Getting started with Kotlin

Hello World

All Kotlin programs start at the main function. Here is an example of a simple Kotlin "Hello World" program:

package my.program

fun main(args: Array<String>) {
    println("Hello, world!")
}

Place the above code into a file named Main.kt (this filename is entirely arbitrary) When targeting the JVM, the function will be compiled as a static method in a class with a name derived from the filename. In the above example, the main class to run would be my.program.MainKt.

To change the name of the class that contains top-level functions for a particular file, place the following annotation at the top of the file above the package statement:

@file:JvmName("MyApp")

In this example, the main class to run would now be my.program.MyApp.

See also:

• Package level functions including @JvmName annotation.
  
• Annotation use-site targets

Hello World using a Companion Object

Similar to using an Object Declaration, you can define the main function of a Kotlin program using a Companion Object of a class.

package my.program

class App {
    companion object {
        @JvmStatic fun main(args: Array<String>) {
            println("Hello World")
        }
    }
}

The class name that you will run is the name of your class, in this case is my.program.App. The advantage to this method over a top-level function is that the class name to run is more self-evident, and any other functions you add are scoped into the class App. This is similar to the Object Declaration example, other than you are in control of instantiating any classes to do further work.

A slight variation that instantiates the class to do the actual "hello":

class App {
    companion object {
        @JvmStatic fun main(args: Array<String>) {
            App().run()
        }
    }

    fun run() {
        println("Hello World")
    }
}

See also:

• Static Methods including the @JvmStatic annotation

Hello World using an Object Declaration

You can alternatively use an Object Declaration that contains the main function for a Kotlin program.

package my.program

object App {
    @JvmStatic fun main(args: Array<String>) {
        println("Hello World")
    }
}

The class name that you will run is the name of your object, in this case is my.program.App.

The advantage to this method over a top-level function is that the class name to run is more self-evident, and any other functions you add are scoped into the class App. You then also have a singleton instance of App to store state and do other work.

See also:

• Static Methods including the @JvmStatic annotation

Main methods using varargs

All of these main method styles can also be used with varargs:

package my.program

fun main(vararg args: String) {
    println("Hello, world!")
}

Compile and Run Kotlin Code in Command Line

As java provide two different commands to compile and run Java code. Same as Kotlin also provide you different commands.

javac to compile java files. java to run java files.

Same as kotlinc to compile kotlin files kotlin to run kotlin files.

Reading input from Command Line

The arguments passed from the console can be received in the Kotlin program and it can be used as an input. You can pass N (1 2 3 and so on) numbers of arguments from the command prompt.

A simple example of a command-line argument in Kotlin.

fun main(args: Array<String>) {

    println("Enter Two number")
    var (a, b) = readLine()!!.split(' ') // !! this operator use for NPE(NullPointerException).

    println("Max number is : ${maxNum(a.toInt(), b.toInt())}")
}

fun maxNum(a: Int, b: Int): Int {

    var max = if (a > b) {
        println("The value of a is $a");
        a
    } else {
        println("The value of b is $b")
        b
    }

    return max;

}

Here, Enter two number from the command line to find the maximum number. Output :

Enter Two number
71 89 // Enter two number from command line

The value of b is 89
Max number is: 89

For !! Operator Please check Null Safety.

Note: Above example compile and run on Intellij.

Compiling Kotlin

Kotlin has a standard IDE plugin for Eclipse and IntelliJ. Kotlin can also be compiled using Maven, using Ant, and using Gradle, or through the command line.

It is worth noting in $ kotlinc Main.kt will output a java class file, in this case MainKt.class (Note the Kt appended to the class name). However if one was to run the class file using $ java MainKt java will throw the following exception:

Exception in thread "main" java.lang.NoClassDefFoundError: kotlin/jvm/internal/Intrinsics
    at MainKt.main(Main.kt)
Caused by: java.lang.ClassNotFoundException: kotlin.jvm.internal.Intrinsics
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 1 more

In order to run the resulting class file using Java, one must include the Kotlin runt-time jar file to the current class path. java -cp .:/path/to/kotlin/runtime/jar/kotlin-runtime.jar MainKt

Remarks

Kotlin is a statically-typed object-oriented programming language developed by JetBrains primarily targeting the JVM. Kotlin is developed with the goals of being quick to compile, backwards-compatible, very type safe, and 100% interoperable with Java. Kotlin is also developed with the goal of providing many of the features wanted by Java developers. Kotlin's standard compiler allows it to be compiled both into Java bytecode for the JVM and into JavaScript.

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