How to parse JSON in Java - GeeksforGeeks

07 Aug, 2019
JSON (JavaScript Object Notation) is a lightweight, text-based, language-independent data exchange format that is easy for humans and machines to read and write. JSON can represent two structured types: objects and arrays. An object is an unordered collection of zero or more name/value pairs. An array is an ordered sequence of zero or more values. The values can be strings, numbers, booleans, null, and these two structured types.
Below is a simple example from Wikipedia that shows JSON representation of an object that describes a person. The object has string values for first name and last name, a number value for age, an object value representing the person’s address, and an array value of phone number objects.
{
    "firstName": "John",
    "lastName": "Smith",
    "age": 25,
    "address": {
        "streetAddress": "21 2nd Street",
        "city": "New York",
        "state": "NY",
        "postalCode": 10021
    },
    "phoneNumbers": [
        {
            "type": "home",
            "number": "212 555-1234"
        },
        {
            "type": "fax",
            "number": "646 555-4567" 
        }
    ] 
}

JSON Processing in Java : The Java API for JSON Processing JSON.simple is a simple Java library that allow parse, generate, transform, and query JSON.
Getting Started : You need to download the json-simple-1.1 jar and put it in your CLASSPATH before compiling and running the below example codes.
For importing jar in IDE like eclipse, refer here.
If you are using maven you may use the following maven link https://mvnrepository.com/artifact/com.googlecode.json-simple/json-simple/1.1.1
Json-Simple API : It provides object models for JSON object and array structures. These JSON structures are represented as object models using types JSONObject and JSONArray. JSONObject provides a Map view to access the unordered collection of zero or more name/value pairs from the model. Similarly, JSONArray provides a List view to access the ordered sequence of zero or more values from the model.
Write JSON to a file
Let us see an example that writes above JSON data into a file “JSONExample.json”, with help of JSONObject and JSONArray.
// Java program for write JSON to a file  import java.io.FileNotFoundException;import java.io.PrintWriter;import java.util.LinkedHashMap;import java.util.Map;import org.json.simple.JSONArray;import org.json.simple.JSONObject;  public class JSONWriteExample{    public static void main(String[] args) throws FileNotFoundException     {        // creating JSONObject        JSONObject jo = new JSONObject();                  // putting data to JSONObject        jo.put("firstName", "John");        jo.put("lastName", "Smith");        jo.put("age", 25);                  // for address data, first create LinkedHashMap        Map m = new LinkedHashMap(4);        m.put("streetAddress", "21 2nd Street");        m.put("city", "New York");        m.put("state", "NY");        m.put("postalCode", 10021);                  // putting address to JSONObject        jo.put("address", m);                  // for phone numbers, first create JSONArray         JSONArray ja = new JSONArray();                  m = new LinkedHashMap(2);        m.put("type", "home");        m.put("number", "212 555-1234");                  // adding map to list        ja.add(m);                  m = new LinkedHashMap(2);        m.put("type", "fax");        m.put("number", "212 555-1234");                  // adding map to list        ja.add(m);                  // putting phoneNumbers to JSONObject        jo.put("phoneNumbers", ja);                  // writing JSON to file:"JSONExample.json" in cwd        PrintWriter pw = new PrintWriter("JSONExample.json");        pw.write(jo.toJSONString());                  pw.flush();        pw.close();    }}
Output from file “JSONExample.json” :
{
     "lastName":"Smith",
    "address":{
        "streetAddress":"21 2nd Street",
         "city":"New York",
         "state":"NY",
         "postalCode":10021
    },
     "age":25,
     "phoneNumbers":[
            {
            "type":"home", "number":"212 555-1234"
            },
         {
            "type":"fax", "number":"212 555-1234"
         }
     ],
     "firstName":"John"
}

Note : In JSON, An object is an unordered set of name/value pairs, so JSONObject doesn’t preserve the order of an object’s name/value pairs, since it is (by definition) not significant. Hence in our output file, order is not preserved.
Read JSON from a file
Let us see an example that read JSON data from above created file “JSONExample.json” with help of JSONParser, JSONObject and JSONArray.
// Java program to read JSON from a file  import java.io.FileReader;import java.util.Iterator;import java.util.Map;  import org.json.simple.JSONArray;import org.json.simple.JSONObject;import org.json.simple.parser.*;  public class JSONReadExample {    public static void main(String[] args) throws Exception     {        // parsing file "JSONExample.json"        Object obj = new JSONParser().parse(new FileReader("JSONExample.json"));                  // typecasting obj to JSONObject        JSONObject jo = (JSONObject) obj;                  // getting firstName and lastName        String firstName = (String) jo.get("firstName");        String lastName = (String) jo.get("lastName");                  System.out.println(firstName);        System.out.println(lastName);                  // getting age        long age = (long) jo.get("age");        System.out.println(age);                  // getting address        Map address = ((Map)jo.get("address"));                  // iterating address Map        Iterator<Map.Entry> itr1 = address.entrySet().iterator();        while (itr1.hasNext()) {            Map.Entry pair = itr1.next();            System.out.println(pair.getKey() + " : " + pair.getValue());        }                  // getting phoneNumbers        JSONArray ja = (JSONArray) jo.get("phoneNumbers");                  // iterating phoneNumbers        Iterator itr2 = ja.iterator();                  while (itr2.hasNext())         {            itr1 = ((Map) itr2.next()).entrySet().iterator();            while (itr1.hasNext()) {                Map.Entry pair = itr1.next();                System.out.println(pair.getKey() + " : " + pair.getValue());            }        }    }}
Output:
John
Smith
25
streetAddress : 21 2nd Street
postalCode : 10021
state : NY
city : New York
number : 212 555-1234
type : home
number : 212 555-1234
type : fax

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