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Let’s now create a Simple Calculator App using Android Studio and Java. This app will allow users to perform basic arithmetic operations (addition, subtraction, multiplication, and division).

Step-by-Step Instructions:

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1. Set Up Your Android Project

1. Open Android Studio.
2. Select New Project.
3. Choose Empty Activity and click Next.
4. Name your project CalculatorApp.
5. Select Java as the programming language.
6. Click Finish to create the project.

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2. Design the Layout (XML)

We will design a simple calculator layout with number buttons (0-9), operator buttons (+, −, ×, ÷), a clear button, and an equal button.

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <EditText
        android:id="@+id/display1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="16dp"
        android:focusable="true"
        android:focusableInTouchMode="true"
        android:textSize="32sp"
        android:gravity="end"
        android:hint="Num 1"/>

    <EditText
        android:id="@+id/display2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/display1"
        android:layout_margin="16dp"
        android:focusable="true"
        android:focusableInTouchMode="true"
        android:textSize="32sp"
        android:gravity="end"
        android:hint="Num 2"/>

    <TextView
        android:id="@+id/result_display"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/display2"
        android:layout_margin="16dp"
        android:textSize="32sp"
        android:gravity="end"
        android:text=""/>

    <GridLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/result_display"
        android:padding="16dp"
        android:rowCount="5"
        android:columnCount="4">

        <Button android:id="@+id/button_1" android:text="1" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_2" android:text="2" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_3" android:text="3" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_4" android:text="4" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_5" android:text="5" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_6" android:text="6" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_7" android:text="7" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_8" android:text="8" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_9" android:text="9" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_0" android:text="0" style="@style/ButtonStyle"/>

        <Button android:id="@+id/button_dot" android:text="." style="@style/ButtonStyle"/>

        <Button android:id="@+id/button_clear" android:text="C" style="@style/ButtonStyle"/>

        <Button android:id="@+id/button_plus" android:text="+" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_minus" android:text="-" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_multiply" android:text="×" style="@style/ButtonStyle"/>
        <Button android:id="@+id/button_divide" android:text="÷" style="@style/ButtonStyle"/>

    </GridLayout>

</RelativeLayout>

This layout contains:

• A TextView to display the calculation result.
• Buttons for numbers, operators (+, -, ×, ÷), and clear.

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Add styles

styles.xml

<?xml version=”1.0″ encoding=”utf-8″?>
<resources>
<style name=”ButtonStyle”>

</style>
</resources>

You can customize if you want.

3. Implement the Calculator Logic in Java

Now we’ll handle the button clicks and implement the calculation logic.

MainActivity.java

package com.example.simplecalculator;

import android.os.Bundle;
import android.text.InputFilter;
import android.text.Spanned;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    private EditText display1, display2;
    private TextView resultDisplay;
    private String operator;
    private double valueOne, valueTwo;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        display1 = findViewById(R.id.display1);
        display2 = findViewById(R.id.display2);
        resultDisplay = findViewById(R.id.result_display);

        // Set filters to allow only numbers and one dot
        setInputFilter(display1);
        setInputFilter(display2);

        // Number buttons
        setNumberButton(R.id.button_0, "0");
        setNumberButton(R.id.button_1, "1");
        setNumberButton(R.id.button_2, "2");
        setNumberButton(R.id.button_3, "3");
        setNumberButton(R.id.button_4, "4");
        setNumberButton(R.id.button_5, "5");
        setNumberButton(R.id.button_6, "6");
        setNumberButton(R.id.button_7, "7");
        setNumberButton(R.id.button_8, "8");
        setNumberButton(R.id.button_9, "9");
        setNumberButton(R.id.button_dot, "."); // Added dot button

        // Operator buttons
        findViewById(R.id.button_plus).setOnClickListener(v -> setOperator("+"));
        findViewById(R.id.button_minus).setOnClickListener(v -> setOperator("-"));
        findViewById(R.id.button_multiply).setOnClickListener(v -> setOperator("×"));
        findViewById(R.id.button_divide).setOnClickListener(v -> setOperator("÷"));

        // Clear button
        findViewById(R.id.button_clear).setOnClickListener(v -> clear());
    }

    private void setInputFilter(EditText editText) {
        editText.setFilters(new InputFilter[]{new DecimalInputFilter()});
    }

    private class DecimalInputFilter implements InputFilter {
        @Override
        public CharSequence filter(CharSequence source, int start, int end,
                                   Spanned dest, int dstart, int dend) {
            // If the input is empty, allow it
            if (source.length() == 0) {
                return null;
            }
            String input = dest.toString().substring(0, dstart) + source.toString() + dest.toString().substring(dend);
            // Regex to check for valid decimal number
            if (input.matches("^[0-9]*(\\.[0-9]+)?$")) {
                return null; // Accept the input
            }
            return ""; // Reject the input
        }
    }

    private void setNumberButton(int buttonId, String number) {
        findViewById(buttonId).setOnClickListener(v -> {
            // Determine which EditText is currently focused and append the number
            if (display1.hasFocus()) {
                // Allow only one dot in the first input
                if (!display1.getText().toString().contains(".") || !number.equals(".")) {
                    display1.append(number);
                }
            } else {
                // Allow only one dot in the second input
                if (!display2.getText().toString().contains(".") || !number.equals(".")) {
                    display2.append(number);
                }
            }
        });
    }

    private void setOperator(String op) {
        if (!display1.getText().toString().isEmpty()) {
            valueOne = Double.parseDouble(display1.getText().toString());
            operator = op;

            // Calculate immediately after setting the operator
            if (!display2.getText().toString().isEmpty()) {
                valueTwo = Double.parseDouble(display2.getText().toString());
                calculate(); // Perform the calculation
            }

            display2.requestFocus(); // Set focus to the second display for the next number
        }
    }

    private void calculate() {
        if (!display2.getText().toString().isEmpty() && operator != null) {
            valueTwo = Double.parseDouble(display2.getText().toString());
            double result = 0;

            switch (operator) {
                case "+":
                    result = valueOne + valueTwo;
                    break;
                case "-":
                    result = valueOne - valueTwo;
                    break;
                case "×":
                    result = valueOne * valueTwo;
                    break;
                case "÷":
                    if (valueTwo != 0) {
                        result = valueOne / valueTwo;
                    } else {
                        resultDisplay.setText("Error");
                        return;
                    }
                    break;
            }
            resultDisplay.setText(String.valueOf(result)); // Update result display
            clearInputs(); // Clear inputs for next calculation
        }
    }

    private void clearInputs() {
        display1.setText(""); // Clear first input
        display2.setText(""); // Clear second input
        operator = null; // Reset operator
    }

    private void clear() {
        clearInputs(); // Reset everything
        resultDisplay.setText(""); // Clear result display
        valueOne = 0;
        valueTwo = 0;
    }
}

4. Run the App

1. Connect an Android device or use an emulator.
2. Click the Run button in Android Studio.
3. The app will launch, allowing you to:

• Enter numbers via the number buttons.
• Perform arithmetic operations via the operator buttons (+, −, ×, ÷).
• Use the C button to clear the input.

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Output:

Key Points of the App:

• TextView displays the current number or result.
• Buttons for numbers (0-9), operators, and clear.
• Arithmetic operations are handled in Java using a simple switch-case structure.
• Clear resets the calculator to its initial state.

This project is a great way to learn how to handle user input, manage basic UI elements, and perform simple logic in an Android app.