Explain the Liskov Substitution Principle (LSP). What happens if it's violated? (Square/Rectangle problem)
Liskov Substitution Principle
LSP states that objects of a subtype should be usable wherever objects of the base type are expected without violating the correctness assumptions of the client. The classic Square/Rectangle example demonstrates the problem: mathematically a square is a rectangle, but if Rectangle exposes independently mutable width and height, a Square cannot honor that behavioral contract. The issue is not inheritance syntax; it is incompatible behavioral expectations.
- 1
LSP is about behavioral substitutability.
- 2
A subtype must preserve the base type's meaningful guarantees.
- 3
Violations often produce surprising conditionals, exceptions or incorrect results.
- 4
The Square/Rectangle issue is caused by incompatible mutation contracts.
- 5
Composition or separate abstractions can model the domain more safely.
Follow-up Questions
- How do preconditions and postconditions relate to LSP?
- How can composition solve the Square/Rectangle problem?
- Is every IS-A relationship automatically LSP compliant?
- How can you detect LSP violations during design?