How do I convert Map into JSON?

This example show you how to convert a java.util.Map into JSON string and back to Map again.

package org.kodejava.example.google.gson;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;

public class MapToJson {
    public static void main(String[] args) {
        Map<String, String> colours = new HashMap<>();
        colours.put("BLACK", "#000000");
        colours.put("RED", "#FF0000");
        colours.put("GREEN", "#008000");
        colours.put("BLUE", "#0000FF");
        colours.put("YELLOW", "#FFFF00");
        colours.put("WHITE", "#FFFFFF");

        // Convert a Map into JSON string.
        Gson gson = new Gson();
        String json = gson.toJson(colours);
        System.out.println("json = " + json);

        // Convert JSON string back to Map.
        Type type = new TypeToken<Map<String, String>>(){}.getType();
        Map<String, String> map = gson.fromJson(json, type);
        for (String key : map.keySet()) {
            System.out.println("map.get = " + map.get(key));
        }
    }
}

Here is the result of the program:

json = {"RED":"#FF0000","WHITE":"#FFFFFF","BLUE":"#0000FF","BLACK":"#000000","YELLOW":"#FFFF00","GREEN":"#008000"}
map.get = #FF0000
map.get = #FFFFFF
map.get = #0000FF
map.get = #000000
map.get = #FFFF00
map.get = #008000

Maven Dependencies

<!-- http://repo1.maven.org/maven2/com/google/code/gson/gson/2.8.0/gson-2.8.0.jar -->
<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.0</version>
</dependency>

How do I convert collections into JSON?

This example show you how to convert Java collections object into JSON string. For Student class use in this example you can find it the previous example on How do I convert object into JSON?.

package org.kodejava.example.google.gson;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.List;

public class CollectionToJson {
    public static void main(String[] args) {
        // Converts a collection of string object into JSON string.
        List<String> names = new ArrayList<String>();
        names.add("Alice");
        names.add("Bob");
        names.add("Carol");
        names.add("Mallory");

        Gson gson = new Gson();
        String jsonNames = gson.toJson(names);
        System.out.println("jsonNames = " + jsonNames);

        // Converts a collection Student object into JSON string
        Student a = new Student("Alice", "Apple St", getDOB(2000, 10, 1));
        Student b = new Student("Bob", "Banana St", null);
        Student c = new Student("Carol", "Grape St", getDOB(2000, 5, 21));
        Student d = new Student("Mallory", "Mango St", null);

        List<Student> students = new ArrayList<Student>();
        students.add(a);
        students.add(b);
        students.add(c);
        students.add(d);

        gson = new Gson();
        String jsonStudents = gson.toJson(students);
        System.out.println("jsonStudents = " + jsonStudents);

        // Converts JSON string into a collection of Student object.
        Type type = new TypeToken<List<Student>>() {}.getType();
        List<Student> studentList = gson.fromJson(jsonStudents, type);

        for (Student student : studentList) {
            System.out.println("student.getName() = " + student.getName());
        }
    }

    private static Date getDOB(int year, int month, int date) {
        Calendar calendar = Calendar.getInstance();
        calendar.set(Calendar.YEAR, year);
        calendar.set(Calendar.MONTH, month - 1);
        calendar.set(Calendar.DATE, date);
        calendar.set(Calendar.HOUR, 0);
        calendar.set(Calendar.MINUTE, 0);
        calendar.set(Calendar.SECOND, 0);
        return calendar.getTime();
    }
}

Here is the result of our program:

jsonNames = ["Alice","Bob","Carol","Mallory"]
jsonStudents = [{"name":"Alice","address":"Apple St","dateOfBirth":"Nov 1, 3900 12:00:00 AM"},{"name":"Bob","address":"Banana St"},{"name":"Carol","address":"Grape St","dateOfBirth":"Jun 21, 3900 12:00:00 AM"},{"name":"Mallory","address":"Mango St"}]
student.getName() = Alice
student.getName() = Bob
student.getName() = Carol
student.getName() = Mallory

Maven Dependencies

<!-- http://repo1.maven.org/maven2/com/google/code/gson/gson/2.8.0/gson-2.8.0.jar -->
<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.0</version>
</dependency>

How do I convert array into JSON?

In the example below you can see how to convert an array into JSON string. We serialize the array to JSON using the Gson.toJson() method. To deserialize a string of JSON into array we use the Gson.fromJson() method.

package org.kodejava.example.google.gson;

import com.google.gson.Gson;

public class ArrayToJson {
    public static void main(String[] args) {
        int[] numbers = {1, 1, 2, 3, 5, 8, 13};
        String[] days = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};

        // Create a new instance of Gson
        Gson gson = new Gson();

        // Convert numbers array into JSON string.
        String numbersJson = gson.toJson(numbers);

        // Convert strings array into JSON string
        String daysJson = gson.toJson(days);
        System.out.println("numbersJson = " + numbersJson);
        System.out.println("daysJson = " + daysJson);

        // Convert from JSON string to a primitive array of int.
        int[] fibonacci = gson.fromJson(numbersJson, int[].class);
        for (int number : fibonacci) {
            System.out.print(number + " ");
        }
        System.out.println("");

        // Convert from JSON string to a string array.
        String[] weekDays = gson.fromJson(daysJson, String[].class);
        for (String weekDay : weekDays) {
            System.out.print(weekDay + " ");
        }
        System.out.println("");

        // Converting multidimensional array into JSON
        int[][] data = {{1, 2, 3}, {3, 4, 5}, {4, 5, 6}};
        String json = gson.toJson(data);
        System.out.println("Data = " + json);

        // Convert JSON string into multidimensional array of int.
        int[][] dataMap = gson.fromJson(json, int[][].class);
        for (int[] i : dataMap) {
            for (int j : i) {
                System.out.print(j + " ");
            }
            System.out.println("");
        }
    }
}

Here is our code result:

numbersJson = [1,1,2,3,5,8,13]
daysJson = ["Sun","Mon","Tue","Wed","Thu","Fri","Sat"]
1 1 2 3 5 8 13 
Sun Mon Tue Wed Thu Fri Sat 
Data = [[1,2,3],[3,4,5],[4,5,6]]
1 2 3 
3 4 5 
4 5 6

Maven Dependencies

<!-- http://repo1.maven.org/maven2/com/google/code/gson/gson/2.8.0/gson-2.8.0.jar -->
<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.0</version>
</dependency>