public static char[] randomChange(String str){
        char[] chars=convertStringtoChar(str);
        Random generator = new Random();
        generator.setSeed(4);
        char ch1=(char)('a'+(Math.abs(generator.nextInt())%26));
        char ch2=(char)('a'+(Math.abs(generator.nextInt())%26));
        for(int i=0;i<chars.length;i++){
            if(chars[i]==ch1){
                chars[i]=ch2;
            }
        }
        return chars;
    }