<resources>
    <string name="app_name">GFG App</string>
    <string name="qr_code">qr_code</string>
    <string name="enter_your_info">Enter your info</string>
    <string name="generate_qr_code">Generate QR Code</string>
</resources>

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <!--We are using this image 
        view to display our QR code-->
    <ImageView
        android:id="@+id/idIVQrcode"
        android:layout_width="300dp"
        android:layout_height="300dp"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="50dp"
        android:contentDescription="@string/qr_code" />

    <!--Edit text to enter text 
        for creating a QR code-->
    <EditText
        android:id="@+id/idEdt"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/idIVQrcode"
        android:layout_marginStart="20dp"
        android:layout_marginTop="30dp"
        android:layout_marginEnd="20dp"
        android:autofillHints=""
        android:hint="@string/enter_your_info"
        android:inputType="text" />

    <!--Button for creating a QR code-->
    <Button
        android:id="@+id/idBtnGenerateQR"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/idEdt"
        android:layout_marginStart="20dp"
        android:layout_marginTop="30dp"
        android:layout_marginEnd="20dp"
        android:text="@string/generate_qr_code" />

</RelativeLayout>

import android.graphics.Bitmap;
import android.graphics.Point;
import android.os.Bundle;
import android.text.TextUtils;
import android.util.Log;
import android.view.Display;
import android.view.View;
import android.view.WindowManager;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import com.google.zxing.WriterException;
import androidmads.library.qrgenearator.QRGContents;
import androidmads.library.qrgenearator.QRGEncoder;

public class MainActivity extends AppCompatActivity {

    // variables for imageview, edittext,
    // button, bitmap and qrencoder.
    private ImageView qrCodeIV;
    private EditText dataEdt;
    private Button generateQrBtn;
    Bitmap bitmap;
    QRGEncoder qrgEncoder;

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

        // initializing all variables.
        qrCodeIV = findViewById(R.id.idIVQrcode);
        dataEdt = findViewById(R.id.idEdt);
        generateQrBtn = findViewById(R.id.idBtnGenerateQR);

        // initializing onclick listener for button.
        generateQrBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (TextUtils.isEmpty(dataEdt.getText().toString())) {

                    // if the edittext inputs are empty then execute 
                    // this method showing a toast message.
                    Toast.makeText(MainActivity.this, "Enter some text to generate QR Code", Toast.LENGTH_SHORT).show();
                } else {
                    // below line is for getting 
                    // the windowmanager service.
                    WindowManager manager = (WindowManager) getSystemService(WINDOW_SERVICE);

                    // initializing a variable for default display.
                    Display display = manager.getDefaultDisplay();

                    // creating a variable for point which 
                    // is to be displayed in QR Code.
                    Point point = new Point();
                    display.getSize(point);

                    // getting width and 
                    // height of a point
                    int width = point.x;
                    int height = point.y;

                    // generating dimension from width and height.
                    int dimen = width < height ? width : height;
                    dimen = dimen * 3 / 4;

                    // setting this dimensions inside our qr code
                    // encoder to generate our qr code.
                    qrgEncoder = new QRGEncoder(dataEdt.getText().toString(), null, QRGContents.Type.TEXT, dimen);
                    try {
                        // getting our qrcode in the form of bitmap.
                        bitmap = qrgEncoder.encodeAsBitmap();
                        // the bitmap is set inside our image 
                        // view using .setimagebitmap method.
                        qrCodeIV.setImageBitmap(bitmap);
                    } catch (WriterException e) {
                        // this method is called for 
                        // exception handling.
                        Log.e("Tag", e.toString());
                    }
                }
            }
        });
    }
}