Java, choose subsequence

Java, choose subsequence

I have a problem with a part of my program. In the following code 27 is
the number of letters of our alphabet. The aim is : for every repetition
of the outer for we take te last n characters of text_generated, and for
every letter of the alphabet we count the number of occurrences in
text_formatted of the n+1 characters obtained by adding the character to
the last n characters of text_generated; then we choose the letter with
the biggest number of occurrences and append it to text_generated. The
result I obtain is something like ***aaaaaaaaaaaaaaaaaaa, why ? The code
int index;
int[] occurrences = new int[27];
int count;
for(int a = 0; a < m; a++){ // m is the number of characters
the user wants to add
for(int b = 0; b < 27; b++){
StringBuffer curr_word = new
StringBuffer(text_generated.substring(text_generated.length()-n,
text_generated.length()));
count = 0;
for(int c = 0; c <= text_formatted.length() -n-1;c++){
if(text_formatted.substring(c,c+n+1).equals(curr_word.append(array[b])))
count += 1;
}
occurrences[b] = count;
}
index = 0;
for(int d = 1; d < 27; d++){
if(occurrences[d] > occurrences[index])
index = d;
}
text_generated = text_generated.append(array[index]);
}