How to build a simple Calculator app using Android Studio?

Pre-requisites:

• Android App Development Fundamentals for Beginners
• Guide to Install and Set up Android Studio
• Android | Starting with first app/android project
• Android | Running your first Android app

Create a simple calculator which can perform basic arithmetic operations like addition, subtraction, multiplication, or division depending upon the user input. A sample video is given below to get an idea about what we are going to do in this article. Note that we are going to implement this project using the Java language. 

Step by Step Implementation

Step 1: Create a New Project

To create a new project in Android Studio please refer to How to Create/Start a New Project in Android Studio. Note that select Java as the programming language.

Step 2: Working with the activity_main.xml file

Navigate to the app > res > layout > activity_main.xml and add the below code to that file. Below is the code for the activity_main.xml file. 

XML

<?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="#8BC34A"
    android:backgroundTint="@android:color/darker_gray"
    tools:context=".MainActivity">

    <!-- Text View to display our basic heading of "calculator"-->
    <TextView
        android:layout_width="194dp"
        android:layout_height="43dp"
        android:layout_marginStart="114dp"
        android:layout_marginLeft="114dp"
        android:layout_marginTop="58dp"
        android:layout_marginEnd="103dp"
        android:layout_marginRight="103dp"
        android:layout_marginBottom="502dp"
        android:scrollbarSize="30dp"
        android:text=" Calculator"
        android:textAppearance="@style/TextAppearance.AppCompat.Body1"
        android:textSize="30dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <!-- Edit Text View to input the values -->
    <EditText
        android:id="@+id/num1"
        android:layout_width="364dp"
        android:layout_height="28dp"
        android:layout_marginStart="72dp"
        android:layout_marginTop="70dp"
        android:layout_marginEnd="71dp"
        android:layout_marginBottom="416dp"
        android:background="@android:color/white"
        android:ems="10"
        android:onClick="clearTextNum1"
        android:inputType="number"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <!-- Edit Text View to input 2nd value-->
    <EditText
        android:id="@+id/num2"
        android:layout_width="363dp"
        android:layout_height="30dp"
        android:layout_marginStart="72dp"
        android:layout_marginTop="112dp"
        android:layout_marginEnd="71dp"
        android:layout_marginBottom="374dp"
        android:background="@android:color/white"
        android:ems="10"
        android:onClick="clearTextNum2"
        android:inputType="number"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <!-- Text View to display result -->
    <TextView
        android:id="@+id/result"
        android:layout_width="356dp"
        android:layout_height="71dp"
        android:layout_marginStart="41dp"
        android:layout_marginTop="151dp"
        android:layout_marginEnd="48dp"
        android:layout_marginBottom="287dp"
        android:background="@android:color/white"
        android:text="result"
        android:textColorLink="#673AB7"
        android:textSize="25sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <!-- A button to perform 'sum' operation -->
    <Button
        android:id="@+id/sum"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="292dp"
        android:layout_marginEnd="307dp"
        android:layout_marginBottom="263dp"
        android:backgroundTint="@android:color/holo_red_light"
        android:onClick="doSum"
        android:text="+"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <!-- A button to perform subtraction operation. -->

    <!-- A button to perform division. -->

    <Button
        android:id="@+id/sub"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="210dp"
        android:layout_marginTop="292dp"
        android:layout_marginEnd="113dp"
        android:layout_marginBottom="263dp"
        android:backgroundTint="@android:color/holo_red_light"
        android:onClick="doSub"
        android:text="-"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.507" />

    <Button
        android:id="@+id/div"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="307dp"
        android:layout_marginTop="292dp"
        android:layout_marginEnd="16dp"
        android:layout_marginBottom="263dp"
        android:backgroundTint="@android:color/holo_red_light"
        android:onClick="doDiv"
        android:text="/"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <!-- A button to perform multiplication. -->
    <Button
        android:id="@+id/mul"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="356dp"
        android:layout_marginEnd="307dp"
        android:layout_marginBottom="199dp"
        android:backgroundTint="@android:color/holo_red_light"
        android:onClick="doMul"
        android:text="x"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <!-- A button to perform a modulus function. -->

    <!-- A button to perform a power function. -->

    <Button
        android:id="@+id/button"
        android:layout_width="103dp"
        android:layout_height="46dp"
        android:layout_marginStart="113dp"
        android:layout_marginTop="356dp"
        android:layout_marginEnd="206dp"
        android:layout_marginBottom="199dp"
        android:backgroundTint="@android:color/holo_red_light"
        android:onClick="doMod"
        android:text="%(mod)"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.515" />

    <Button
        android:id="@+id/pow"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="113dp"
        android:layout_marginTop="292dp"
        android:layout_marginEnd="210dp"
        android:layout_marginBottom="263dp"
        android:backgroundTint="@android:color/holo_red_light"
        android:onClick="doPow"
        android:text="n1^n2"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.507" />

</androidx.constraintlayout.widget.ConstraintLayout>

After using this code the UI will be like as follows:

Step 3: Working with the MainActivity.java file

Open the MainActivity.java file there within the class, and make a method named doSum(View v). In this method, first of all, we have to link two EditText with variables so that we can use them for our input. So link those edit box with variables we have written

"EditText e1=(EditText )findViewById(R.id.num1);"

Here num1 is id for the textbox and we are just giving a variable name ‘e1’ to text box with id ‘num1’. Similarly, we have to use the same statement for the second textbox with the variable name ‘e2’. For the third text box, we have used

"TextView t1=(TextView) findViewById(R.id.result);"

Here we have used TextView because we only have to display text avoiding it being user-changeable. Now we have to input numbers in form of string using the getText() function. The input statement will be 

"String s11=e1.getText().toString();"

Here s11 stores the number entered in textbox 1. We have to do the same with another Textbox(e2). Now store the number in int form and apply addition. store the added value in another variable. To display stored in sum we have to use setText() as follows: 

result.setText(final_sum.toString())

final_sum stores the sum and it’s necessary to convert it to string(.toString()). Below is the code for the MainActivity.java file. Comments are added inside the code to understand the code in more detail.

File: MainActivity.java

Java

package com.example.calculator2;

import android.os.Bundle;

import com.google.android.material.snackbar.Snackbar;

import androidx.appcompat.app.AppCompatActivity;

import android.text.TextUtils;
import android.view.View;

import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;

import com.example.calculator2.databinding.ActivityMainBinding;

import android.view.Menu;
import android.view.MenuItem;

import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    private AppBarConfiguration appBarConfiguration;
    private ActivityMainBinding binding;
    public EditText e1, e2;
    TextView t1;
    int num1, num2;

    public boolean getNumbers() {

        //checkAndClear();
        // defining the edit text 1 to e1
        e1 = (EditText) findViewById(R.id.num1);

        // defining the edit text 2 to e2
        e2 = (EditText) findViewById(R.id.num2);

        // defining the text view to t1
        t1 = (TextView) findViewById(R.id.result);

        // taking input from text box 1
        String s1 = e1.getText().toString();

        // taking input from text box 2
        String s2 = e2.getText().toString();



         if(s1.equals("Please enter value 1") && s2.equals(null))
         {
             String result = "Please enter value 2";
             e2.setText(result);
             return false;
         }
        if(s1.equals(null) && s2.equals("Please enter value 2"))
        {
            String result = "Please enter value 1";
            e1.setText(result);
            return false;
        }
        if(s1.equals("Please enter value 1") || s2.equals("Please enter value 2"))
        {
            return false;
        }

         if((!s1.equals(null) && s2.equals(null))|| (!s1.equals("") && s2.equals("")) ){

            String result = "Please enter value 2";

            e2.setText(result);
            return false;
        }
        if((s1.equals(null) && !s2.equals(null))|| (s1.equals("") && !s2.equals("")) ){
            //checkAndClear();
            String result = "Please enter value 1";
            e1.setText(result);
            return false;
        }
        if((s1.equals(null) && s2.equals(null))|| (s1.equals("") && s2.equals("")) ){
            //checkAndClear();
            String result1 = "Please enter value 1";
            e1.setText(result1);
            String result2 = "Please enter value 2";
            e2.setText(result2);
            return false;
        }

        else {
            // converting string to int.
            num1 = Integer.parseInt(s1);

            // converting string to int.
            num2 = Integer.parseInt(s2);


        }

        return true;
    }

    public void doSum(View v) {

        // get the input numbers
        if (getNumbers()) {
            int sum = num1 + num2;
            t1.setText(Integer.toString(sum));
        }
       else
        {
            t1.setText("Error Please enter Required Values");
        }

    }
    public void clearTextNum1(View v) {

        // get the input numbers
      e1.getText().clear();
    }
    public void clearTextNum2(View v) {

        // get the input numbers
        e2.getText().clear();
    }
    public void doPow(View v) {

        //checkAndClear();
        // get the input numbers
        if (getNumbers()) {
            double sum = Math.pow(num1, num2);
            t1.setText(Double.toString(sum));
        }
        else
        {
            t1.setText("Error Please enter Required Values");
        }
    }

    // a public method to perform subtraction
    public void doSub(View v) {
        //checkAndClear();
        // get the input numbers
        if (getNumbers()) {
            int sum = num1 - num2;
            t1.setText(Integer.toString(sum));
        }
        else
        {
            t1.setText("Error Please enter Required Values");
        }
    }

    // a public method to perform multiplication
    public void doMul(View v) {
        //checkAndClear();
        // get the input numbers
        if (getNumbers()) {
            int sum = num1 * num2;
            t1.setText(Integer.toString(sum));
        }
        else
        {
            t1.setText("Error Please enter Required Values");
        }
    }

    // a public method to perform Division
    public void doDiv(View v) {
        //checkAndClear();
        // get the input numbers
        if (getNumbers()) {

            // displaying the text in text view assigned as t1
            double sum = num1 / (num2 * 1.0);
            t1.setText(Double.toString(sum));
        }
        else
        {
            t1.setText("Error Please enter Required Values");
        }
    }

    // a public method to perform modulus function
    public void doMod(View v) {
        //checkAndClear();
        // get the input numbers
        if (getNumbers()) {
            double sum = num1 % num2;
            t1.setText(Double.toString(sum));
        }
        else
        {
            t1.setText("Error Please enter Required Values");
        }
    }


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        e1 = (EditText) findViewById(R.id.num1);
        // defining the edit text 2 to e2
        e2 = (EditText) findViewById(R.id.num2);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public boolean onSupportNavigateUp() {
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_content_main);
        return NavigationUI.navigateUp(navController, appBarConfiguration)
                || super.onSupportNavigateUp();
    }
}

Output: