How to parse JSON in Java

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.

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// 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.

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// 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

This article is contributed by Gaurav Miglani. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to [e-mail]. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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