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：