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()); } @Nullable public 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：
