Vysvětlete Liskovův princip substituce do hloubky.

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Accepted Answer

The **Liskov Substitution Principle (LSP)** states that objects of a subtype must be substitutable for their base type **without breaking the program's correctness**. If code works with a `Base`, it must work with any `Subclass` — same behavioral expectations, no surprises.

## Kanonické porušení: Square extends Rectangle

```java
class Rectangle {
    protected int w, h;
    void setWidth(int w)  { this.w = w; }
    void setHeight(int h) { this.h = h; }
    int area() { return w * h; }
}

class Square extends Rectangle {       // "a square IS-A rectangle"... seemingly
    void setWidth(int w)  { this.w = w; this.h = w; }   // keep sides equal
    void setHeight(int h) { this.w = h; this.h = h; }
}

void resize(Rectangle r) {
    r.setWidth(5);
    r.setHeight(4);
    assert r.area() == 20;   // holds for Rectangle... FAILS for Square (area=16)
}
```

`Square` *breaks the contract* callers rely on: setting width and height independently. It passes the "is-a" English test but **violates LSP**.

## Co LSP skutečně požaduje

```text
A subtype must NOT:
  ✗ strengthen preconditions  (demand more than the base, e.g. reject valid inputs)
  ✗ weaken postconditions     (promise less than the base)
  ✗ break invariants the base guarantees
  ✗ throw new unexpected exceptions for previously valid calls
A subtype MAY:
  ✓ weaken preconditions (accept more)   ✓ strengthen postconditions (promise more)
```

Toto je **contravariance of inputs, covariance of outputs** — the formal core of LSP.

## Jak opravit příklad

Drop the inheritance: `Square` and `Rectangle` are both `Shape`s with an `area()`, but neither is the other. Model behavior, not naive taxonomy.

## Proč to má smysl

LSP je to, co činí polymorfismus *bezpečným*: bez něj se každá kontrola `if (x instanceof Square)` vrací, což popírá smysl subtypingu.

Znovudefiniuje návrh dědičnosti kolem **kontraktů chování**, ne slovníku — skutečný důvod, proč "favor composition" existuje, je ten, že mnoho je-a vztahů tiše porušuje LSP.