How to decode this json object data in java

According the java doc, JSONValue.parse returns

Instance of the following: org.json.simple.JSONObject, org.json.simple.JSONArray, java.lang.String, java.lang.Number, java.lang.Boolean, null

In your case it should be a JSONObject,so you can cast it to a JSONObject and use method in JSONObject to retrive email.

Object obj=JSONValue.parse(result);
JSONObject jsonObject=(JSONObject)obj;
JSONObject data=(JSONObject)jsonObject.get("data");
String email= (String)data.get("email");

In recent version JSONObject has been deprecated, use JsonObject instead, which don't bother using so many casting.

ObjectMapper can also be used:

use the pom to download the dependency.

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.1.2</version>
 </dependency>

Code:-

import com.fasterxml.jackson.databind.ObjectMapper;

    public class HelloWorld{
    
         public static void main(String []args){
            String dataJson = "{\"data\":{\"id\":2,\"email\":\"[email protected]\",\"first_name\":\"Janet\",\"last_name\":\"Weaver\",\"avatar\":\"https://s3.amazonaws.com/uifaces/faces/twitter/josephstein/128.jpg\"},\"ad\":{\"company\":\"StatusCode Weekly\",\"url\":\"http://statuscode.org/\",\"text\":\"A weekly newsletter\n" + 
                    "focusing on software development, infrastructure, the server,\n" + 
                    "performance, and the stack end of things.\"}}";
            
            ObjectMapper objectMapper = new ObjectMapper();
            try {
                A a = objectMapper.readValue(dataJson, A.class);
    
                System.out.println("email  = " + a.data.email);
            } catch (Exception e) {
                e.printStackTrace();
            }
         }
    }
    
    
    class A{
        B data;
    }
    class B{
        int id;
        String email;
        String first_name;
        String last_name;
        String avatar;
        String ad;
        String url;
        String text;
        
    }

Download dependency https://mvnrepository.com/artifact/org.json/json/20180813 and Use below code -

import org.json.JSONObject;
public class TestJson {
    public static void main(String[] args) {
    Runtime rt = Runtime.getRuntime();
    Process pr = rt.exec("curl -s -S https://reqres.in/api/users/2");
    BufferedReader br = new BufferedReader(new InputStreamReader(pr.getInputStream()));
    String result = br.readLine();
    JSONObject obj = new JSONObject(result);
    String email = obj.getJSONObject("data").getString("email");
    System.out.println(email);
    }
}

I think that in general case you have to use some JSON parser framework like Jackson. But in case you have to find only one value and do not care about json validation or other aspects, then you could use simple RegExp:

public static String getEmail(String json) {
    Pattern pattern = Pattern.compile("\"email\"\\s*:\\s*\"(?<email>[^\"]+)\"");
    Matcher matcher = pattern.matcher(json);
    return matcher.find() ? matcher.group("email") : null;
}

or event simplier str.indexOf():

public static String getEmail(String json) {
    int pos = json.indexOf("\"email\"");
    pos = pos == -1 ? pos : json.indexOf('"', pos + 7);
    return pos == -1 ? null : json.substring(pos + 1, json.indexOf('"', pos + 1));
}