For the sake of the example lets assume you have a class Person with just a name.

private class Person {
    public String name;

    public Person(String name) {
        this.name = name;
    }
}

Jackson (Maven)

My personal favourite and probably the most widely used.

ObjectMapper mapper = new ObjectMapper();

// De-serialize to an object
Person user = mapper.readValue("{\"name\": \"John\"}", Person.class);
System.out.println(user.name); //John

// Read a single attribute
JsonNode nameNode = mapper.readTree("{\"name\": \"John\"}");
System.out.println(nameNode.get("name").asText());

Google GSON (Maven)

Gson g = new Gson();

// De-serialize to an object
Person person = g.fromJson("{\"name\": \"John\"}", Person.class);
System.out.println(person.name); //John

// Read a single attribute
JsonObject jsonObject = new JsonParser().parseString("{\"name\": \"John\"}").getAsJsonObject();
System.out.println(jsonObject.get("name").getAsString()); //John

Org.JSON (Maven)

This suggestion is listed here simply because it appears to be quite popular due to stackoverflow reference to it. I would not recommend using it as it is more a proof-of-concept project than an actual library.

JSONObject obj = new JSONObject("{\"name\": \"John\"}");

System.out.println(obj.getString("name")); //John

If it's a deeper key/value you're after and you're not dealing with arrays of keys/values at each level, you could recursively search the tree:

public static String recurseKeys(JSONObject jObj, String findKey) throws JSONException {
    String finalValue = "";
    if (jObj == null) {
        return "";
    }

    Iterator<String> keyItr = jObj.keys();
    Map<String, String> map = new HashMap<>();

    while(keyItr.hasNext()) {
        String key = keyItr.next();
        map.put(key, jObj.getString(key));
    }

    for (Map.Entry<String, String> e : (map).entrySet()) {
        String key = e.getKey();
        if (key.equalsIgnoreCase(findKey)) {
            return jObj.getString(key);
        }

        // read value
        Object value = jObj.get(key);

        if (value instanceof JSONObject) {
            finalValue = recurseKeys((JSONObject)value, findKey);
        }
    }

    // key is not found
    return finalValue;
}

Usage:

JSONObject jObj = new JSONObject(jsonString);
String extract = recurseKeys(jObj, "extract");

Using Map code from https://stackoverflow.com/a/4149555/2301224

Here are couple of alternatives versions with Jackson (since there are more than one ways you might want data as):

  ObjectMapper mapper = new ObjectMapper(); // just need one
  // Got a Java class that data maps to nicely? If so:
  FacebookGraph graph = mapper.readValue(url, FaceBookGraph.class);
  // Or: if no class (and don't need one), just map to Map.class:
  Map<String,Object> map = mapper.readValue(url, Map.class);

And specifically the usual (IMO) case where you want to deal with Java objects, can be made one liner:

FacebookGraph graph = new ObjectMapper().readValue(url, FaceBookGraph.class);

Other libs like Gson also support one-line methods; why many examples show much longer sections is odd. And even worse is that many examples use obsolete org.json library; it may have been the first thing around, but there are half a dozen better alternatives so there is very little reason to use it.