Cipher

import java.util.stream.*;

public class Cipher {

private String key;
public Cipher() {
    this("aaaaaaaaaa");
}

public Cipher(String key) {
    this.key = key;
}

public String getKey() {
    return key;
}

public String encode(String plainText) {
    return shift(plainText, false);
}

public String decode(String cipherText) {
    return shift(cipherText, true);
}

private String shift(String plainText, boolean down){
    StringBuilder result = new StringBuilder();
    for(int i=0; i<plainText.length(); i++){
        char currentLett = plainText.charAt(i);
        int transform = (down? -1: 1) * (key.charAt(i% key.length()) - 'a');
        int newTrasf = ((currentLett - 'a' + transform)+26)% 26 + 'a';
        result.append((char) newTrasf);
    }
    return result.toString();
}

}