How to send data back to the Main Activity in Android using Kotlin?

This example demonstrates how to send data back to the Main Activity in Android using Kotlin

Step 1 − Create a new project in Android Studio, go to File ⇒ New Project and fill all required details to create a new project.

Step 2 − Add the following code to res/layout/activity_main.xml.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:gravity="center_horizontal"
   android:orientation="vertical"
   tools:context=".MainActivity">
   <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_marginTop="100dp"
      android:layout_marginBottom="50dp"
      android:text="Tutorials Point"
      android:textAlignment="center"
      android:textColor="@android:color/holo_green_dark"
      android:textSize="32sp"
      android:textStyle="bold" />
   <TextView
      android:id="@+id/textViewNumbers"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Numbers: "
      android:textColor="@android:color/black"
      android:textSize="24sp"
      android:textStyle="bold" />
   <Button
      android:id="@+id/buttonAdd"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_marginTop="10dp"
      android:text="add" />
   <Button
      android:id="@+id/buttonSubtract"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_marginTop="5dp"
      android:text="subtract" />
</LinearLayout>

Step 3 − Add the following code to src/MainActivity.kt

import android.app.Activity
import android.content.Intent
import android.os.Bundle
import android.widget.Button
import android.widget.EditText
import android.widget.TextView
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
class MainActivity : AppCompatActivity() {
   private lateinit var textViewResult: TextView
   private lateinit var editTextNumber1: EditText
   private lateinit var editTextNumber2: EditText
   private lateinit var button: Button
   override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_main)
      title = "KotlinApp"
      textViewResult = findViewById(R.id.textViewResult)
      editTextNumber1 = findViewById(R.id.editTextNumber1)
      editTextNumber2 = findViewById(R.id.editTextNumber2)
      button = findViewById(R.id.btnOpenActivity2)
      button.setOnClickListener {
         if ((editTextNumber1.text.toString() == "" || editTextNumber2.text.toString() == "")) {
            Toast.makeText(this@MainActivity, "Please insert numbers", Toast.LENGTH_SHORT).show()
         } else {
            val number1 = Integer.parseInt(editTextNumber1.text.toString())
            val number2 = Integer.parseInt(editTextNumber2.text.toString())
            val intent = Intent(this@MainActivity, SecondActivity::class.java)
            intent.putExtra("number1", number1)
            intent.putExtra("number2", number2)
            startActivityForResult(intent, 1)
         }
      }
   }
   override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
      super.onActivityResult(requestCode, resultCode, data)
      if (requestCode == 1) {
         if (resultCode == Activity.RESULT_OK) {
            val result = data!!.getIntExtra("result", 0)
            textViewResult.text = "" + result
         }
         if (resultCode == Activity.RESULT_CANCELED) {
            textViewResult.text = "Nothing selected"
         }
      }
   }
}

Step 4 − Create a new activity and add the following code −

activity_second.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:gravity="center_horizontal"
   android:orientation="vertical"
   tools:context=".MainActivity">
   <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_marginTop="100dp"
      android:layout_marginBottom="50dp"
      android:text="Tutorials Point"
      android:textAlignment="center"
      android:textColor="@android:color/holo_green_dark"
      android:textSize="32sp"
      android:textStyle="bold" />
   <TextView
      android:id="@+id/textViewNumbers"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Numbers: "
      android:textColor="@android:color/black"
      android:textSize="24sp"
      android:textStyle="bold" />
   <Button
      android:id="@+id/buttonAdd"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_marginTop="10dp"
      android:text="add" />
   <Button
      android:id="@+id/buttonSubtract"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_marginTop="5dp"
      android:text="subtract" />
</LinearLayout>

SecondActivity.kt

import android.app.Activity
import android.content.Intent
import android.os.Bundle
import android.widget.Button
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity
class SecondActivity : AppCompatActivity() {
   lateinit var textViewNumber: TextView
   lateinit var buttonAdd: Button
   lateinit var buttonSubtract: Button
   override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_second)
      val intent = intent
      val number1 = intent.getIntExtra("number1", 0)
      val number2 = intent.getIntExtra("number2", 0)
      textViewNumber = findViewById(R.id.textViewNumbers)
      textViewNumber.text = "Numbers: $number1, $number2"
      buttonAdd = findViewById(R.id.buttonAdd)
      buttonSubtract = findViewById(R.id.buttonSubtract)
      buttonAdd.setOnClickListener {
         val result = number1 + number2
         val resultIntent = Intent()
         resultIntent.putExtra("result", result)
         setResult(Activity.RESULT_OK, resultIntent)
         finish()
      }
      buttonSubtract.setOnClickListener {
         val result = number1 - number2
         val resultIntent = Intent()
         resultIntent.putExtra("result", result)
         setResult(Activity.RESULT_OK, resultIntent)
         finish()
      }
   }
}

Step 5 − Add the following code to androidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="com.example.q11">
   <application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:roundIcon="@mipmap/ic_launcher_round"
      android:supportsRtl="true"
      android:theme="@style/AppTheme">
      <activity android:name=".MainActivity">
         <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
         </intent-filter>
      </activity>
   </application>
</manifest>

Let's try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from android studio, open one of your project's activity files and click the Run icon from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen