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This article explains how to convert a Hashmap to a JSON object in Java. We will see a detailed example of creating a hashmap and then converting it to a JSON object.

Hashmap and JSON are both very commonly used by developers as they help us create a simple structure that can be easily used to store as well as transfer data.

new JSONObject(hashmap)Convert Hashmap to JSON object

The most traditional way to convert a hashmap to a JSON object is to call JSONObject JSONObject()and then pass the hashmap.

Let's look at an example that creates a hashmap and then prints it in JSON format.

import java.util.HashMap;
import java.util.Map;
import org.json.simple.JSONObject;

public class Main {
  public static void main(String[] args) {
    Map<String, Object> map = new HashMap();

    map.put("key1", "value1");
    map.put("key2", "value2");

    JSONObject json = new JSONObject(map);
    System.out.printf("JSON: %s", json);
  }

Output:

JSON: {"key1":"value1","key2":"value2"}

It is important to note that Map<String, Object>Map takes a string, ie key, and an object Object, ie value. This means that we can pass any valid object as a value into Map and it will be converted into a JSON object.

Here is an example that takes a string and a Arraylistas a value.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
import org.json.simple.JSONObject;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> stringArrayList = new ArrayList<>();
    stringArrayList.add("firstString");
    stringArrayList.add("secondString");
    stringArrayList.add("thirdString");

    Map<String, Object> map = new HashMap();
    map.put("key1", "value1");
    map.put("key2", "value2");
    map.put("stringList", stringArrayList);

    JSONObject json = new JSONObject(map);
    System.out.printf("JSON: %s", json);
  }

Output:

JSON: {"key1":"value1","key2":"value2","stringList":["firstString","secondString","thirdString"]}

JacksonLibrary to convert Hashmap to JSON object

In Java, there are some libraries that can help us convert Hashmap to JSON object and they are very flexible.

Jackson is one of these libraries that takes a Java mapand converts it into JSON format.

Let's not forget to handle that JsonProcessingException, because ObjectMapper().writeValueAsString(map)when an incompatible data format is found, an exception will be thrown.

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.util.HashMap;
import java.util.Map;

public class Main {
  public static void main(String[] args) throws JsonProcessingException {
    Map<String, Object> map = new HashMap();
    map.put("key1", "value1");
    map.put("key2", "value2");

    String json = new ObjectMapper().writeValueAsString(map);
    System.out.printf("JSON: %s", json);
  }

Output:

JSON: {"key1":"value1","key2":"value2"}

GSONLibrary to convert Hashmap to JSON object

GsonThe library is one of the most commonly used libraries to convert hashmap to JSON object. It provides easy methods to handle our hashmap and JSON.

GsonThe class has a method toJsonTreewhich takes our Map and converts it into a JSON tree. But since we want a JSON object, we can use toJSONObject()to make the JSON tree into a JSON object.

import com.google.gson.Gson;
import com.google.gson.JsonObject;
import java.util.HashMap;
import java.util.Map;

public class Main {
  public static void main(String[] args) {
    Map<String, Object> map = new HashMap();
    map.put("key1", "value1");
    map.put("key2", "value2");

    Gson gson = new Gson();
    JsonObject json = gson.toJsonTree(map).getAsJsonObject();

    System.out.printf("JSON: %s", json);
  }

Output:

JSON: {"key1":"value1","key2":"value2"}