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?.



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>();

        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>();

        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


How do I create type specific collections?

package org.kodejava.example.fundamental;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class TypeSpecificCollection {
    public static void main(String[] args) {
        // Using a Generic can enable us to create a type specific collection
        // object. In the example below we create a Map whose key is an Integer
        // a have the value of a String.
        Map<Integer, String> grades = new HashMap<Integer, String>();
        grades.put(1, "A");
        grades.put(2, "B");
        grades.put(3, "C");
        grades.put(4, "D");
        grades.put(5, "E");

        // A value obtained from type specific collection doesn't not need to
        // be casted, it knows the type returned.
        String value = grades.get(1);
        System.out.println("value = " + value);

        // Creating a List that will contains a String only values.
        List<String> dayNames = new ArrayList<String>();

        // We also don't need to cast the retrieved value because it knows the
        // returned type object.
        String firstDay = dayNames.get(0);
        System.out.println("firstDay = " + firstDay);

How do I use for-each in Java?

Using for-each command to iterate arrays or a list can simplified our code. Below is an example how to do it in Java. The first loop is for iterating array and the second for iterating a list containing a some names.

package org.kodejava.example.lang;

import java.util.ArrayList;
import java.util.List;

public class ForEachExample {
    public static void main(String[] args) {
        Integer[] numbers = {10, 100, 1000, 10000, 100000, 1000000};

        for (Integer i : numbers) {
            System.out.println("Number: " + i);

        List<String> names = new ArrayList<String>();

        for (String name : names) {
            System.out.println("Name: " + name);

How do I Convert Array to Collection?

package org.kodejava.example.util;

import java.util.Arrays;
import java.util.List;

public class ArrayToListExample {
    public static void main(String[] args) {
        // Creates an array of object, in this case we create an
        // Integer array.
        Integer[] numbers = {1, 1, 2, 3, 5, 8, 13, 21, 34, 55};

        // Convert the created array above to collection, in this
        // example we convert it to a List.
        List list = Arrays.asList(numbers);

        // We've got a list of our array here and iterate it.
        for (int i = 0; i < list.size(); i++) {
            System.out.print(list.get(i) + ", ");

How do I convert collection to ArrayList?

package org.kodejava.example.util;

import java.util.ArrayList;
import java.util.LinkedList;

public class CollectionToArrayList {
    public static void main(String[] args) {
        // We create LinkedList collection type at put some values
        // in it. Here we put A, B, C and D letter into it.
        LinkedList<String> linkedList = new LinkedList<String>();

        // Let say you want to convert it to other type of collection,
        // for instance here we convert it into ArrayList. To do it
        // we can pass the collection created above as a parameter to
        // ArrayList constructor.
        ArrayList<String> arrayList = new ArrayList<String>(linkedList);

        // Now we have converted the collection into ArrayList and
        // printed what is inside.
        for (String s : arrayList) {
            System.out.println("s = " + s);

How do I create an empty collection object?

Sometimes you need to return an empty collection from your Java methods. The java.util.Collections utility class have three different static constants for creating empty List, Set and Map.

  • Collections.EMPTY_LIST
  • Collections.EMPTY_SET
  • Collections.EMPTY_MAP

There are also methods when you want to create type-safe empty collections.

  • Collections.emptyList()
  • Collections.emptySet()
  • Collections.emptyMap()

Bellow it the code example.

package org.kodejava.example.util;

import java.util.*;

public class EmptyCollectionDemo {
    public static void main(String args[]) {
        List list = Collections.EMPTY_LIST;
        System.out.println("list.size()  = " + list.size());
        Set set = Collections.EMPTY_SET;
        System.out.println("set.size()   = " + set.size());
        Map map = Collections.EMPTY_MAP;
        System.out.println("map.size()   = " + map.size());

        // For the type-safe example use the following methods.
        List<String> strings = Collections.emptyList();
        System.out.println("strings      = " + strings.size());

        Set<Long> longs = Collections.emptySet();
        System.out.println("longs.size() = " + longs.size());

        Map<String, Date> dates = Collections.emptyMap();
        System.out.println("dates.size() = " + dates.size());

The output are:

list.size()  = 0
set.size()   = 0
map.size()   = 0
strings      = 0
longs.size() = 0
dates.size() = 0

How do I convert a collection object into an array?

To convert collection-based data into array we can use toArray() or toArray(T[] a) method provided by the implementation of Collection interface such as java.util.ArrayList.

package org.kodejava.example.util;

import java.util.List;
import java.util.ArrayList;

public class CollectionToArrayExample {
    public static void main(String[] args) {
        List<String> words = new ArrayList<>();

        String[] array = words.toArray(new String[words.size()]);
        for (String word : array) {

Our sample code result is shown below:


How do I convert an array into a collection object?

To convert array based data into List / Collection based we can use java.util.Arrays class. This class provide a static method asList(T... a) that converts array into List / Collection.

package org.kodejava.example.util;

import java.util.Arrays;
import java.util.List;

public class ArraysExample {
    public static void main(String[] args) {
        String[] words = {"Happy", "New", "Year", "2006"};
        List<String> list = Arrays.asList(words);

        for (String word : list) {

The results of our code are:


How do I use ArrayList class?

In this example we will learn how to use the java.util.ArrayList class. An ArrayList is part of the Java Collection Framework. By using this class we can create a dynamic size array of data, which means we can add or remove elements from the array dynamically.

In the code below we will see the demonstration on how to create an instance of ArrayList, add some elements, remove elements and iterate through the entire ArrayList elements either using a for-loop or using the for-each syntax.

We can also see how to convert the ArrayList into an array object using the toArray() method. And we use the Arrays.toString() utility method when we want to print the content of an array.

When creating a class instance it is a good practice to use the interface as the type of variable instead of the concrete type directly. This can make us easily update our code if we don’t want to use ArrayList anymore sometime in the future.

And here is the code snippet:

package org.kodejava.example.util;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class ArrayListExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();

        // Add items into ArrayList
        list.add("Item 1");
        list.add("Item 2");
        list.add("Item 3");

        // Remove the third item from ArrayList, first index = 0

        // Iterate ArrayList item using for loop statement
        for (int i = 0; i < list.size(); i++) {
            String item = list.get(i);
            System.out.println("Item = " + item);

        // Iterate ArrayList item using for-each statement
        for (String item : list) {
            System.out.println("Item = " + item);

        // Convert ArrayList into array of object
        String[] array = list.toArray(new String[list.size()]);
        System.out.println("Items = " + Arrays.toString(array));

Executing the program will give us the following output printed in our console.

Item = Item 1
Item = Item 2
Item = Item 1
Item = Item 2
Items = [Item 1, Item 2]