For anyone else who might need this:

String jsonString = "[\"string1\",\"string2\",\"string3\"]";
ObjectMapper mapper = new ObjectMapper();
List<String> strings = mapper.readValue(jsonString, List.class);

You are asking Jackson to parse a StudentList. Tell it to parse a List (of students) instead. Since List is generic you will typically use a TypeReference

List<Student> participantJsonList = mapper.readValue(jsonString, new TypeReference<List<Student>>(){});

You've tagged Jackson but are using Gson in your example. I'm going to go with Jackson

String json = "{\"data\":[\"my/path/old\",\"my/path/new\"]}"; // or wherever you're getting it from

Create your ObjectMapper

ObjectMapper mapper = new ObjectMapper();

Read the JSON String as a tree. Since we know it's an object, you can cast the JsonNode to an ObjectNode.

ObjectNode node = (ObjectNode)mapper.readTree(json);

Get the JsonNode named data

JsonNode arrayNode = node.get("data");

Parse it into an ArrayList<String>

ArrayList<String> data = mapper.readValue(arrayNode.traverse(), new TypeReference<ArrayList<String>>(){});

Printing it

System.out.println(data);

gives

[my/path/old, my/path/new]

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);

Instead of getting a List<String> from the JSON Object, you can access the array of fields in the same way you are accessing the array of errors:

public static String getField(JSONObject json, String errorsIndex, String fieldIndex) {
    JSONObject errorJson = json.getJSONArray("errors").getJSONObject(errorIndex);
    String value = errorJson.getJSONArray("fields").getString(fieldIndex);
    return value;
}

Note that get(fieldIndex) has changed to getString(fieldIndex). That way you don't have to cast an Object to a String.

I'll elaborate a bit more on ChrisR awesome answer and bring images from his awesome reference.

A valid JSON always starts with either curly braces { or square brackets [, nothing else.

{ will start an object:

{ "key": value, "another key": value }

Hint: although javascript accepts single quotes ', JSON only takes double ones ".

[ will start an array:

[value, value]

Hint: spaces among elements are always ignored by any JSON parser.

And value is an object, array, string, number, bool or null:

So yeah, ["a", "b"] is a perfectly valid JSON, like you could try on the link Manish pointed.

Here are a few extra valid JSON examples, one per block:

{}

[0]

{"__comment": "json doesn't accept comments and you should not be commenting even in this way", "avoid!": "also, never add more than one key per line, like this"}

[{   "why":null} ]

{
  "not true": [0, false],
  "true": true,
  "not null": [0, 1, false, true, {
    "obj": null
  }, "a string"]
}

Take a look at this tutorial. Also you can parse above json like :

JSONArray arr = new JSONArray(yourJSONresponse);
List<String> list = new ArrayList<String>();
for(int i = 0; i < arr.length(); i++){
    list.add(arr.getJSONObject(i).getString("name"));
}