Explain the purpose and features of methods that are accessible only within the class, and why they are used.

Liskov Substitution Principle (LSP) relates to object-oriented programming principles and is closely connected with inheritance. It states that objects of a subclass should be interchangeable with objects of the superclass without violating the correctness of the program.

That is, if class B inherits from class A, then objects of class B should behave so that they can be used in place of objects of class A without unexpected errors or behavior changes.

This helps create class hierarchies where subclasses extend the functionality of base classes without breaking their contract.

Example:

class Bird {
  void fly() {
    print('Bird is flying');
  }
}

class Sparrow extends Bird {
  @override
  void fly() {
    print('Sparrow is flying');
  }
}

// Violation of LSP if subclass cannot fly
class Ostrich extends Bird {
  @override
  void fly() {
    throw Exception('Ostrich cannot fly');
  }
}

Here, Ostrich violates the Liskov principle because it cannot replace Bird without changing behavior.