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>