How to insert random character between character Java

For example, my password is “mycleverpassword”. Then, insert random character between “m” and “y”, “y” and “c”, “c” and “l”, and until the last character. I'm not quite sure how to create a random character and have them be slotted in between each word. We are currently learning about interfaces too in class. We are then also suppose to undo the encryption.