Java to kotlin part -1

Hello Everyone,

The purpose of this post to teach you kotlin syntax and new features based upon your current knowledge of java.

Kotlin is a statically-typed programming language that runs on the Java virtual machine and also can be compiled to JavaScript source code or use the LLVM compiler infrastructure. Its primary development is from a team of JetBrains programmers based in Saint Petersburg, Russia. As of Android studio, 3.0 kotlin is a fully supported programming language as well you can also write a server-side application in kotlin. you can also use kotlin to write build script in the gradle build system.

The best things you can directly use Java classes and library into kotlin and vice-versa

so let’s jump into code.

# print hello world


public static void main(String[] args) {

    System.out.println("Hello World!");


fun main(args : Array)
    println("Hello World!")

# using variable 


public static void main(String[] args) {

    final int a =1;
    final int b = 2;
    final int c;
    c = 3;
    int d =4;

    System.out.println(String.format("a = %d, b = %d, c = %d, d = %d", a, b, c, d));


fun main(args : Array)
    val a: Int = 1  // immediate assignment
    val b = 2   // `Int` type is inferred
    val c: Int  // Type required when no initializer is provided
    c = 3       // deferred assignment
    var d = 4  //Mutable variable
    println("a = $a, b = $b, c = $c, d = $d")

#using function with return and no parameter 


public String message()
    return "Hello World";


fun message() : String
    return "Hello World"

#using function with void type and parameter 


public void message(String message)


fun message(message : String) : Unit
    println("Hello World")


fun message(message : String)
    println("Hello World")

#using conditional expression


public int maxValue(int a, int b)
    if(a > b){
        return a;
    }else {
        return b;


fun maxValue(a: Int, b: Int): Int {
    if (a > b) {
        return a
    } else {
        return b

#using switch case(in case of kotlin we can using ‘when’)


switch (args)
    case 1:
    case 2:
           System.out.println("args is not 1 or 2");



    1 -> println("One")
    2 -> println("Two")
    else -> {
        println("args is not 1 or 2")

#using for loop


String[] arrays = {"Apple", "Mango", "Orange"};
for(int i=0;i<arrays.length;i++)
    System.out.println(String.format("item at %d is %s",i,arrays[i]));


val items = arrayOf("Apple","Mango","Orange");
for (index in items.indices)
    println("item at $index is ${items[index]}")

#using while loop


String[] arrays = {"Apple", "Mango", "Orange"};
int index = 0;
while(index < arrays.length)
    System.out.println(String.format("item at %d is %s",index,arrays[index]));


val items = arrayOf("Apple","Mango","Orange");
var index =0
while (index < items.size)
    println("item at $index is ${items[index]}")

#using lambda


String[] strArray =  {"Avocado","Mango","Apple","Orange","banana","Guava"};
List fruits = Arrays.asList(strArray);>s.startsWith("A")).sorted().map(s->s.toUpperCase())


val fruits = arrayOf("Avocado","Mango","Apple","Orange","banana","Guava")
 fruits.filter { it.startsWith("a") }.sortedBy { it }.
         map { it.toUpperCase() }.forEach{ println(it)}

#using null values and checking for null


public Integer parseInt(String str)
    try {
        return Integer.parseInt(str);
    }catch(NumberFormatException ex)
        return null;


fun parseInt(str: String) : Int?
    return str.toIntOrNull()
//incase if you call parseInt method with null input
fun parseInt(str: String?) : Int?
    return str?.toIntOrNull()

#using type checks


public Integer getStringLength(Object obj)
    if(obj instanceof String)
        return ((String) obj).length();

    return null;


fun getStringLength(obj: Any): Int? {
    if (obj is String) {
        // `obj` is automatically cast to `String` in this branch
        return obj.length

    // `obj` is still of type `Any` outside of the type-checked branch
    return null

Ref –  Kotlin docs


Thanks for reading 🙂

Expose RESTful APIs using spring boot in 7 minutes

Hello Friends,

Today we are going to see how to expose standalone RESTful web services, the purpose of a post is to enable a reader to write their own restful web services.

Yes, that’s right! , I’m sure if you see complete video of this post you can write your RESTful web service.. ok enough talk 🙂 let’s start.


Hope you like this post. Thanks 🙂



Spring 5 reactive web application using kotlin

Spring has introduced webFlux new reactive web framework to support reactive programming.

Reactive programming?

“Reactive programming is about processing an asynchronous stream of data items, where applications react to the data items as they occur. A stream of data is essentially a sequence of data items occurring over time. This model is more memory efficient because the data is processed as streams, as compared to iterating over the in-memory data” (define at Oracle docs) .

For example, in an imperative programming setting, a = b +c would mean that a is being assigned the result of  b + c in the instant the expression is evaluated , and letter the values of b and/or c can be changed with no effect on the value of a, However, in reactive programming, the valud of a is automatically updated whenever the values of b and/or  c changes, without the program having to re-execute the sentence a= b + c to determine the presently assigned value of a (click here for more detail) .

Prerequisites :

Spring boot
Spring 5 webFlux and Reactive basic(RxJava or Project Reactor)
Spring data

let generate spring boot template from Spring Initializr

Screen Shot 2017-11-26 at 11.13.34 AM

now import the generated project in your IDE and create Customer data class, check below example:-

Screen Shot 2017-11-26 at 11.35.48 AM

git link

@Document applied at the class level to indicate this class is a candidate for mapping to the database. You can specify the name of the collection where the database will be stored.

@Field applied at a variable level to define column name.

data class is specialized for hold data.

then created ‘Repository’ class look like below.

Screen Shot 2017-11-26 at 10.09.42 AM

git link

above class, I’ve extended ReactiveMongoReposiotry instead of MongoRepository because we are using Reactive features like Mono and Flux.

once ‘CustomerRepo’ repository classes created then create a handler for it, below code sample.

Screen Shot 2017-11-26 at 10.08.20 AM

git link

ServerRequest and ServerResponse introduce in spring 5, we can extract request param, path variable, header information and body details from ServerRequest.

Mono is a specialized Publisher that emits at most one item and then optionally terminates with an onComplete signal or an onError signal. click here for more detail.

Flux is a standard ‘Publisher’ representing an asynchronous sequence of 0 to N emitted items, optionally terminated by either a completion signal or an error. Thus, the possible values of a flux are a value, a completion signal, or an error. As in the Reactive Streams spec, these 3 types of signal translate to calls to a downstream object’s ‘onNext’, ‘onComplete’ or ‘onError’ methods. click here for more detail.

now we have handler and repository we need to expose endpoint, there is the magic of webFlux, you not need to create a separate class for it also get a ride from so much boilerplate, see below snippet.

Screen Shot 2017-11-26 at 10.07.39 AM

git link

Let me explain line by line,

@Configuration annotation indicates that a class declares one or more @Bean methods and may be processed by the Spring container to generate bean definitions and service requests for those beans at runtime.

@EnableReactiveMongoRepositories annotation is for Spring boot to enable reactive mongo repositories. if no base package is configured, in the case of spring boot application it is by default enable if you have include starter library.

@Bean is a method-level annotation and a direct analog of the XML element. The annotation supports most of the attributes offered by.

RouterFunction Central entry point to Spring’s functional web framework. Exposes routing functionality, such as to create a RouterFunction given a RequestPredicate and HandlerFunction, and to do further subroutine on an existing routing function.

HandlerFunction  Incoming HTTP requests are handled by a HandlerFunction, which is essentially a function that takes a ServerRequest and returns a Mono
let’s test whatever we have written, see below code snippet.

Screen Shot 2017-11-28 at 4.59.39 AM

git link

By default Spring webFulx include asynchronous natty server.

@RunWith(SpringRunner::class) Indicates that the class should use Spring’s JUnit facilities.

@SpringBootTest provides the following features over and above the regular Spring TestContext Framework,

  • Uses SpringBootContextLoader as the default ContextLoader when no specific @ContextConfiguration(loader=…) is defined.
  • Automatically searches for a @SpringBootConfiguration when nested @Configuration is not used, and no explicit classes are specified.
  • Allows custom Environment properties to be defined using the properties attribute.
  • Provides support for different webEnvironment modes, including the ability to start a fully running container listening on a defined or random port.
  • Registers a TestRestTemplate bean for use in web tests that are using a fully running container.

WebTestClient Non-blocking, reactive client for testing web servers. It uses the reactive WebClient internally to perform requests and provides a fluent API to verify responses.

WebTestClient can connect to any server over an HTTP connection. It can also bind directly to WebFlux applications using mock request and response objects, without the need for an HTTP server.


There are lots more features provided by Spring 5 and project reactor, I’ve introduced very basic sample on it. sample source available on my github .

Create spring beans using YAML

Spring highly adopted annotation based configuration and it is happily accepted by developer communities and why not, no one wants to struggle with XML tags but that’s not enough somewhere we still miss XML configuration or you can say external file configuration as powerful as XML configuration. especially when we write an application that has to alter behaviors without compilation.

But still writing an XML configuration is not readable and not easily understandable for beginners, so I’ve written experimental plugins for spring boot that convert YAML definition to Spring Beans.

here is an example of bean definition in XML and same in YAML.


Screen Shot 2017-11-04 at 6.45.07 PM


Screen Shot 2017-11-04 at 6.44.14 PM

above code snippet, you can see a line of code and readability, let jump in a code example.

below code snippet, only need to add annotation “@ImportYamlResource” that will work same as “@ImportResource”

Screen Shot 2017-11-05 at 10.38.22 AM

below an example link, you can check out a project from git


download library 


Note ->  YAML-spring-beans is a just experimental library, still there are lots of features not included

feedback will be much appreciated 🙂

Configure GitLab Runner with shell

Hi ,

I’ve found many examples where people configure with docker, in some cases if we need simple configuration for internal uses, we have also an alternative that can help to configure your GitLab CI runner easily.

In my case, i’m using Centos.

  1. added gitlab multi runner to yum repository

# curl -L | sudo bash

2.after added to repository install gitlab-multi runner
#sudo yum install gitlab-ci-multi-runner

once gitlab runner install, it will create “gitlab-runner” user in your machine

may find here “/home/gitlab-runner” , gitlab-runner user do not have permission you need to add it to user group or something else whatever you need.

you can also see multiple commands provided by gitlab runner

#gitlab-runner –help

now ,we need to configure it with our gitlab hosted repository

#sudo gitlab-ci-multi-runner register 

Please enter the gitlab-ci coordinator URL (e.g. )
Please enter the gitlab-ci token for this runner
Please enter the gitlab-ci description for this runner
INFO[0034] fcf5c619 Registering runner… succeeded
Please enter the executor: shell, docker, docker-ssh, ssh?
running already the config should be automatically reloaded!


after gitlab runner successfully configure you will see “admin area ->Runner” in gitlab

something similar to below images

Screen Shot 2016-10-08 at 5.29.43 PM.png

that is not completed yet you also need to configure Ci with include this file in your project root directory “”

added some configuration related to your project build and unit test running etc..

here is simple example i’ve added in “.gitlab-ci.yml” file

– cd spirng-mvc
– echo “making build”
– ./gradlew clean
– ./gradlew build

once configuration done, every commit we can run our build

Screen Shot 2016-10-08 at 5.55.33 PM.png

may I have skip few steps, let us know if needed will update.

Thank for reading 🙂

Strategy pattern in java

Hi Friends,

today i want to share strategy pattern, of course this is very well known and mostly used pattern. and their is lots of cases where we need to use strategic pattern.

#what is Strategy pattern?

The strategy pattern(also known as the policy pattern) is a software design pattern that enables an algorithm’s behavior to be selected at runtime. The strategy pattern. defines a family of algorithms, encapsulates each algorithm, and makes the algorithms interchangeable within that family. more

#When to use?

when we have multiple algorithm for a specific task and want to decides the actual implementation to be used at runtime


Suppose there is one software startup company and they have limited resource to done their work but all resource have multi-talent to do multiple roles,  like four resources (EmployeeA, EmployeeB, EmployeeC, EmployeeD) each one can able to done coding, testing,content writing etc. and any work can assign to anyone . its depend on availability.

Screen Shot 2015-10-22 at 3.10.43 AM

you can find example on Github link



Chat application over xmpp protocol

Hi folk,

Today I’m going to share simple chat application that is based on xmpp protocol.

i know that you have lots of question  about xmpp , so simply i’m start with some definition or basic question.

# what is xmpp ?

xmpp is the extensible messaging and presence protocol, a set of open technologies for instant messaging,presence,multi-party chat, voice and video calls, collaboration, lightweight middleware, content syndication,and generalized routing of xml data.

XMPP was originally developed in the Jabber open-source community to provide an open, decentralized alternative to the closed instant messaging services at that time. click here for more detail.

# Why xmpp?

#Open standard – its gives you the choice and control about how you access your data and service.

#multi platform support – you can create chat application for the multiple platform over the xmpp protocol.

# xmpp implementation :

For the xmpp chat application we have to implement chat client and  configure server. firstly we are talking about chat client then we going to implement server configuration.


The following assumes that you have already basic knowledge in Android development with android studio.

1.Android studio

2.Smack 4.1 library (smack 4.1 supporting android for previous version we need to implement asmack  library).

#Example :

step 1 > Create a project in android studio(File -> New Project(add Application name,project location) -> check phone and teblet and select minimum sdk -> select activity type -> define activity name ->finish) for more detail how to manage a project in android studio click here

add library detail in build.gradle file as below.


step 2 > after adding library into project , now connect to openfire server and login as user.


above this code example i have disable ssl and DIGEST-MD5 . According to your need you can enable it.

if you want to create user through your application then use AccountManager classto do it as for exp :

Account_manager step 3 > after successfully connected to chat server now create a chat with another user and receive there message.


Above this code i’ve created ‘doChat’ method where we need to pass userId(userId withwhome we want to chat) and implement ChatMessageListner to recive user message.

I hope a tutorial is helpful.

Thanks for reading 🙂