the caller · fixed code
s.setWidth(5);
s.setHeight(4);
// expect area == 20
pass in a substitute
what the bench sees
width × height
— pick a substitute
The substitution test
The caller code never changes. Pass a Rectangle and it should hold its promise — pass a Square and watch what happens to the width you set.
▸ Watch the box animate when you pass a Square: setHeight(4) quietly drags the width down too.
the caller · fixed code
bird.fly();
// expect it takes off
pass in a substitute
what the bench sees
🐦
a Bird, ready to fly()
— pick a substitute
The substitution test
The base Bird promises fly() works. A Sparrow keeps that promise. A Penguin can't fly — so what does its fly() do?
▸ A subtype that overrides a method just to throw is a degenerate override — a loud LSP smell.
the corrected design — every substitute passes
We stop forcing bad extends trees. Shapes share a tiny immutable Shape interface (just area(), no setters to break). Birds split into a plain Bird and a separate FlyingBird capability. Click any substitute below — they all keep their promises now.
✓ Substitutability restored
Pick a substitute above to see why it now passes the bench with no surprises.
▸ The rule: when a subtype can't keep the base's full contract, prefer a shared interface or composition over inheritance.