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Typing: Sealed Class

Typing: Sealed Class

sealed class TShirtSize {}

class Small extends TShirtSize {}

class Medium extends TShirtSize {}

class ExtraLarge extends TShirtSize {
  final int size;

  ExtraLarge(this.size);
}

void printSizeInfo(TShirtSize theSize) {
  // When not exhaustive, the compiler will refuse to compile
  switch (theSize) {
    case Small _:
      print("Small");
    case Medium _:
      print("Medium");
    case ExtraLarge val:
      print("E" + "x" * val.size + "tra Large");
  }
}

sealed class Role {
  void printInfo() {
    // When not exhaustive, the compiler will refuse to compile
    switch (this) {
      case Teacher teacher:
        print(
            "Name: ${teacher.name}, License: ${teacher.teachingLicense}, Major: ${teacher.teachingMajor}");
        break;
      case Student student:
        print(
            "Student: ${student.name}, ID: ${student.studentId}, School: ${student.school}");
        break;
    }
  }
}

class Teacher extends Role {
  final String name;
  final String teachingLicense;
  final String teachingMajor;

  Teacher(this.name, this.teachingLicense, this.teachingMajor);
}

class Student extends Role {
  final String name;
  final String studentId;
  final String school;

  Student(this.name, this.studentId, this.school);
}

void main() {
  printSizeInfo(Small()); // Output: Small
  printSizeInfo(Medium()); // Output: Medium
  printSizeInfo(ExtraLarge(1)); // Output: Extra Large
  printSizeInfo(ExtraLarge(3)); // Output: Exxxtra Large

  Teacher("John Doe", "123456", "Math")
      .printInfo(); // Output: Name: John Doe, License: 123456, Major: Math
  Student("Jane Doe", "123456", "University of Life")
      .printInfo(); // Output: Student: Jane Doe, ID: 123456, School: University of Life
}

The provided Dart code is straightforward and does the following:

  1. A sealed class TShirtSize is declared, restricting inheritance to a specific set of classes defined in the same file.
  2. Three classes Small, Medium, and ExtraLarge are defined that extend the TShirtSize sealed class. These represent specific t-shirt sizes.
  3. The ExtraLarge class has an integer size field, assigned via the constructor.
  4. The printSizeInfo function accepts an instance of TShirtSize and uses an exhaustive switch case to print different outputs depending on the actual class of the instance. Dart’s exhaustiveness check on sealed classes ensures that all subclasses are covered.
  5. A sealed class Role is declared with a printInfo method, which prints different information based on whether the instance is a Teacher or a Student.
  6. The Teacher and Student classes extend the Role sealed class. Each class has unique fields like name, teachingLicense, teachingMajor for Teacher and name, studentId, school for Student.
  7. In the main function, instances of Small, Medium, and ExtraLarge are created and passed to the printSizeInfo function, printing “Small”, “Medium”, “Extra Large” and “Exxxtra Large” respectively.
  8. The main function also creates instances of Teacher and Student, and calls their printInfo method, printing “Name: John Doe, License: 123456, Major: Math” and “Student: Jane Doe, ID: 123456, School: University of Life” respectively.

The key takeaways from this code are the use of sealed classes for type safety and pattern matching, exhaustive switch cases for complete handling of all subclasses, and efficient printing of information based on the class of the instance.

Last updated on
Typing: ResultTyping: tryCatch