How to assign value type byte array in java to a button

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Java

algorithm

GUI

netbeans

I’m trying to develop a system that will ask the user to enter a text in JTextBox (plainText) then it will click on key JButton (key_128) then it will encrypt the user text and show the output in another JTextBox(encryptedtext). the code now asking the user to enter the key. how i can change it to static value in JButton (key_128)

byte[]  plain = plaintext.getText().getBytes();
        byte[] K = time_128.getText().getBytes();
         byte[] encrypted = encrypt(plain, K);
         String a = new String(bytesToHex(encrypted));
         encryptedtext.setText(a);

What I have tried:

i tried to use binaryCodec but that didn't work :(