Question 185581 in Json Question posted in
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Json – Why Iterator suddenly printed the map elements in reverse order?

Suppose we have a block of JAVA code which is iterating through the JSON file.

JAVAfile

        // getting phoneNumbers
        JSONArray ja = (JSONArray) jo.get("phoneNumbers");//jo is the JSONObject of the given JSON 
        //Iterating through phonenumbers 
        Iterator itr2 = ja.iterator();
          
        while (itr2.hasNext()) 
        {
            Iterator<Map.Entry>itr1 = ((Map)itr2.next()).entrySet().iterator();
            while (itr1.hasNext()) {
                Map.Entry pair = itr1.next();
                System.out.println(pair.getKey() + " : " + pair.getValue());
            }
        }

JSONfile

{
    "firstName": "Souvik",
    "lastName": "Dey",
    "address": {
        "streetAddress": "D P Rd",
        "city": "Barasat",
        "state": "WB",
        "postalCode": 700126
    },
    "age": 23,
    "phoneNumbers": [
        {
            "type": "personal",
            "number": "#######23"
        },
        {
            "type": "office",
            "number": "########"
        }
    ]
}

Question

Why am I getting this:

Output

number : #######23
type : personal
number : ########
type : office

Expectations

type : personal
number : #######23
type : office
number : ########

Tags:

2

Answers


  1. 0

    You can use Jackson to parse the JSON file into a JsonNode, grab the phoneNumbers field and iterate through the nodes. Jackson uses a LinkedHashMap under the hood, so they original order of they keys should be preserved.

    import java.io.*;
import java.util.Iterator;
import java.util.Map.Entry;
import com.fasterxml.jackson.core.exc.StreamReadException;
import com.fasterxml.jackson.databind.*;

public class JsonReaderExample {
    private static ObjectMapper objectMapper = new ObjectMapper();

    public static void main(String[] args) {
        try {
            ClassLoader classLoader = JsonReaderExample.class.getClassLoader();
            InputStream inputStream = classLoader.getResourceAsStream("input.json");
            JsonNode person = objectMapper.readTree(inputStream);
            JsonNode phoneNumbers = person.get("phoneNumbers");
            if (phoneNumbers.isArray()) {
                for (final JsonNode phoneNumber : phoneNumbers) {
                    Iterator<Entry<String, JsonNode>> it = phoneNumber.fields();
                    while (it.hasNext()) {
                        Entry<String, JsonNode> entry = it.next();
                        System.out.printf("%s: %s%n", entry.getKey(), entry.getValue());
                    }
                }
            }
        } catch (StreamReadException e) {
            e.printStackTrace();
        } catch (DatabindException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

    Your safest bet would be to deserialize the JSON as a POJO. You can use Jackson to read the JSON data and parse it into a Person.

    import java.io.*;
import com.fasterxml.jackson.core.exc.StreamReadException;
import com.fasterxml.jackson.databind.*;

public class JsonReaderExample {
    private static ObjectMapper objectMapper = new ObjectMapper();

    public static void main(String[] args) {
        try {
            ClassLoader classLoader = JsonReaderExample.class.getClassLoader();
            InputStream inputStream = classLoader.getResourceAsStream("input.json");
            Person person = objectMapper.readValue(inputStream, Person.class);

            for (PhoneNumber phoneNumber : person.phoneNumbers) {
                System.out.printf("type: %s%n", phoneNumber.type);
                System.out.printf("number: %s%n", phoneNumber.number);
            }
        } catch (StreamReadException e) {
            e.printStackTrace();
        } catch (DatabindException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static class Person {
        public String firstName;
        public String lastName;
        public Address address;
        public int age;
        public PhoneNumber[] phoneNumbers;
    }

    private static class PhoneNumber {
        public String type;
        public String number;
    }

    private static class Address {
        public String streetAddress;
        public String city;
        public String state;
        public int postalCode;
    }
}

    Output:

    type: personal
number: #######23
type: office
number: ########
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  2. 0

    Map elements in JSON are not regarded as ordered data. It comes out in a different order for reasons similar to the ordering of a HashMap’s contents. You shouldn’t expect a HashMap to iterate in the same order that you put content into it, and the same is true for mapped content from a JSON source.

    If you want to preserve the order of mapped data in JSON, then you should transform the mapped data to an array and store that in JSON.

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