Get specific value from JSONArray based on key and filter in Java/Android

This article details how to efficiently parse a JSON array (JSONArray) in Java and Android development, extract the value corresponding to a specific key, and implement data filtering on this basis. The content covers JSON object traversal, value acquisition, exception handling, and code implementation combined with actual scenarios (such as filtering longitude and latitude information based on email addresses). It aims to provide a tutorial with a clear structure and easy to understand.

1. Understand JSON data structure

JSON (JavaScript Object Notation) is a lightweight data exchange format that is easy for humans to read and write, and easy for machines to parse and generate. In Java and Android development, we often need to process JSON data from the server.

There are two main basic structures of JSON data:

• JSON Object (JSONObject) : An unordered collection of "key/value" pairs. The key is a string and the value can be a string, a number, a boolean, another JSON object, a JSON array, or null. In Java it usually corresponds to the org.json.JSONObject class.
• JSON array (JSONArray) : An ordered collection of values. Each value can be any JSON data type (string, number, boolean, JSON object, JSON array, etc.). In Java it usually corresponds to the org.json.JSONArray class.

This tutorial will focus on how to handle a JSONArray containing multiple JSON objects.

2. Parse JSONArray and extract data

Suppose we have a JSONArray containing location information for multiple users. Each object contains fields such as num, email, lati (latitude), longt (longitude), and time. Our goal is to parse this array and extract the lati and longt values.

2.1 Convert JSON string to JSONArray object

First, you need to convert the received JSON string into a JSONArray object.

 import org.json.JSONArray;
import org.json.JSONObject;
import org.json.JSONException;
import java.util.ArrayList;
import java.util.List;

public class JsonParserExample {

    // Assume this is the JSON string you got from the API private static final String JSON_DATA = "["  
            "{"  
            " \"num\": \"34304\","  
            " \"email\": \"<a class='\\\"__cf_email__\\\"' data-cfemail='\\\"2f4e4e4e4e6f424e4643014e4b4b5d4a5c5c\\\"' href="%5C%5C%5C%22/cdn-cgi/l/email-protection%5C%5C%5C%22">[email protected]</a>\","  
            " \"lati\": \"8888888\","  
            " \"longt\": \"88888888\","  
            " \"time\": \"2022-12-08 21:15:39\""  
            "},"  
            "{"  
            " \"num\": \"34303\","  
            " \"email\": \"<a class='\\\"__cf_email__\\\"' data-cfemail='\\\"2c4d4d4d4d6c414d4540024d48485e495f5f\\\"' href="%5C%5C%5C%22/cdn-cgi/l/email-protection%5C%5C%5C%22">[email protected]</a>\","  
            " \"lati\": \"8888\","  
            " \"longt\": \"88888\","  
            " \"time\": \"8888888\""  
            "},"  
            "{"  
            " \"num\": \"34302\","  
            " \"email\": \"<a class='\\\"__cf_email__\\\"' data-cfemail='\\\"3c5e5e5e5e7c515d5550125d58584e594f4f\\\"' href="%5C%5C%5C%22/cdn-cgi/l/email-protection%5C%5C%5C%22">[email protected]</a>\","  
            " \"lati\": \"8888\","  
            " \"longt\": \"88888\","  
            " \"time\": \"8888888\""  
            "},"  
            "{"  
            " \"num\": \"34301\","  
            " \"email\": \"<a class='\\\"__cf_email__\\\"' data-cfemail='\\\"7716161616371a161e1b5916131305120404\\\"' href="%5C%5C%5C%22/cdn-cgi/l/email-protection%5C%5C%5C%22">[email protected]</a>\","  
            " \"lati\": \"8888\","  
            " \"longt\": \"88888\","  
            " \"time\": \"8888888\""  
            "},"  
            "{"  
            " \"num\": \"34300\","  
            " \"email\": \"<a class='\\\"__cf_email__\\\"' data-cfemail='\\\"1f7c7c7c7c5f727e7673317e7b7b6d7a6c6c\\\"' href="%5C%5C%5C%22/cdn-cgi/l/email-protection%5C%5C%5C%22">[email protected]</a>\","  
            " \"lati\": \"8888\","  
            " \"longt\": \"88888\","  
            " \"time\": \"8888888\""  
            "}"  
            "]";

    public static void main(String[] args) {
        try {
            JSONArray jsonArray = new JSONArray(JSON_DATA);
            System.out.println("Successfully converted JSON string to JSONArray, containing " jsonArray.length() " elements.");
        } catch (JSONException e) {
            e.printStackTrace();
            System.err.println("JSON parsing error: " e.getMessage());
        }
    }
}

2.2 Traverse JSONArray and obtain JSONObject

JSONArray is actually a list of JSONObjects. We need to traverse this array and process each JSONObject.

 // Assuming you already have a JSONArray jsonArray
for (int i = 0; i <h3> 2.3 Extract specific values ​​from JSONObject</h3><p> JSONObject provides multiple methods to obtain values ​​of different types:</p>

• getString(String key): Get the string value.
• getInt(String key): Get the integer value.
• getDouble(String key): Get a double-precision floating point value.
• getBoolean(String key): Get Boolean value.
• getJSONObject(String key): Get the nested JSONObject.
• getJSONArray(String key): Get the nested JSONArray.

Corresponding to the get method, there is also the opt method (such as optString, optInt, etc.). The opt method does not throw JSONException when the key does not exist, but returns a default value (for example, optString returns an empty string, optInt returns 0, optJSONObject returns null), which is very useful when dealing with optional fields.

In our example, lati and longt are both string types, but we usually need to convert them to double types to represent geographic coordinates.

 // ... Inside the for loop above...
String latiStr = jsonObject.getString("lati");
String longtStr = jsonObject.getString("longt");

try {
    double latitude = Double.parseDouble(latiStr);
    double longitude = Double.parseDouble(longtStr);
    System.out.println("Parsed Lati: " latitude ", Parsed Longt: " longitude);
    // You can store latitude and longitude into a custom object or list here } catch (NumberFormatException e) {
    System.err.println("Latitude or longitude value format error: " latiStr ", " longtStr);
}

3. Apply filtering logic: filter data based on Email

The original question asked to filter data based on specific email addresses. In our JSON data, the email field is an HTML tag that contains the actual email address. We need to extract the plain text email address from this HTML tag first, and then filter it.

3.1 Extract plain text email address

Since the email field is an HTML structure such as [email protected], directly obtaining getString("email") will get the entire HTML string. We need to extract [email protected] from this string. A simple way is to use regular expressions or string replacement to remove HTML tags.

 // Auxiliary method: extract plain text email from HTML string private static String extractEmailFromHtml(String htmlEmail) {
    // Remove all HTML tags String plainText = htmlEmail.replaceAll("]*>", "");
    // Replace the HTML entity   with a normal space plainText = plainText.replace(" ", " ");
    return plainText.trim();
}

//... Inside the for loop, after getting the email string...
String rawEmail = jsonObject.getString("email");
String cleanEmail = extractEmailFromHtml(rawEmail);
System.out.println("Cleaned Email: " cleanEmail);

// Suppose the email we want to filter is "[email protected]"
String targetEmail = "[email protected]";

if (cleanEmail.equals(targetEmail)) {
    // Match successfully, extract latitude and longitude String latiStr = jsonObject.getString("lati");
    String longtStr = jsonObject.getString("longt");
    try {
        double latitude = Double.parseDouble(latiStr);
        double longitude = Double.parseDouble(longtStr);
        System.out.println("--- Find data matching the mailbox---");
        System.out.println("Email: " cleanEmail);
        System.out.println("Lati: " latitude);
        System.out.println("Longt: " longitude);
        // Add matching latitude and longitude to the result list } catch (NumberFormatException e) {
        System.err.println("The format of the filtered latitude or longitude value is incorrect: " latiStr ", " longtStr);
    }
}

4. Complete sample code

Combining all the above steps, here is a complete Java code example that demonstrates how to parse a JSON array, extract latitude and longitude, and filter based on email address.

In order to better organize the data, we define a simple LatLng class to store latitude and longitude.

import org.json.JSONArray;
import org.json.JSONObject;
import org.json.JSONException;
import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

// Simulate the LatLng class in Android class LatLng {
    public double latitude;
    public double longitude;

    public LatLng(double latitude, double longitude) {
        this.latitude = latitude;
        this.longitude = longitude;
    }

    @Override
    public String toString() {
        return "LatLng{"  
               "latitude=" latitude  
               ", longitude=" longitude  
               '}';
    }
}

public class JsonDataProcessor {

    private static final String JSON_DATA = "["  
            "{"  
            " \"num\": \"34304\","  
            " \"email\": \"<a class='\\\"__cf_email__\\\"' data-cfemail='\\\"2f4e4e4e4e6f424e4643014e4b4b5d4a5c5c\\\"' href="%5C%5C%5C%22/cdn-cgi/l/email-protection%5C%5C%5C%22">[email protected]</a>\","  
            " \"lati\": \"8888888\","  
            " \"longt\": \"88888888\","  
            " \"time\": \"2022-12-08 21:15:39\""  
            "},"  
            "{"  
            " \"num\": \"34303\","  
            " \"email\": \"<a class='\\\"__cf_email__\\\"' data-cfemail='\\\"2c4d4d4d4d6c414d4540024d48485e495f5f\\\"' href="%5C%5C%5C%22/cdn-cgi/l/email-protection%5C%5C%5C%22">[email protected]</a>\","  
            " \"lati\": \"8888\","  
            " \"longt\": \"88888\","  
            " \"time\": \"8888888\""  
            "},"  
            "{"  
            " \"num\": \"34302\","  
            " \"email\": \"<a class='\\\"__cf_email__\\\"' data-cfemail='\\\"3c5e5e5e5e7c515d5550125d58584e594f4f\\\"' href="%5C%5C%5C%22/cdn-cgi/l/email-protection%5C%5C%5C%22">[email protected]</a>\","  
            " \"lati\": \"8888\","  
            " \"longt\": \"88888\","  
            " \"time\": \"8888888\""  
            "},"  
            "{"  
            " \"num\": \"34301\","  
            " \"email\": \"<a class='\\\"__cf_email__\\\"' data-cfemail='\\\"7716161616371a161e1b5916131305120404\\\"' href="%5C%5C%5C%22/cdn-cgi/l/email-protection%5C%5C%5C%22">[email protected]</a>\","  
            " \"lati\": \"8888\","  
            " \"longt\": \"88888\","  
            " \"time\": \"8888888\""  
            "},"  
            "{"  
            " \"num\": \"34300\","  
            " \"email\": \"<a class='\\\"__cf_email__\\\"' data-cfemail='\\\"1f7c7c7c7c5f727e7673317e7b7b6d7a6c6c\\\"' href="%5C%5C%5C%22/cdn-cgi/l/email-protection%5C%5C%5C%22">[email protected]</a>\","  
            " \"lati\": \"8888\","  
            " \"longt\": \"88888\","  
            " \"time\": \"8888888\""  
            "}"  
            "]";

    /**
     * Extract plain text email addresses from HTML strings.
     * For example, extract "[email protected]" from "<a ...>[email protected]</a>"
     *
     * @param htmlEmail HTML string containing the email address * @return Plain text email address, or an empty string if it cannot be extracted.
     */
    private static String extractEmailFromHtml(String htmlEmail) {
        // Use regular expressions to match the text content inside the <a> tag // Pattern pattern = Pattern.compile("(?)(.*?)(?=</a>

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