Ni nini vitu visivyobadilika na kwa nini kuavitumia?

Question

Accepted Answer

**Kitu kisivyobadilika** hakiwezi kubadiliwaa baada ya kujenengwa — kila sehemu imewekwa mara moja na hakuna mbinu zinazobadilisha hali. Ili ku"badilisha" kitu, unajenga kitu kipya.

## Kujenga aina isivyobadilika

```java
public final class Money {              // final: can't be subclassed to add mutation
    private final int cents;            // final fields, set once
    private final String currency;

public Money(int cents, String currency) {
        this.cents = cents;
        this.currency = currency;
    }

public Money plus(Money other) {    // returns a NEW object, no mutation
        return new Money(this.cents + other.cents, currency);
    }
    public int cents() { return cents; }
}
```

`plus` haibadiliwi `this`; inarudisha `Money` mpya.

## Kwa nini kutokubadilikana kunasaidia

```text
✓ Thread-safe by default → no shared mutable state, no locks needed
✓ Safe to share/cache    → no defensive copies
✓ Valid forever          → if valid at construction, stays valid
✓ Usable as map keys     → hashCode never changes
```

## Marekebisho

```text
⚠️ Creates new objects on every change → more allocation/GC pressure
⚠️ Awkward for large objects updated frequently (mitigated by builders / persistent
   data structures)
```

Jinga kwa *kina* cha kutokubadilikana: sehemu `final` inayoelekeza kwenye orodha inayobadilika bado inabadilika kupitia kumbukumbu hiyo. Nakili kwa njia ya ushindi au tumia zbimu zisizobadilika.

## Kwa nini ni muhimu

Kutokubadilikana kuondoa darasa lote la matatizo: hakuna kitu kinachoweza kubadilisha kitu chako nyuma yako, kwa hivyo kufikiria na sambamba huwa rahisi sana.

Ndilo sababu aina za thamani (kamba, `LocalDate`, rekodi) havibadiliki katika lugha za sasa — zinagawanywa, kuzikumbuka, na kutumiwa kama funguo kwa usalama bila kunakili kwa njia ya ushindi.