Encryption and Decryption Application in Android using Caesar Cipher Algorithm

public class utility {

    // Declaration of all the required variables
    private static String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    private static int index;
    private static int updated_index;
    private static int final_index;
    private static int index_p_t_l;
    private static int index_s_t_l;
    private static String plainTxt;
    private static String cipherTxt;
    private static String finalTxt;

    // code for encryption
    public static String encrypt1(String plaintext, int encrptionKey) {
        reset();
        // plaintext is converted to uppercase 
        // so that it is easy to convert according
        // to Caesar Cipher algorithm
        plaintext = plaintext.toUpperCase();
        // using a for loop the use index and 
        // change text with help of it
        for (index = 0; index < plaintext.length(); index++) {
            // checking the condition for plaintext to be
            // null at some character position
            if (plaintext.charAt(index) != ' ') {
                index_p_t_l = alphabet.indexOf(plaintext.charAt(index));
                // index is being updated 
                // so that next and final index
                // be used for ciphertext
                updated_index = encrptionKey + alphabet.indexOf(plaintext.charAt(index));
                if (updated_index >= alphabet.length()) {
                    final_index = updated_index - alphabet.length();
                } else
                    final_index = updated_index;
                // substring is used so that every character
                // can be separately converted to ciphertext
                cipherTxt = alphabet.substring(final_index, final_index + 1);
                finalTxt = finalTxt + cipherTxt;
            }
        }
        // returning the 
        // final changed text
        return finalTxt;
    }

    // code for decryption
    public static String decrypt1(String ciphertext, int decryptionKey) {
        reset();
        ciphertext = ciphertext.toUpperCase();
        // using a for loop the use index and 
        // change text with help of it
        for (index = 0; index < ciphertext.length(); index++) {
            if (ciphertext.charAt(index) != ' ') {
                index_p_t_l = alphabet.indexOf(ciphertext.charAt(index));
                index_s_t_l = index_p_t_l;
                // index is updated with help of decryption
                // key which we provided as input
                updated_index = alphabet.indexOf(ciphertext.charAt(index)) - decryptionKey;
                if (updated_index < 0) {
                    final_index = updated_index + alphabet.length();
                } else
                    final_index = updated_index;
                // reverse of encryption is done as 
                // substring here is used to convert
                // each ciphertext character to plaintext 
                plainTxt = alphabet.substring(final_index, final_index + 1);
                finalTxt += plainTxt;
            }
        }
        // returning the 
        // final changed text
        return finalTxt;
    }

    // method to reset the text
    // in the output textview
    private static void reset() {
        finalTxt = "";
    }
}

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

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    // declaring all essential variables
    private Button encrypt, decrypt;
    private EditText message, cipher, key;
    private TextView screen_output;
    private static final String alphabetString = "abcdefghijklmnopqrstuvwxyz";

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

        // findViewById is the method that 
        // finds the View by the ID it is given
        encrypt = findViewById(R.id.btnencrypt);
        decrypt = findViewById(R.id.btndecrypt);
        screen_output = findViewById(R.id.tV1);
        message = findViewById(R.id.inputMessage);
        cipher = findViewById(R.id.ciphEdt);
        key = findViewById(R.id.key_dt);

        // setting onCLickListener on encrypt button
        encrypt.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                encrypt12(message.getText().toString(), Integer.parseInt(key.getText().toString()));
            }
        });

        // setting onCLickListener on decrypt button
        decrypt.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                decrypt12(cipher.getText().toString(), Integer.parseInt(key.getText().toString()));
            }
        });
    }

    // method to show the final output on the output
    // textView when decrypt button is clicked
    public void decrypt12(String cipher, int key) {
        screen_output.setText((utility.decrypt1(cipher, key).toLowerCase()));
    }

    // method to show the final output on the output
    // textView when encrypt button is clicked
    public String encrypt12(String message, int shiftkey) {
        message = message.toLowerCase();
        String cipherText = "";
        for (int i = 0; i < message.length(); i++) {
            int charPosition = alphabetString.indexOf(message.charAt(i));
            int keyval = (shiftkey + charPosition) % 26;
            char replaceVAL = alphabetString.charAt(keyval);
            cipherText += replaceVAL;
            screen_output.setText(cipherText);
            cipher.setText(cipherText);
        }

        // returning the final ciphertext
        return cipherText;
    }
}