How to convert hashmap to JSON object in Java

How to convert or cast hashmap to JSON object in Java, and again convert JSON object to JSON string?

You can use:

new JSONObject(map);

Caution: This will only work for a Map<String, String>!

Other functions you can get from its documentation
http://stleary.github.io/JSON-java/index.html

Gson can also be used to serialize arbitrarily complex objects.

Here is how you use it:

Gson gson = new Gson(); 
String json = gson.toJson(myObject);

Gson will automatically convert collections to JSON arrays. Gson can serialize private fields and automatically ignores transient fields.

You can convert Map to JSON using Jackson as follows:

Map<String,Object> map = new HashMap<>();
//You can convert any Object.
String[] value1 = new String[] { "value11", "value12", "value13" };
String[] value2 = new String[] { "value21", "value22", "value23" };
map.put("key1", value1);
map.put("key2", value2);
map.put("key3","string1");
map.put("key4","string2");

String json = new ObjectMapper().writeValueAsString(map);
System.out.println(json);

Maven Dependencies for Jackson :

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.5.3</version>
    <scope>compile</scope>
</dependency>

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.5.3</version>
    <scope>compile</scope>
</dependency>

If you are using `JSONObject` library, you can convert map to `JSON` as follows:

JSONObject Library: import org.json.JSONObject;

Map<String, Object> map = new HashMap<>();
// Convert a map having list of values.
String[] value1 = new String[] { "value11", "value12", "value13" };
String[] value2 = new String[] { "value21", "value22", "value23" };
map.put("key1", value1);
map.put("key2", value2);

JSONObject json = new JSONObject(map);
System.out.println(json);

Maven Dependencies for `JSONObject` :

<dependency>
    <groupId>org.json</groupId>
    <artifactId>json</artifactId>
    <version>20140107</version>
</dependency>

Hope this will help. Happy coding.

Example using json

Map<String, Object> data = new HashMap<String, Object>();
    data.put( "name", "Mars" );
    data.put( "age", 32 );
    data.put( "city", "NY" );
    JSONObject json = new JSONObject();
    json.putAll( data );
    System.out.printf( "JSON: %s", json.toString(2) );

output::

JSON: {
  "age": 32,
  "name": "Mars",
  "city": "NY"
}

You can also try to use Google's GSON.Google's GSON is the best library available to convert Java Objects into their JSON representation.

http://code.google.com/p/google-gson/

In my case I didn't want any dependancies. Using Java 8 you can get JSON as a string this simple:

Map<String, Object> map = new HashMap<>();
map.put("key", "value");
map.put("key2", "value2");
String json = "{"+map.entrySet().stream()
    .map(e -> "\""+ e.getKey() + "\":\"" + String.valueOf(e.getValue()) + "\"")
    .collect(Collectors.joining(", "))+"}";

How to convert hashmap to JSON object in Java

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