Kotlin - Basic Syntax

Kotlin Program Entry Point

An entry point of a Kotlin application is the main() function. A function can be defined as a block of code designed to perform a particular task.

Let's start with a basic Kotlin program to print "Hello, World!" on the standard output:

fun main() {
   var string: String  = "Hello, World!"
   println("$string")
}

When you run the above Kotlin program, it will generate the following output:

Hello, World!

Entry Point with Parameters

Another form of main() function accepts a variable number of String arguments as follows:

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

When you run the above Kotlin program, it will generate the following output:

Hello, World!

If you have observed, its clear that both the programs generate same output, so it is very much optional to pass a parameter in main() function starting from Kotlin version 1.3.

print() vs println()

The print() is a function in Kotlin which prints its argument to the standard output, similar way the println() is another function which prints its argument on the standard output but it also adds a line break in the output.

Let's try the following program to understand the difference between these two important functions:

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

    print("Hello,")
    print(" world!")
}

When you run the above Kotlin program, it will generate the following output:

Hello, 
 world!
Hello, world!

Both the functions (print() and println()) can be used to print numbers as well as strings and at the same time to perform any mathematical calculations as below:

fun main(args: Array<String>){
    println( 200 )
    println( "200" )
    println( 2 + 2 )

    print(4*3)
}

When you run the above Kotlin program, it will generate the following output:

200
200
4
12

Semicolon (;) in Kotlin

Kotlin code statements do not require a semicolon (;) to end the statement like many other programming languages, such as Java, C++, C#, etc. do need it.

Though you can compile and run a Kotlin program with and without semicolon successfully as follows:

fun main() {
    println("I'm without semi-colon")
    
    println("I'm with semi-colon");
}

When you run the above Kotlin program, it will generate the following output:

I'm without semi-colon
I'm with semi-colon

So as a good programming practice, it is not recommended to add a semicolon in the end of a Kotlin statement.

Packages in Kotlin

Kotlin code is usually defined in packages though package specification is optional. If you don't specify a package in a source file, its content goes to the default package.

If we specify a package in Kotlin program then it is specified at the top of the file as follows:

package org.tutorialspoint.com

fun main() {
    println("Hello, World!")
}

When you run the above Kotlin program, it will generate the following output:

Hello, World!

fun main() {
    println("1"); println("2")
}