How do you design an immutable class? Why immutability helps concurrency?

Immutable classes

Immutable objects never change after construction—safe to share across threads without locks.

final class Money {
  private final BigDecimal amount;
  Money(BigDecimal a) { this.amount = a; }
  Money add(Money other) { return new Money(amount.add(other.amount)); }
}

• Make class final, fields final, no setters.
• Defensive copy mutable inputs (Date, collections).
• Record classes (Java 16+) for data carriers.