Kotlin’s extensions for each class

Extensions in Kotlin are very powerful mechanism. It allows for add any method to any of existing classes. Each instance has (as in Java) equals, toString and hashCode methods, but there is much more in Kotlin.Example classesLet’s define some simple cl…

Extensions in Kotlin are very powerful mechanism. It allows for add any method to any of existing classes. Each instance has (as in Java) equals, toString and hashCode methods, but there is much more in Kotlin.

Example classes

Let’s define some simple classes describing person: normal class and data class.

class PersonJaxb {
    var firstName: String? = null
    var lastName: String? = null
    var age: Int? = null
}

data class Person(val firstName: String, val lastName: String, val age: Int)

 

Normal class extensions

All instances have methods described below.

apply method

I often work with jaxb classes similar to PersonJaxb, which has not all arg constructor and all fields must be set via setters. Kotlin helps to deal with it via apply method. Target instance is provided as delagate to closure so we could define all fields values in it and returns this. The signature is T.apply(f: T.() -> Unit): T.

@Test
fun applyTest() {
    //when
    val person = PersonJaxb().apply {
        firstName = "John"
        lastName = "Smith"
        age = 20
    }

//then
assertEquals(20, person.age)
assertEquals(“John”, person.firstName)
assertEquals(“Smith”, person.lastName)
}

 

let method

Another extension is let method which is similar to map operation for collections. It has signature T.let(f: (T) -> R): R. this is passed as parameter to given closure/function.

@Test
fun letTest() {
    //when
    val fullName = Person("John", "Smith", 20).let {
        "${it.firstName} ${it.lastName}"
    }

//then
assertEquals(“John Smith”, fullName)
}

 

run method

run method looks like merge of apply and let methods: access to this is via delegate as in apply, but it also returns value as in let method. It has signature T.run(f: T.() -> R): R.

@Test
fun runTest() {
    //when
    val fullName = Person("John", "Smith", 20).run {
        "$firstName $lastName"
    }

//then
assertEquals(“John Smith”, fullName)
}

 

to method

Each instance has also defined to infix operator, which is used to create Pair. Pairs is helpful to create map entries. It has signature A.to(that: B): Pair<A, B>.

@Test
fun toTest() {
    //when
    val pair = Person("John", "Smith", 20) to 5

//then
assertEquals(Person(“John”, “Smith”, 20), pair.first)
assertEquals(5, pair.second)
}

 

Data class methods

Data class instances have also some other helpful methods (which are not extensions, but are generated for us).

componentX methods

Data class Person has three fields and it has component method generated for each of them: component1 for firstName, component2 for lastName and component3 for age.

@Test
fun componentsTest() {
    //when
    val p = Person("John", "Smith", 20)

//then
assertEquals(“John”, p.component1())
assertEquals(“Smith”, p.component2())
assertEquals(20, p.component3())
}

Why is it helpful? componentX methods are used in extracting (similar to Scala case classes extracting mechanism), e. g.:

@Test
fun extractingTest() {
    //when
    val (first, last, age) = Person("John", "Smith", 20)

//then
assertEquals(20, age)
assertEquals(“John”, first)
assertEquals(“Smith”, last)
}

 

copy method

copy method allows to create new instance based on current instance.

@Test
fun copyTest() {
    //when
    val person = Person("John", "Smith", 20).copy(lastName = "Kowalski", firstName = "Jan")

//then
assertEquals(Person(“Jan”, “Kowalski”, 20), person)
}

 

Summary

Kotlin’s extensions for each instances are very simple and help to solve many problems. The code written with these extensions is much more readable and concise than written in Java.

Sources are available here.

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Multi module Gradle project with IDE support

This article is a short how-to about multi-module project setup with usage of the Gradle automation build tool.

Here's how Rich Seller, a StackOverflow user, describes Gradle:
Gradle promises to hit the sweet spot between Ant and Maven. It uses Ivy's approach for dependency resolution. It allows for convention over configuration but also includes Ant tasks as first class citizens. It also wisely allows you to use existing Maven/Ivy repositories.
So why would one use yet another JVM build tool such as Gradle? The answer is simple: to avoid frustration involved by Ant or Maven.

Short story

I was fooling around with some fresh proof of concept and needed a build tool. I'm pretty familiar with Maven so created project from an artifact, and opened the build file, pom.xml for further tuning.
I had been using Grails with its own build system (similar to Gradle, btw) already for some time up then, so after quite a time without Maven, I looked on the pom.xml and found it to be really repulsive.

Once again I felt clearly: XML is not for humans.

After quick googling I found Gradle. It was still in beta (0.8 version) back then, but it's configured with Groovy DSL and that's what a human likes :)

Where are we

In the time Ant can be met but among IT guerrillas, Maven is still on top and couple of others like for example Ivy conquer for the best position, Gradle smoothly went into its mature age. It's now available in 1.3 version, released at 20th of November 2012. I'm glad to recommend it to anyone looking for relief from XML configured tools, or for anyone just looking for simple, elastic and powerful build tool.

Lets build

I have already written about basic project structure so I skip this one, reminding only the basic project structure:
<project root>

├── build.gradle
└── src
├── main
│ ├── java
│ └── groovy

└── test
├── java
└── groovy
Have I just referred myself for the 1st time? Achievement unlocked! ;)

Gradle as most build tools is run from a command line with parameters. The main parameter for Gradle is a 'task name', for example we can run a command: gradle build.
There is no 'create project' task, so the directory structure has to be created by hand. This isn't a hassle though.
Java and groovy sub-folders aren't always mandatory. They depend on what compile plugin is used.

Parent project

Consider an example project 'the-app' of three modules, let say:
  1. database communication layer
  2. domain model and services layer
  3. web presentation layer
Our project directory tree will look like:
the-app

├── dao-layer
│ └── src

├── domain-model
│ └── src

├── web-frontend
│ └── src

├── build.gradle
└── settings.gradle
the-app itself has no src sub-folder as its purpose is only to contain sub-projects and build configuration. If needed it could've been provided with own src though.

To glue modules we need to fill settings.gradle file under the-app directory with a single line of content specifying module names:
include 'dao-layer', 'domain-model', 'web-frontend'
Now the gradle projects command can be executed to obtain such a result:
:projects

------------------------------------------------------------
Root project
------------------------------------------------------------

Root project 'the-app'
+--- Project ':dao-layer'
+--- Project ':domain-model'
\--- Project ':web-frontend'
...so we know that Gradle noticed the modules. However gradle build command won't run successful yet because build.gradle file is still empty.

Sub project

As in Maven we can create separate build config file per each module. Let say we starting from DAO layer.
Thus we create a new file the-app/dao-layer/build.gradle with a line of basic build info (notice the new build.gradle was created under sub-project directory):
apply plugin: 'java'
This single line of config for any of modules is enough to execute gradle build command under the-app directory with following result:
:dao-layer:compileJava
:dao-layer:processResources UP-TO-DATE
:dao-layer:classes
:dao-layer:jar
:dao-layer:assemble
:dao-layer:compileTestJava UP-TO-DATE
:dao-layer:processTestResources UP-TO-DATE
:dao-layer:testClasses UP-TO-DATE
:dao-layer:test
:dao-layer:check
:dao-layer:build

BUILD SUCCESSFUL

Total time: 3.256 secs
To use Groovy plugin slightly more configuration is needed:
apply plugin: 'groovy'

repositories {
mavenLocal()
mavenCentral()
}

dependencies {
groovy 'org.codehaus.groovy:groovy-all:2.0.5'
}
At lines 3 to 6 Maven repositories are set. At line 9 dependency with groovy library version is specified. Of course plugin as 'java', 'groovy' and many more can be mixed each other.

If we have settings.gradle file and a build.gradle file for each module, there is no need for parent the-app/build.gradle file at all. Sure that's true but we can go another, better way.

One file to rule them all

Instead of creating many build.gradle config files, one per each module, we can use only the parent's one and make it a bit more juicy. So let us move the the-app/dao-layer/build.gradle a level up to the-app/build-gradle and fill it with new statements to achieve full project configuration:
def langLevel = 1.7

allprojects {

apply plugin: 'idea'

group = 'com.tamashumi'
version = '0.1'
}

subprojects {

apply plugin: 'groovy'

sourceCompatibility = langLevel
targetCompatibility = langLevel

repositories {
mavenLocal()
mavenCentral()
}

dependencies {
groovy 'org.codehaus.groovy:groovy-all:2.0.5'
testCompile 'org.spockframework:spock-core:0.7-groovy-2.0'
}
}

project(':dao-layer') {

dependencies {
compile 'org.hibernate:hibernate-core:4.1.7.Final'
}
}

project(':domain-model') {

dependencies {
compile project(':dao-layer')
}
}

project(':web-frontend') {

apply plugin: 'war'

dependencies {
compile project(':domain-model')
compile 'org.springframework:spring-webmvc:3.1.2.RELEASE'
}
}

idea {
project {
jdkName = langLevel
languageLevel = langLevel
}
}
At the beginning simple variable langLevel is declared. It's worth knowing that we can use almost any Groovy code inside build.gradle file, statements like for example if conditions, for/while loops, closures, switch-case, etc... Quite an advantage over inflexible XML, isn't it?

Next the allProjects block. Any configuration placed in it will influence - what a surprise - all projects, so the parent itself and sub-projects (modules). Inside of the block we have the IDE (Intellij Idea) plugin applied which I wrote more about in previous article (look under "IDE Integration" heading). Enough to say that with this plugin applied here, command gradle idea will generate Idea's project files with modules structure and dependencies. This works really well and plugins for other IDEs are available too.
Remaining two lines at this block define group and version for the project, similar as this is done by Maven.

After that subProjects block appears. It's related to all modules but not the parent project. So here the Groovy language plugin is applied, as all modules are assumed to be written in Groovy.
Below source and target language level are set.
After that come references to standard Maven repositories.
At the end of the block dependencies to groovy version and test library - Spock framework.

Following blocks, project(':module-name'), are responsible for each module configuration. They may be omitted unless allProjects or subProjects configure what's necessary for a specific module. In the example per module configuration goes as follow:
  • Dao-layer module has dependency to an ORM library - Hibernate
  • Domain-model module relies on dao-layer as a dependency. Keyword project is used here again for a reference to other module.
  • Web-frontend applies 'war' plugin which build this module into java web archive. Besides it referes to domain-model module and also use Spring MVC framework dependency.

At the end in idea block is basic info for IDE plugin. Those are parameters corresponding to the Idea's project general settings visible on the following screen shot.


jdkName should match the IDE's SDK name otherwise it has to be set manually under IDE on each Idea's project files (re)generation with gradle idea command.

Is that it?

In the matter of simplicity - yes. That's enough to automate modular application build with custom configuration per module. Not a rocket science, huh? Think about Maven's XML. It would take more effort to setup the same and still achieve less expressible configuration quite far from user-friendly.

Check the online user guide for a lot of configuration possibilities or better download Gradle and see the sample projects.
As a tasty bait take a look for this short choice of available plugins:
  • java
  • groovy
  • scala
  • cpp
  • eclipse
  • netbeans
  • ida
  • maven
  • osgi
  • war
  • ear
  • sonar
  • project-report
  • signing
and more, 3rd party plugins...