Hello I am an absolute beginner with Kotlin and Android Studio. I want to hear what I could do better for the next time or which flaws does my code have. The code does what it should be.

the MainActivity file:

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import android.widget.Button
import android.widget.TextView
import android.widget.Toast
class MainActivity : AppCompatActivity(), View.OnClickListener {
 override fun onCreate(savedInstanceState: Bundle?) {
 super.onCreate(savedInstanceState)
 setContentView(R.layout.activity_main)
 val button0 = findViewById<Button>(R.id.button0)
 val button1 = findViewById<Button>(R.id.button1)
 val button2 = findViewById<Button>(R.id.button2)
 val button3 = findViewById<Button>(R.id.button3)
 val button4 = findViewById<Button>(R.id.button4)
 val button5 = findViewById<Button>(R.id.button5)
 val button6 = findViewById<Button>(R.id.button6)
 val button7 = findViewById<Button>(R.id.button7)
 val button8 = findViewById<Button>(R.id.button8)
 val button9 = findViewById<Button>(R.id.button9)
 val buttonPlus = findViewById<Button>(R.id.buttonPlus)
 val buttonMinus = findViewById<Button>(R.id.buttonMinus)
 val buttonMultiply = findViewById<Button>(R.id.buttonMultiply)
 val buttonDivide = findViewById<Button>(R.id.buttonDivide)
 val buttonClear = findViewById<Button>(R.id.buttonClear)
 val buttonEqual = findViewById<Button>(R.id.buttonEqual)
 button0.setOnClickListener(this)
 button1.setOnClickListener(this)
 button2.setOnClickListener(this)
 button3.setOnClickListener(this)
 button4.setOnClickListener(this)
 button5.setOnClickListener(this)
 button6.setOnClickListener(this)
 button7.setOnClickListener(this)
 button8.setOnClickListener(this)
 button9.setOnClickListener(this)
 buttonPlus.setOnClickListener(this)
 buttonMinus.setOnClickListener(this)
 buttonMultiply.setOnClickListener(this)
 buttonDivide.setOnClickListener(this)
 buttonClear.setOnClickListener(this)
 buttonEqual.setOnClickListener(this)
 }
 var equOnScreen: String? = null
 override fun onClick(p0: View?) {
 val resultOnScreen = findViewById<TextView>(R.id.resultOnScreen)
 val message: String = "Please write two numbers and an operator between them"
 if(equOnScreen.isNullOrBlank() || equOnScreen == message) {
 equOnScreen = ""
 }
 when (p0?.id) {
 R.id.button0 -> equOnScreen = "$equOnScreen" + "0" + " "
 R.id.button1 -> equOnScreen = "$equOnScreen" + "1" + " "
 R.id.button2 -> equOnScreen = "$equOnScreen" + "2" + " "
 R.id.button3 -> equOnScreen = "$equOnScreen" + "3" + " "
 R.id.button4 -> equOnScreen = "$equOnScreen" + "4" + " "
 R.id.button5 -> equOnScreen = "$equOnScreen" + "5" + " "
 R.id.button6 -> equOnScreen = "$equOnScreen" + "6" + " "
 R.id.button7 -> equOnScreen = "$equOnScreen" + "7" + " "
 R.id.button8 -> equOnScreen = "$equOnScreen" + "8" + " "
 R.id.button9 -> equOnScreen = "$equOnScreen" + "9" + " "
 R.id.buttonPlus -> equOnScreen = "$equOnScreen" + "+" + " "
 R.id.buttonMinus -> equOnScreen = "$equOnScreen" + "-" + " "
 R.id.buttonMultiply -> equOnScreen = "$equOnScreen" + "*" + " "
 R.id.buttonDivide -> equOnScreen = "$equOnScreen" + "/" + " "
 R.id.buttonClear -> equOnScreen = ""
 R.id.buttonEqual -> {
 if(stringdataIntoCalc(equOnScreen!!) != null) {
 equOnScreen = stringdataIntoCalc(equOnScreen!!).toString() + " "
 }
 else equOnScreen = message
 }
 }
 resultOnScreen.text = equOnScreen
 }
 fun stringdataIntoCalc(string : String) : Int? {
 var stringList : MutableList<String> = string.split(" ").toMutableList()
 if(stringList.size != 4) {return null}
 var first : String = stringList[0]
 var second : String = stringList[2]
 var sign : String = stringList[1]
 var result : Int? = null
 when(sign) {
 "+" -> result = first.toInt() + second.toInt()
 "-" -> result = first.toInt() - second.toInt()
 "*" -> result = first.toInt() * second.toInt()
 "/" -> result = first.toInt() / second.toInt()
 }
 if(result != null) {
 return result
 }
 return null
 }
 }

Here is the xml file:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:app="http://schemas.android.com/apk/res-auto"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 tools:context=".MainActivity"
 android:orientation="vertical"
 android:background="@color/black"
 >
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="350dp"
 android:orientation="vertical"
 android:background="@color/hellblauoderso"
 >
 <TextView
 android:id="@+id/resultOnScreen"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:text="Result"
 android:gravity="center"
 android:textSize="50dp"
 android:textStyle="bold"
 />
 </LinearLayout>
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:orientation="vertical"
 >
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:layout_weight="1"
 android:gravity="center|bottom"
 android:orientation="horizontal"
 android:paddingVertical="0dp"
 >
 <Button
 android:id="@+id/button1"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="1" />
 <Button
 android:id="@+id/button2"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="2" />
 <Button
 android:id="@+id/button3"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="3" />
 <Button
 android:id="@+id/button4"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="4" />
 </LinearLayout>
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:layout_weight="1"
 android:gravity="center|bottom"
 android:orientation="horizontal"
 android:paddingVertical="0dp"
 >
 <Button
 android:id="@+id/button5"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="5" />
 <Button
 android:id="@+id/button6"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="6" />
 <Button
 android:id="@+id/button7"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="7" />
 <Button
 android:id="@+id/button8"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="8" />
 </LinearLayout>
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:layout_weight="1"
 android:gravity="center|bottom"
 android:orientation="horizontal"
 android:paddingVertical="0dp"
 >
 <Button
 android:id="@+id/button9"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="9" />
 <Button
 android:id="@+id/button0"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="0" />
 <Button
 android:id="@+id/buttonPlus"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="+" />
 <Button
 android:id="@+id/buttonMinus"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="-" />
 </LinearLayout>
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:layout_weight="1"
 android:gravity="center|bottom"
 android:orientation="horizontal"
 android:paddingVertical="0dp"
 >
 <Button
 android:id="@+id/buttonMultiply"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="*" />
 <Button
 android:id="@+id/buttonDivide"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="/" />
 <Button
 android:id="@+id/buttonEqual"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="="
 android:textSize="25dp" />
 <Button
 android:id="@+id/buttonClear"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="clear" />
 </LinearLayout>
 </LinearLayout>
</LinearLayout>

Hello I am an absolute beginner with Kotlin and Android Studio. I want to hear what I could do better for the next time or any flaws my code has.

The code implements a four-function calculator (+ - ✕ ÷).

The MainActivity file:

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import android.widget.Button
import android.widget.TextView
import android.widget.Toast
class MainActivity : AppCompatActivity(), View.OnClickListener {
 override fun onCreate(savedInstanceState: Bundle?) {
 super.onCreate(savedInstanceState)
 setContentView(R.layout.activity_main)
 val button0 = findViewById<Button>(R.id.button0)
 val button1 = findViewById<Button>(R.id.button1)
 val button2 = findViewById<Button>(R.id.button2)
 val button3 = findViewById<Button>(R.id.button3)
 val button4 = findViewById<Button>(R.id.button4)
 val button5 = findViewById<Button>(R.id.button5)
 val button6 = findViewById<Button>(R.id.button6)
 val button7 = findViewById<Button>(R.id.button7)
 val button8 = findViewById<Button>(R.id.button8)
 val button9 = findViewById<Button>(R.id.button9)
 val buttonPlus = findViewById<Button>(R.id.buttonPlus)
 val buttonMinus = findViewById<Button>(R.id.buttonMinus)
 val buttonMultiply = findViewById<Button>(R.id.buttonMultiply)
 val buttonDivide = findViewById<Button>(R.id.buttonDivide)
 val buttonClear = findViewById<Button>(R.id.buttonClear)
 val buttonEqual = findViewById<Button>(R.id.buttonEqual)
 button0.setOnClickListener(this)
 button1.setOnClickListener(this)
 button2.setOnClickListener(this)
 button3.setOnClickListener(this)
 button4.setOnClickListener(this)
 button5.setOnClickListener(this)
 button6.setOnClickListener(this)
 button7.setOnClickListener(this)
 button8.setOnClickListener(this)
 button9.setOnClickListener(this)
 buttonPlus.setOnClickListener(this)
 buttonMinus.setOnClickListener(this)
 buttonMultiply.setOnClickListener(this)
 buttonDivide.setOnClickListener(this)
 buttonClear.setOnClickListener(this)
 buttonEqual.setOnClickListener(this)
 }
 var equOnScreen: String? = null
 override fun onClick(p0: View?) {
 val resultOnScreen = findViewById<TextView>(R.id.resultOnScreen)
 val message: String = "Please write two numbers and an operator between them"
 if(equOnScreen.isNullOrBlank() || equOnScreen == message) {
 equOnScreen = ""
 }
 when (p0?.id) {
 R.id.button0 -> equOnScreen = "$equOnScreen" + "0" + " "
 R.id.button1 -> equOnScreen = "$equOnScreen" + "1" + " "
 R.id.button2 -> equOnScreen = "$equOnScreen" + "2" + " "
 R.id.button3 -> equOnScreen = "$equOnScreen" + "3" + " "
 R.id.button4 -> equOnScreen = "$equOnScreen" + "4" + " "
 R.id.button5 -> equOnScreen = "$equOnScreen" + "5" + " "
 R.id.button6 -> equOnScreen = "$equOnScreen" + "6" + " "
 R.id.button7 -> equOnScreen = "$equOnScreen" + "7" + " "
 R.id.button8 -> equOnScreen = "$equOnScreen" + "8" + " "
 R.id.button9 -> equOnScreen = "$equOnScreen" + "9" + " "
 R.id.buttonPlus -> equOnScreen = "$equOnScreen" + "+" + " "
 R.id.buttonMinus -> equOnScreen = "$equOnScreen" + "-" + " "
 R.id.buttonMultiply -> equOnScreen = "$equOnScreen" + "*" + " "
 R.id.buttonDivide -> equOnScreen = "$equOnScreen" + "/" + " "
 R.id.buttonClear -> equOnScreen = ""
 R.id.buttonEqual -> {
 if(stringdataIntoCalc(equOnScreen!!) != null) {
 equOnScreen = stringdataIntoCalc(equOnScreen!!).toString() + " "
 }
 else equOnScreen = message
 }
 }
 resultOnScreen.text = equOnScreen
 }
 fun stringdataIntoCalc(string : String) : Int? {
 var stringList : MutableList<String> = string.split(" ").toMutableList()
 if(stringList.size != 4) {return null}
 var first : String = stringList[0]
 var second : String = stringList[2]
 var sign : String = stringList[1]
 var result : Int? = null
 when(sign) {
 "+" -> result = first.toInt() + second.toInt()
 "-" -> result = first.toInt() - second.toInt()
 "*" -> result = first.toInt() * second.toInt()
 "/" -> result = first.toInt() / second.toInt()
 }
 if(result != null) {
 return result
 }
 return null
 }
 }

Here is the XML file:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:app="http://schemas.android.com/apk/res-auto"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 tools:context=".MainActivity"
 android:orientation="vertical"
 android:background="@color/black"
 >
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="350dp"
 android:orientation="vertical"
 android:background="@color/hellblauoderso"
 >
 <TextView
 android:id="@+id/resultOnScreen"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:text="Result"
 android:gravity="center"
 android:textSize="50dp"
 android:textStyle="bold"
 />
 </LinearLayout>
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:orientation="vertical"
 >
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:layout_weight="1"
 android:gravity="center|bottom"
 android:orientation="horizontal"
 android:paddingVertical="0dp"
 >
 <Button
 android:id="@+id/button1"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="1" />
 <Button
 android:id="@+id/button2"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="2" />
 <Button
 android:id="@+id/button3"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="3" />
 <Button
 android:id="@+id/button4"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="4" />
 </LinearLayout>
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:layout_weight="1"
 android:gravity="center|bottom"
 android:orientation="horizontal"
 android:paddingVertical="0dp"
 >
 <Button
 android:id="@+id/button5"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="5" />
 <Button
 android:id="@+id/button6"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="6" />
 <Button
 android:id="@+id/button7"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="7" />
 <Button
 android:id="@+id/button8"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="8" />
 </LinearLayout>
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:layout_weight="1"
 android:gravity="center|bottom"
 android:orientation="horizontal"
 android:paddingVertical="0dp"
 >
 <Button
 android:id="@+id/button9"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="9" />
 <Button
 android:id="@+id/button0"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="0" />
 <Button
 android:id="@+id/buttonPlus"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="+" />
 <Button
 android:id="@+id/buttonMinus"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="-" />
 </LinearLayout>
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:layout_weight="1"
 android:gravity="center|bottom"
 android:orientation="horizontal"
 android:paddingVertical="0dp"
 >
 <Button
 android:id="@+id/buttonMultiply"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="*" />
 <Button
 android:id="@+id/buttonDivide"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="/" />
 <Button
 android:id="@+id/buttonEqual"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="="
 android:textSize="25dp" />
 <Button
 android:id="@+id/buttonClear"
 style="?android:attr/buttonStyleSmall"
 android:layout_width="100dp"
 android:layout_height="wrap_content"
 android:layout_gravity="center_horizontal|center"
 android:text="clear" />
 </LinearLayout>
 </LinearLayout>
</LinearLayout>