java 获取下一个字母（传大写返回大写，传小写返回小写）

public static String getNextUpEn(String en){
        char lastE = 'a';
        char st = en.toCharArray()[0];
        if(Character.isUpperCase(st)){
            if(en.equals("Z")){
                return "A";
            }
            if(en==null || en.equals("")){
                return "A";
            }
            lastE = 'Z';
        }else{
            if(en.equals("z")){
                return "a";
            }
            if(en==null || en.equals("")){
                return "a";
            }
            lastE = 'z';
        }
        int lastEnglish = (int)lastE;
        char[] c = en.toCharArray();
        if(c.length>1){
            return null;
        }else{
            int now = (int)c[0];
            if(now >= lastEnglish)
                return null;
            char uppercase = (char)(now+1);
            return String.valueOf(uppercase);
        }
    }