Kotlin main method explained

The main() method is a function that must be included in a Kotlin application.

Like in Java, the main() method is the entry point of an application written in Kotlin.

A Java application requires you to have a main() method inside a class as follows:

class Scratch {
    public static void main(String[] args) {
        // Your app code here
        System.out.println("Hello World");
    }
}

But in Kotlin, you don’t need to create a class for the main() method.

The main() method in Kotlin has two constructors: one with the args argument and one without.

You can write the Kotlin main() method as follows:

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

// or

fun main(args: Array<String>) {
    println("Hello World!")
    println(args.contentToString())
}

Without a main() method, an error will be thrown during runtime, saying Function 'main' not found.

In Android, the main() method is replaced with the MainActivity class that’s generated by Android Studio when you create a new Android project.

An example MainActivity class might look as follows:

package com.example.kotlinbasic

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}

Then, the MainActivity class should be registered in the AndroidManifest.xml file as the activity to run for MAIN intent as shown below:

<activity
    android:name=".MainActivity"
    android:exported="true" >
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />

        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

This is why when developing Android applications, you don’t see the main() method inside the Kotlin file.

Instead, you have a MainActivity class with the onCreate() method.

The MainActivity class would then be registered in the AndroidManifest.xml file as the entry point with the <intent-filter> tag.