Cad é an difríochtaí idir is-a agus has-a?

Question

Accepted Answer

**Is-a** is a relationship of *type* — modeled with **inheritance** (a `Car` *is a* `Vehicle`). **Has-a** is a relationship of *containment* — modeled with **composition** (a `Car` *has an* `Engine`). Ag roghnú an chinn cheart is cinneadh modelála lárnach a bhíonn ann.

## The two relationships

```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
}
```

## The classic test

Ask: "Is X *a kind of* Y, or does X *have/use* a 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) ✅
```

## Pitfall

Baineann daoine úsáid as oidirchealadh chun cód a athúsáid, fiú nuair is compositionis-a an gaol i ndáiríre. Mura ndéanfá an subclass a chur in ionad an bhunscoil i ngach áit, is dócha nach is-a a bhíonn ann — úsáid composition.

## Cén fáth a bhíonn sé tábhachtach

Is an t-idirdhealú seo an riail dhéanta phraicticiúil taobh thiar de "favor composition over inheritance": roghnaigh an gaol atá fíor, ní an ceann a shábhálann roinnt línte.

Ar an dóigh cheart a dhéanamh de, coinníonn sé líonraithí fúarchéimithe agus macánta, agus coisceann sé Liskov violations áit ar féidir le "subtype" gan a bheith ina ionad dá thuismitheoir.