Ni nini tofauti kati ya mahusiano ya is-a na has-a?

Question

Accepted Answer

**Is-a** ni mahusiano ya *aina* — yaonekanwa kwa **urithi** (kitu `Car` *ni* `Vehicle`). **Has-a** ni mahusiano ya *kuwa na* — yaonekanwa kwa **composition** (kitu `Car` *kina* `Engine`). Kuchagua kitanzi sahihi ni uamuzi muhimu wa kuiga.

## Mahusiano mawili

```java
// IS-A → inheritance
class Vehicle { void move() {} }
class Car extends Vehicle { }        // a Car IS A Vehicle

// HAS-A → composition
class Engine { void start() {} }
class Car2 {
    private Engine engine = new Engine();   // a Car HAS AN Engine
    void start() { engine.start(); }        // delegate to the part
}
```

## Mtihani wa kawaida

Uliza: "Je, X *ni aina ya* Y, au X *ina/hutumia* Y?"

```text
A Dog        IS-A Animal        → inheritance ✅
A Car        HAS-A Engine       → composition ✅
A Square     IS-A Shape         → inheritance ✅
A Manager    HAS Employees      → composition (a list) ✅
A Stack      HAS-A list (not IS-A) → composition (see earlier pitfall) ✅
```

## Ndoto

Watu wanakaribiana na urithi ili *kutumia tena kod*, hata wakati mahusiano yaliyoo ni has-a. Ikiwa haungekuwa na njia ya kubadili darasa dogo kwa msingi kila mahali, labda si is-a — tumia composition.

## Kwa nini ni muhimu

Tofauti hii ni sheria ya uamuzi wa vitendo nyuma ya "pendekeza composition juu ya urithi": chagua mahusiano yanayotaka, si ile inayookoa mistari michache.

Kupata kazi sawa inahifadhi hierarchies nyingi na adimu, na inazuia ukiukaji wa Liskov ambapo "subtype" haiwezi kweli kusimama mahali pa mzazi wake.