Optional

// Old way — null checks everywhere, easy to forget one
User user = findUser(id);
if (user != null) {
    Address addr = user.getAddress();
    if (addr != null) {
        String city = addr.getCity();
    }
}

Optional<String> present = Optional.of("hello");  // non-null value
Optional<String> maybe   = Optional.ofNullable(nullableValue); // may be null
Optional<String> empty   = Optional.empty();

Optional<User> optUser = findUser(id);

// Safe get with default
User user = optUser.orElse(User.ANONYMOUS);
User user = optUser.orElseGet(() -> createDefaultUser()); // lazy

// Throw if absent
User user = optUser.orElseThrow(() -> new UserNotFoundException(id));

// Execute if present
optUser.ifPresent(u -> System.out.println(u.getName()));

// Transform if present
Optional<String> name = optUser.map(User::getName);

// Chain — flatMap avoids Optional<Optional<T>>
Optional<String> city = optUser
    .flatMap(User::getAddress)
    .map(Address::getCity);

// ❌ Don't call get() without isPresent() check
optUser.get(); // throws NoSuchElementException if empty

// ❌ Don't use Optional as a method parameter
void process(Optional<User> user) {} // use overloads instead

// ❌ Don't use Optional for fields
private Optional<String> name; // just use @Nullable String name