How to avoid Java gracefully: NullPointerException (null pointer exception)

Table of contents

1: Null pointer problem

2: Solution

The first way:

The second way:

The third way:

The fourth way:

---

1: Null pointer problem

Java has no pointers, so we often say that "Java pointer" means "Java reference". A null pointer is a null reference. A Java null pointer exception means that the reference itself is null, but a method is called. At this time, a null pointer exception will occur.

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: Solution

The first way:

Use if to make non-null judgment

/**
     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());
                }
            }
        }
    }
}

The second way:

Use Optional.ofNuallable for processing

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);

    }
}

The third way:

Use Assert assertion

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);
    }
}

The fourth way:

Use @Nullable annotation (the code will be prompted)

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());
    }

}

hint: