Apa iku obyek immutable lan mengapa nggunakake?

Question

Accepted Answer

Obyek **immutable** ora bisa diowahi sawisé dikonstruksi — saben kolom diatur sedhela, lan ora ana metode sing ngrubah status. Kanggo "ngrubah" obyek kasebut, sampeyan kudu nggawe obyek anyar.

## Nggawe tipe immutable

```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` ora ngrubah `this`; nggawe `Money` anyar.

## Mengapa immutability nulungi

```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
```

## Tukar-menukar

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

Omongé *shallow* immutability: kolom `final` sing nunjuk list sing bisa diowahi isih bisa diowahi lewat referensi kasebut. Salin kanthi defensif utawa gunakake koleksi sing ora bisa diubah.

## Mengapa iki penting

Immutability ngilangi kabeh kelas bug: ora ana sing bisa ngrubah obyek tanpa kakarepan, mula penalaran lan concurrency dadi luwih gampang.

Iki alasan mengapa tipe nilai (string, `LocalDate`, record) immutable ing basa modern — digunakake bareng, di-cache, lan digunakake minangka kunci kanthi aman tanpa panyalinian defensif.