How do you design an immutable class in Java?

Make the class final so a subclass cannot add mutable behaviour. Make fields private and final. Do not expose setters. If a field refers to a mutable object, copy it on the way in and on the way out, or store an unmodifiable snapshot that nobody else holds.

public final class Order {
    private final String id;
    private final List<String> items;

    public Order(String id, List<String> items) {
        this.id = id;
        this.items = List.copyOf(items);
    }

    public List<String> items() {
        return items;
    }
}

Where immutability leaks

• Returning a mutable internal collection lets callers change the object
• Storing a caller's mutable array or list without copying keeps an alias
• A subclass can add mutability unless the class is final or uses a private constructor
• Publishing this during construction can expose a partially built object

Records are a concise fit for shallow immutable data carriers. They generate a constructor, accessors, equals, hashCode and toString. A record only makes its fields final; any referenced mutable object still needs defensive copying.