泛谈Java中的不可变数据结构

Address address = new Address();
address.setCity("Sydney");
// address is in invalid state now, since the country hasn't been set.
Address address = new Address("Sydney", "Australia");
// Address is valid and doesn't have setters, so the address object is always valid.

public boolean isOverseas(Address address) {
 if(address.getCountry().equals("Australia") == false) {
  address.setOverseas(true); // address has now been mutated!
  return true;
 } else {
  return false;
 }
}

final Person person = new Person("John");

person.setName("Cindy");

person.getAddresses().add(new Address("Sydney"));

public final class Person {// final class, can't be overridden by subclasses
 private final String name;  // final for safe publication in multithreaded applications
 private final List<Address> addresses;
 public Person(String name, List<Address> addresses) {
  this.name = name;
  this.addresses = List.copyOf(addresses); // makes a copy of the list to protect from outside mutations (Java 10+).
    // Otherwise, use Collections.unmodifiableList(new ArrayList<>(addresses));
 }
 public String getName() {
  return this.name; // String is immutable, okay to expose
 }
 public List<Address> getAddresses() {
  return addresses; // Address list is immutable
 }
}
public final class Address { // final class, can't be overridden by subclasses
 private final String city; // only immutable classes
 private final String country;
 public Address(String city, String country) {
  this.city = city;
  this.country = country;
 }
 public String getCity() {
  return city;
 }
 public String getCountry() {
  return country;
 }
}

import java.util.List;
final Person person = new Person("John", List.of(new Address(“Sydney”, "Australia"));

final List<Address> addresses = new ArrayList<>();
addresses.add(new Address("Sydney", "Australia"));
final Person person = new Person("John", addressList);
addresses.clear();