An example of creating a JSON string representation of an object with a nested object using the Gson library from Google. You will need to add the google-gson library as a dependency. Available here -> https://code.google.com/p/google-gson/

JavaToGSONExample.java

package net.tomlins.testapplication.json;

import com.google.gson.*;

public class JavaObjToGSONStringExample {

    public static void main(String... args) {

        JavaObjToGSONStringExample app = new JavaObjToGSONStringExample();
        System.out.println(app.toGSONStringExample());
    }

    public String toGSONStringExample() {

        // Create a GSon object
        Gson gson = new Gson();

        // Create a simple Employee object
        Employee emp = new Employee();
        emp.setName("Jason");
        emp.setPosition("Java Developer");
        emp.setSalary(1000);

        // Create a simple Address object for the Employee object
        Address empAddress = new Address();
        empAddress.setHouseNumber(10);
        empAddress.setStreet("Java Street");
        empAddress.setSuburb("Grange");
        empAddress.setCity("Brisbane");
        empAddress.setCountry("Australia");

        // Set the employees address
        emp.setAddress(empAddress);

        // Get our GSon string representation of the Employee object
        String sGobj = gson.toJson(emp);

        return sGobj;

    }

    /**
     * Simple Employee POJO
     */
    public class Employee {

        private String name;
        private String position;
        private int salary;
        private Address address;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getPosition() {
            return position;
        }

        public void setPosition(String position) {
            this.position = position;
        }

        public int getSalary() {
            return salary;
        }

        public void setSalary(int salary) {
            this.salary = salary;
        }

        public Address getAddress() {
            return address;
        }

        public void setAddress(Address address) {
            this.address = address;
        }
    }

    /**
     * Simple Address POJO
     */
    public class Address {

        private int houseNumber;
        private String street;
        private String suburb;
        private String city;
        private String country;

        public int getHouseNumber() {
            return houseNumber;
        }

        public void setHouseNumber(int houseNumber) {
            this.houseNumber = houseNumber;
        }

        public String getStreet() {
            return street;
        }

        public void setStreet(String street) {
            this.street = street;
        }

        public String getSuburb() {
            return suburb;
        }

        public void setSuburb(String suburb) {
            this.suburb = suburb;
        }

        public String getCity() {
            return city;
        }

        public void setCity(String city) {
            this.city = city;
        }

        public String getCountry() {
            return country;
        }

        public void setCountry(String country) {
            this.country = country;
        }
    }
}