Use GSON library for that. Here is the sample code

List<String> foo = new ArrayList<String>();
foo.add("A");
foo.add("B");
foo.add("C");

String json = new Gson().toJson(foo );

Here is the maven dependency for Gson

<dependencies>
    <!--  Gson: Java to Json conversion -->
    <dependency>
        <groupId>com.google.code.gson</groupId>
        <artifactId>gson</artifactId>
        <version>2.2.2</version>
        <scope>compile</scope>
    </dependency>
</dependencies>

Or you can directly download jar from here and put it in your class path

http://code.google.com/p/google-gson/downloads/detail?name=gson-1.0.jar&can=4&q=

To send Json to client you can use spring or in simple servlet add this code

response.getWriter().write(json);

Using gson it is much simpler. Use following code snippet:

 // create a new Gson instance
 Gson gson = new Gson();
 // convert your list to json
 String jsonCartList = gson.toJson(cartList);
 // print your generated json
 System.out.println("jsonCartList: " + jsonCartList);

Converting back from JSON string to your Java object

 // Converts JSON string into a List of Product object
 Type type = new TypeToken<List<Product>>(){}.getType();
 List<Product> prodList = gson.fromJson(jsonCartList, type);

 // print your List<Product>
 System.out.println("prodList: " + prodList);

If you need to convert to String use the com.fasterxml.jackson.databind.ObjectMapper

Ex:

        List<Integer> myList = new ArrayList<>();
        myList.add(1);
        myList.add(2);

        ObjectMapper mapper = new ObjectMapper();
        try {
            String json = mapper.writeValueAsString(myList);
            System.out.println("result = " + json);
            //System.out.println(json);
        } catch (JsonProcessingException e) {
            //
        }

You should convert your list to a JSON Array, and just use its toString() function:

JSONArray myArray = new JSONArray(jsonObjlist);

// ...
String arrayToJson = myArray.toString(2);

The int parameter specifies the indent factor to use for formatting.