Step by step beginners tutorial - how to make an android calculator using android studio

In the second tutorial of Android Studio tutorials (you can check them here : Android Studio Tutorials) and the programming ones (you can check them here : Programming Tutorials) we're showing you how to make a simple android calculator that can do basic calculations such as addition, subtracting, multiplication and division. I have tried to explain as much as possible that is why the video is +30minutes long and i kinda got tired in the middle of it so haha ! i am sorry for that

before watching this tutorial you need to download and install android studio ( that's an article that shows how : Download And Install Android Studio, and that is a YouTube video how to download and install android studio)

A video to explain the code : 

The used code :

	<?xml version="1.0" encoding="utf-8"?>
	<androidx.constraintlayout.widget.ConstraintLayout 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"
	android:background="@color/appbg"
	tools:context=".MainActivity">
	<ScrollView
	android:layout_width="match_parent"
	android:layout_height="match_parent">
	<LinearLayout
	android:layout_width="match_parent"
	android:layout_height="wrap_content"
	android:orientation="vertical"
	android:padding="8dp"
	tools:ignore="MissingConstraints">
	<TextView
	android:layout_width="match_parent"
	android:layout_height="wrap_content"
	android:padding="8dp"
	android:text="the Result : "
	android:textColor="@color/black"
	android:textSize="20sp"
	android:textStyle="bold" />
	<TextView
	android:id="@+id/result_tv"
	android:layout_width="match_parent"
	android:layout_height="wrap_content"
	android:gravity="center"
	android:padding="8dp"
	android:textColor="@color/black"
	android:textSize="30sp"
	android:textStyle="bold" />
	<EditText
	android:id="@+id/first_num_et"
	android:layout_width="match_parent"
	android:layout_height="wrap_content"
	android:layout_marginTop="32dp"
	android:layout_marginBottom="16dp"
	android:gravity="center"
	android:hint="first number"
	android:inputType="numberDecimal" />
	<LinearLayout
	android:layout_width="match_parent"
	android:layout_height="wrap_content"
	android:orientation="horizontal"
	android:padding="8dp"
	tools:ignore="MissingConstraints">
	<Button
	android:id="@+id/addbtn"
	android:layout_width="match_parent"
	android:layout_height="match_parent"
	android:layout_marginRight="4dp"
	android:layout_weight="1"
	android:backgroundTint="@color/appbg2"
	android:text="+"
	android:textSize="20sp" />
	<Button
	android:id="@+id/minbtn"
	android:layout_width="match_parent"
	android:layout_height="match_parent"
	android:layout_marginRight="4dp"
	android:layout_weight="1"
	android:backgroundTint="@color/appbg2"
	android:text="-"
	android:textSize="20sp" />
	<Button
	android:id="@+id/multbtn"
	android:layout_width="match_parent"
	android:layout_height="match_parent"
	android:layout_marginRight="4dp"
	android:layout_weight="1"
	android:backgroundTint="@color/appbg2"
	android:text="×"
	android:textSize="20sp" />
	<Button
	android:id="@+id/divbtn"
	android:layout_width="match_parent"
	android:layout_height="match_parent"
	android:layout_marginRight="4dp"
	android:layout_weight="1"
	android:backgroundTint="@color/appbg2"
	android:text="/"
	android:textSize="20sp" />
	</LinearLayout>
	<EditText
	android:id="@+id/second_num_et"
	android:layout_width="match_parent"
	android:layout_height="wrap_content"
	android:layout_marginTop="16dp"
	android:layout_marginBottom="32dp"
	android:gravity="center"
	android:hint="second number"
	android:inputType="numberDecimal" />
	</LinearLayout>
	</ScrollView>
	</androidx.constraintlayout.widget.ConstraintLayout>

view raw activity_main.xml hosted with ❤ by GitHub

	package techituptoday.simple.calculator;
	import androidx.appcompat.app.AppCompatActivity;
	import android.os.Bundle;
	import android.view.View;
	import android.widget.Button;
	import android.widget.EditText;
	import android.widget.TextView;
	import android.widget.Toast;
	public class MainActivity extends AppCompatActivity {
	Button addbtn,minbtn,multbtn,divbtn;
	EditText firstNumET,secondNumET;
	double FirstNum,SecondNum,Result;
	char op;
	TextView resultTv;
	@Override
	protected void onCreate(Bundle savedInstanceState) {
	super.onCreate(savedInstanceState);
	setContentView(R.layout.activity_main);
	addbtn = findViewById(R.id.addbtn);
	minbtn = findViewById(R.id.minbtn);
	multbtn = findViewById(R.id.multbtn);
	divbtn = findViewById(R.id.divbtn);
	firstNumET = findViewById(R.id.first_num_et);
	secondNumET = findViewById(R.id.second_num_et);
	resultTv = findViewById(R.id.result_tv);
	addbtn.setOnClickListener(new View.OnClickListener() {
	@Override
	public void onClick(View view) {
	op='+';
	try {
	FirstNum= Double.parseDouble(firstNumET.getText().toString());
	SecondNum= Double.parseDouble(secondNumET.getText().toString());
	}catch (NullPointerException n){
	Toast.makeText(MainActivity.this, "please write number", Toast.LENGTH_SHORT).show();
	}catch (Exception e){
	Toast.makeText(MainActivity.this, "please write valid numbers", Toast.LENGTH_SHORT).show();
	}
	Result = calculate(FirstNum,SecondNum,op);
	resultTv.setText(String.valueOf(Result));
	}
	});
	minbtn.setOnClickListener(new View.OnClickListener() {
	@Override
	public void onClick(View view) {
	op='-';
	try {
	FirstNum= Double.parseDouble(firstNumET.getText().toString());
	SecondNum= Double.parseDouble(secondNumET.getText().toString());
	}catch (NullPointerException n){
	Toast.makeText(MainActivity.this, "please write number", Toast.LENGTH_SHORT).show();
	}catch (Exception e){
	Toast.makeText(MainActivity.this, "please write valid numbers", Toast.LENGTH_SHORT).show();
	}
	Result = calculate(FirstNum,SecondNum,op);
	resultTv.setText(String.valueOf(Result));
	}
	});
	multbtn.setOnClickListener(new View.OnClickListener() {
	@Override
	public void onClick(View view) {
	op='*';
	try {
	FirstNum= Double.parseDouble(firstNumET.getText().toString());
	SecondNum= Double.parseDouble(secondNumET.getText().toString());
	}catch (NullPointerException n){
	Toast.makeText(MainActivity.this, "please write number", Toast.LENGTH_SHORT).show();
	}catch (Exception e){
	Toast.makeText(MainActivity.this, "please write valid numbers", Toast.LENGTH_SHORT).show();
	}
	Result = calculate(FirstNum,SecondNum,op);
	resultTv.setText(String.valueOf(Result));
	}
	});
	divbtn.setOnClickListener(new View.OnClickListener() {
	@Override
	public void onClick(View view) {
	op='/';
	try {
	FirstNum= Double.parseDouble(firstNumET.getText().toString());
	SecondNum= Double.parseDouble(secondNumET.getText().toString());
	}catch (NullPointerException n){
	Toast.makeText(MainActivity.this, "please write number", Toast.LENGTH_SHORT).show();
	}catch (Exception e){
	Toast.makeText(MainActivity.this, "please write valid numbers", Toast.LENGTH_SHORT).show();
	}
	Result = calculate(FirstNum,SecondNum,op);
	resultTv.setText(String.valueOf(Result));
	}
	});
	}
	public double calculate(double firstnum,double secondnum,char o){
	double res=0;
	switch (o){
	case '+':
	res = firstnum+secondnum;
	break;
	case '-':
	res = firstnum-secondnum;
	break;
	case '*':
	res = firstnum*secondnum;
	break;
	case '/':
	res = firstnum/secondnum;
	break;
	}
	return res;
	}
	}

view raw MainActivity.java hosted with ❤ by GitHub