What are the conditions for LSP compliance?

A subclass is LSP-compliant with its parent when it satisfies these behavioral contracts: (1) Preconditions cannot be strengthened — if the parent accepts any integer, the subclass cannot restrict to positive integers only; (2) Postconditions cannot be weakened — if the parent guarantees a non-null return, the subclass cannot return null; (3) Invariants must be preserved — properties that are always true of the parent must remain true in the subclass; (4) No new exceptions — the subclass cannot throw exceptions that the parent doesn't throw (or subtypes of them); (5) History constraint — if the parent's state only changes in certain ways, the subclass must respect those constraints. These conditions together ensure that code using the parent type works correctly with any subtype.