Immutable classes

The recipe:

• Make the class final, or all constructors private, so behaviour cannot be overridden.
• Make every field private final.
• Provide no mutators.
• Defensively copy any mutable component on the way in and on the way out.

public final class Money {
    private final BigDecimal amount;
    private final Currency currency;

    public Money(BigDecimal amount, Currency currency) {
        this.amount = Objects.requireNonNull(amount);
        this.currency = Objects.requireNonNull(currency);
    }

    public Money plus(Money other) {          // returns a new instance
        if (!currency.equals(other.currency))
            throw new IllegalArgumentException("currency mismatch");
        return new Money(amount.add(other.amount), currency);
    }
}

The final-field guarantee

The memory model gives a special promise: once a constructor completes without letting this escape, every thread sees the correctly initialised values of its final fields, with no synchronisation at all. Let this escape from the constructor and the guarantee is void.

Cost

Every modification allocates. That is usually irrelevant, and where it is not, the pattern is a mutable builder producing an immutable result — String and StringBuilder being the canonical pair.