how to create alert dialog in android studio

package org.geeksforgeeks.navedmalik.alertdialog;
  
import android.content.DialogInterface;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
  
public class MainActivity extends AppCompatActivity {
  
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
  
    // Declare the onBackPressed method
    // when the back button is pressed
    // this method will call
    @Override
    public void onBackPressed()
    {
  
        // Create the object of
        // AlertDialog Builder class
        AlertDialog.Builder builder
            = new AlertDialog
                  .Builder(MainActivity.this);
  
        // Set the message show for the Alert time
        builder.setMessage("Do you want to exit ?");
  
        // Set Alert Title
        builder.setTitle("Alert !");
  
        // Set Cancelable false
        // for when the user clicks on the outside
        // the Dialog Box then it will remain show
        builder.setCancelable(false);
  
        // Set the positive button with yes name
        // OnClickListener method is use of
        // DialogInterface interface.
  
        builder
            .setPositiveButton(
                "Yes",
                new DialogInterface
                    .OnClickListener() {
  
                        @Override
                        public void onClick(DialogInterface dialog,
                                            int which)
                        {
  
                            // When the user click yes button
                            // then app will close
                            finish();
                        }
                    });
  
        // Set the Negative button with No name
        // OnClickListener method is use
        // of DialogInterface interface.
        builder
            .setNegativeButton(
                "No",
                new DialogInterface
                    .OnClickListener() {
  
                        @Override
                        public void onClick(DialogInterface dialog,
                                            int which)
                        {
  
                            // If user click no
                            // then dialog box is canceled.
                            dialog.cancel();
                        }
                    });
  
        // Create the Alert dialog
        AlertDialog alertDialog = builder.create();
  
        // Show the Alert Dialog box
        alertDialog.show();
    }
}