What are immutable objects and why use them?

Question

Accepted Answer

An **immutable object** cannot be changed after it's constructed — every field is set once, and there are no methods that mutate state. To "change" it, you create a new object.

## Building an immutable type

```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` doesn't modify `this`; it returns a fresh `Money`.

## Why immutability helps

```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
```

## Trade-offs

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

Watch out for *shallow* immutability: a `final` field pointing to a mutable list is still mutable through that reference. Defensively copy or use unmodifiable collections.

## Why it matters

Immutability eliminates a whole class of bugs: nothing can change an object out from under you, so reasoning and concurrency get dramatically simpler.

It's why value types (strings, `LocalDate`, records) are immutable in modern languages — shared, cached, and used as keys safely without defensive copying.