If you need to create a JSONObject and put data in it, consider the following example:

// Create a new javax.json.JSONObject instance.
JSONObject first = new JSONObject();

first.put("foo", "bar");
first.put("temperature", 21.5);
first.put("year", 2016);

// Add a second object.
JSONObject second = new JSONObject();
second.put("Hello", "world");
first.put("message", second);

// Create a new JSONArray with some values
JSONArray someMonths = new JSONArray(new String[] { "January", "February" });
someMonths.put("March");
// Add another month as the fifth element, leaving the 4th element unset.
someMonths.put(4, "May");

// Add the array to our object
object.put("months", someMonths);

// Encode
String json = object.toString();

// An exercise for the reader: Add pretty-printing!
/* {
       "foo":"bar",
       "temperature":21.5,
       "year":2016,
       "message":{"Hello":"world"},
       "months":["January","February","March",null,"May"]
   }
*/

If you need to get data from a JSONObject, consider the following example:

String json = "{\"foo\":\"bar\",\"temperature\":21.5,\"year\":2016,\"message\":{\"Hello\":\"world\"},\"months\":[\"January\",\"February\",\"March\",null,\"May\"]}";

// Decode the JSON-encoded string
JSONObject object = new JSONObject(json);

// Retrieve some values
String foo = object.getString("foo");
double temperature = object.getDouble("temperature");
int year = object.getInt("year");

// Retrieve another object
JSONObject secondary = object.getJSONObject("message");
String world = secondary.getString("Hello");

// Retrieve an array
JSONArray someMonths = object.getJSONArray("months");
// Get some values from the array
int nMonths = someMonths.length();
String february = someMonths.getString(1);

JSONObject and JSONArray have a few methods that are very useful while dealing with a possibility that a value your are trying to get does not exist or is of another type.

JSONObject obj = new JSONObject();
obj.putString("foo", "bar");

// For existing properties of the correct type, there is no difference
obj.getString("foo");        // returns "bar"
obj.optString("foo");        // returns "bar"
obj.optString("foo", "tux"); // returns "bar"

// However, if a value cannot be coerced to the required type, the behavior differs
obj.getInt("foo");      // throws JSONException
obj.optInt("foo");      // returns 0
obj.optInt("foo", 123); // returns 123

// Same if a property does not exist
obj.getString("undefined");        // throws JSONException
obj.optString("undefined");        // returns ""
obj.optString("undefined", "tux"); // returns "tux"

The same rules apply to the getXXX / optXXX methods of JSONArray.

Lets assume you have a class called Person with just name

private class Person {
    public String name;

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

Code:

Gson g = new Gson();

Person person = new Person("John");
System.out.println(g.toJson(person)); // {"name":"John"}

Of course the Gson jar must be on the classpath.

Lets assume you have a class called Person with just name

private class Person {
    public String name;

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

Code:

Gson gson = new Gson();
String json = "{\"name\": \"John\"}";

Person person = gson.fromJson(json, Person.class);
System.out.println(person.name); //John

You must have gson library in your classpath.

String json = "{\"name\": \"John\", \"age\":21}";

JsonObject jsonObject = new JsonParser().parse(json).getAsJsonObject();

System.out.println(jsonObject.get("name").getAsString()); //John
System.out.println(jsonObject.get("age").getAsInt()); //21

Pojo Model

public class Model {
    private String firstName;
    private String lastName;
    private int age;
    /* Getters and setters not shown for brevity */        
}

Example: String to Object

Model outputObject = objectMapper.readValue(
     "{\"firstName\":\"John\",\"lastName\":\"Doe\",\"age\":23}",
     Model.class);
System.out.println(outputObject.getFirstName());
//result: John

Example: Object to String

String jsonString = objectMapper.writeValueAsString(inputObject));
//result: {"firstName":"John","lastName":"Doe","age":23}

Details

Import statement needed:

import com.fasterxml.jackson.databind.ObjectMapper;

Maven dependency: jackson-databind

ObjectMapper instance

//creating one
ObjectMapper objectMapper = new ObjectMapper();

• ObjectMapper is threadsafe
• recommended: have a shared, static instance

Deserialization:

<T> T readValue(String content, Class<T> valueType)  

• valueType needs to be specified -- the return will be of this type
• Throws
  • IOException - in case of a low-level I/O problem
  • JsonParseException - if underlying input contains invalid content
  • JsonMappingException - if the input JSON structure does not match object structure

Usage example (jsonString is the input string):

Model fromJson = objectMapper.readValue(jsonString, Model.class);

Method for serialization:

String writeValueAsString(Object value)

• Throws
  • JsonProcessingException in case of an error
  • Note: prior to version 2.1, throws clause included IOException; 2.1 removed it.

Iterate over JSONObject properties

JSONObject obj = new JSONObject("{\"isMarried\":\"true\", \"name\":\"Nikita\", \"age\":\"30\"}");
Iterator<String> keys = obj.keys();//all keys: isMarried, name & age
while (keys.hasNext()) {                      //as long as there is another key
      String key = keys.next();               //get next key 
      Object value = obj.get(key);            //get next value by key
      System.out.println(key + " : " + value);//print key : value
}

Iterate over JSONArray values

JSONArray arr = new JSONArray();        //Initialize an empty array
//push (append) some values in:
arr.put("Stack");
arr.put("Over");
arr.put("Flow");
for (int i = 0; i < arr.length(); i++) {//iterate over all values
    Object value = arr.get(i);          //get value
    System.out.println(value);          //print each value
}

You can use method chaining while working with JSONObject and JSONArray.

JSONObject example

JSONObject obj = new JSONObject();//Initialize an empty JSON object 
//Before: {}
obj.put("name","Nikita").put("age","30").put("isMarried","true");
//After: {"name":"Nikita","age":30,"isMarried":true}

JSONArray

JSONArray arr = new JSONArray();//Initialize an empty array
//Before: []
arr.put("Stack").put("Over").put("Flow");
//After: ["Stack","Over","Flow"]

If you need to add a property with a null value, you should use the predefined static final JSONObject.NULL and not the standard Java null reference.

JSONObject.NULL is a sentinel value used to explicitly define a property with an empty value.

JSONObject obj = new JSONObject();
obj.put("some", JSONObject.NULL);   //Creates: {"some":null}
System.out.println(obj.get("some"));//prints: null

Note

JSONObject.NULL.equals(null); //returns true

Which is a clear violation of Java.equals() contract:

For any non-null reference value x, x.equals(null) should return false

Here is a simple JsonArray which you would like to convert to a Java ArrayList:

{
    "list": [
                "Test_String_1",
                "Test_String_2"
            ] 
}

Now pass the JsonArray 'list' to the following method which returns a corresponding Java ArrayList:

public ArrayList<String> getListString(String jsonList){
    Type listType = new TypeToken<List<String>>() {}.getType();
    //make sure the name 'list' matches the name of 'JsonArray' in your 'Json'.
    ArrayList<String> list = new Gson().fromJson(jsonList, listType);    
    return list;
}

You should add the following maven dependency to your POM.xml file:

<!-- https://mvnrepository.com/artifact/com.google.code.gson/gson -->
<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.7</version>
</dependency>

Or you should have the jar com.google.code.gson:gson:jar:<version> in your classpath.

Suppose you have a pojo class Person

public class Person {
    public String name;

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

And you want to parse it into a JSON array or a map of Person objects. Due to type erasure you cannot construct classes of List<Person> and Map<String, Person> at runtime directly (and thus use them to deserialize JSON). To overcome this limitation jackson provides two approaches - TypeFactory and TypeReference.

TypeFactory

The approach taken here is to use a factory (and its static utility function) to build your type for you. The parameters it takes are the collection you want to use (list, set, etc.) and the class you want to store in that collection.

TypeReference

The type reference approach seems simpler because it saves you a bit of typing and looks cleaner. TypeReference accepts a type parameter, where you pass the desired type List<Person>. You simply instantiate this TypeReference object and use it as your type container.

Now let's look at how to actually deserialize your JSON into a Java object. If your JSON is formatted as an array, you can deserialize it as a List. If there is a more complex nested structure, you will want to deserialize to a Map. We will look at examples of both.

Deserializing JSON array

String jsonString = "[{\"name\": \"Alice\"}, {\"name\": \"Bob\"}]"

TypeFactory approach

CollectionType listType = 
    factory.constructCollectionType(List.class, Person.class);
List<Preson> list = mapper.readValue(jsonString, listType);

TypeReference approach

TypeReference<Person> listType = new TypeReference<List<Person>>() {};
List<Person> list = mapper.readValue(jsonString, listType);

Deserializing JSON map

String jsonString = "{\"0\": {\"name\": \"Alice\"}, \"1\": {\"name\": \"Bob\"}}"

TypeFactory approach

CollectionType mapType = 
    factory.constructMapLikeType(Map.class, String.class, Person.class);
List<Person> list = mapper.readValue(jsonString, mapType);

TypeReference approach

TypeReference<Person> mapType = new TypeReference<Map<String, Person>>() {};
Map<String, Person> list = mapper.readValue(jsonString, mapType);

Details

Import statement used:

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.type.CollectionType;

Instances used:

ObjectMapper mapper = new ObjectMapper();
TypeFactory factory = mapper.getTypeFactory();

Note

While TypeReference approach may look better it has several drawbacks:

1. TypeReference should be instantiated using anonymous class
2. You should provide generic explicity

Failing to do so may lead to loss of generic type argument which will lead to deserialization failure.