Eleza Liskov Substitution Principle kwa kina.

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Accepted Answer

**Liskov Substitution Principle (LSP)** inasema kuwa vitu vya subtype lazima viweze kubadilishwa kwa aina yao ya msingi **bila kuvunja usahihi wa programu**. Ikiwa msimbo unafanya kazi na `Base`, lazima ufanye kazi na `Subclass` yoyote — matarajio sawa ya tabia, hakuna mshangao.

## Ukiukaji wa kawaida: 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` *kinavunja mkataba* wanaotegemea wapiaji: kuweka upana na urefu kwa kujitegemea. Inapita mtihani wa Kiingereza wa "is-a" lakini **inakamatia LSP**.

## LSP inachohitaji halisi

```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)
```

Hii ni **kontravariansa ya ingizo, kovariansa ya pato** — kiini rasmi cha LSP.

## Jinsi ya kurekebisha mfano

Acha urithi: `Square` na `Rectangle` zote ni `Shape`s yenye `area()`, lakini moja si nyingine. Mtunzaji tabia, si taksonomia naive.

## Kwa nini ni muhimu

LSP ndilo linalolifanya polymorphism *salama*: bila hilo, kila `if (x instanceof Square)` anguka kilichambishi nyuma, kukamatia ncha ya aina ndogo.

Inaandika upya muundo wa urithi kuzunguka **mkataba wa tabia**, si fasihi — sababu halisi ya kuwa "pendelea muundo" ipo ni kwamba mahusiano mengi ya is-a kwa kimya yanakamatia LSP.