1：空指针问题

Java是没有指针的，所以我们常说"Java 指针"就是指"Java 的引用"。空指针就是空引用，Java空指针异常就是引用本身为空，但却调用了方法，这个时候就会出现空指针异常

public class demo {public static void main(String[] args) {Address address = new Address();User user = new User("xia", 12, address);String cityName = user.getAddress().getCity().trim();System.out.println(cityName);}
}Address:@Data
@NoArgsConstructor
@AllArgsConstructor
public class Address {private String city;private String address;
}User:@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {private String name;private Integer age;private Address address;
}

2：解决方法

第一种方式：

使用if 来进行非空判断


/**使用if 条件判断，但是这种写法不优美
*/public class demo {public static void main(String[] args) {Address address = new Address();User user = new User("xia", 12, address);if (user != null){Address userAddress = user.getAddress();if (userAddress != null) {String city = address.getCity();if (city != null && !"".equals(city)){System.out.println(user.getAddress().getCity().trim());}}}}
}

第二种方式：

使用Optional.ofNuallable 来进行处理


public class demo {public static void main(String[] args) {Address address = new Address();User user = new User("xia", 12, address);String ored = Optional.ofNullable(user).map(User::getAddress).map(Address::getCity).map(String::trim).orElse("default");System.out.println(ored);}
}

第三种方式：

使用Assert 断言

public class demo {public static void main(String[] args) {Address address = new Address();User user = new User("xia", 12, address);getUserCity(user);try {TimeUnit.SECONDS.sleep(10);} catch (InterruptedException e) {throw new RuntimeException(e);}getUserCity(null);}public static void getUserCity(User user){Assert.notNull(user,"user is null");Address address = user.getAddress();Assert.notNull(address,"address is null");String city = address.getCity();System.out.println(city);}
}

第四种方式：

使用@Nullable 注解（代码会有提示）


public class demo {public static void printString(@Nullable String str){System.out.println(str.toString());}@Nullablepublic static String getString(){return null;}public static void main(String[] args) {Address address = new Address();String str = null;printString(str);getString().toString();User user = new User();System.out.println(user.getAddress().getCity());}}

提示：