses, developers can ensure that only relevant and properly designed classes extend or implement the sealed class, leading to a more maintainable codebase.
   • Improved Code Flexibility: By restricting the subclasses that can extend or implement a sealed class, developers can make necessary changes or refactoring without worrying about breaking any external code. This provides a sense of flexibility while working with sealed classes and ensures that the codebase remains stable and maintainable.

Example Scenario:

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// In com.example.shapes package
public sealed class Shape permits Circle, Rectangle {
    // Class body
}

// In com.example.shapes package
public final class Circle extends Shape {
    // Class body
}

// In com.example.errors package
public final class Rectangle extends Shape {
    // Class body
}

• The sealed class Shape is declared in com.example.shapes.
• The permitted subclasses Circle and Rectangle are declared in com.example.shapes and com.example.errors, respectively.
• The classloader ensures that all these classes are loaded from the same module com.example, maintaining the integrity of the sealed class hierarchy.