Liskov Substitution Principle Summary

LSP Definition

• Subtype must be completely substitutable for its supertype
• Subtype must preserve behavior expected of supertype
• Formal Definition: Let φ(x) be a property provable about objects x of type T. Then φ(y) should be true for objects y of type S where S <: T
• A subclass should pass all test cases of its superclass

Preventing Inheritance

• Use final keyword to prevent inheritance
• Example:

  final class String {  // Cannot be inherited
    // implementation
  }
  

Preventing Method Overriding

• Use final keyword on methods
• Prevents subclasses from changing behavior
• Example:

  class Circle {
    final double getArea() {  // Cannot be overridden
      return Math.PI * r * r;
    }
  }
  

Best Practices

• Design inheritance hierarchies carefully
• Document expected behavior and constraints
• Use final when inheritance not intended
• Test subclasses against superclass contracts
• Consider composition over inheritance
• Make classes immutable when possible
• Write clear specifications for methods