Java assignment, need guidance ...

I'm incredibly new to writing Java code. For my assignment, I have to use arrays to calculate the sum, average, largest, and smallest of any number of test scores. In addition, I have to use a "for loop" to tally up the number of A'S, B'S, C'S, D'S, and F'S (ex. "The number of students with scores of 90-100 (A) is 4", etc. for each letter grade) based on the test scores. All this has to be shown in a message box. I can calculate the sum, average, largest, and smallest parts of this assignment, but I can't get the latter half of the assignment done right. Instead of "The number of students with scores of 90-100 (A) is **(insert calculated number here)**" showing up, I get "The number of students with scores of 90-100 (A) is **0**", and this happens for each letter grade I'm trying to determine. I'm not sure why the **0** is showing up. Here is my code:

import javax.swing.JOptionPane;

public class ArrayProject3 {

public static void main(String[] args) {

String response = JOptionPane.showInputDialog(null, "Number of scores:");

int number = Integer.parseInt(response);

int scores[] = new int[number];

for (int a = 0; a < number; a++) {

response = JOptionPane.showInputDialog(null, "Enter score " + (a+1));

scores[a] = Integer.parseInt(response);

}

int count = 0;

int gradeA = 0;

int gradeB = 0;

int gradeC = 0;

int gradeD = 0;

int gradeF = 0;

int sum = 0;

int largest = scores[0];

int smallest = scores[0];

for (int i = 0; i < scores.length; i++) {

sum = sum + scores[i];

if (scores[i] > largest) {

largest = scores[i];

}

if (scores[i] < smallest) {

smallest = scores[i];

}

if (scores[i] <= 90 && scores[i] >= 100){

gradeA = count++;}

if (scores[i] <= 80 && scores[i] >= 89){

gradeB = count++;}

if (scores[i] <= 70 && scores[i] >= 79){

gradeC = count++;}

if (scores[i] <= 60 && scores[i] >= 69){

gradeD = count++;}

if (scores[i] <= 60){

gradeF = count++;}

}

JOptionPane.showMessageDialog(null, "The sum is " + sum

+ "\nThe average is " + (sum / scores.length)

+ "\nThe largest is " + largest

+ "\nThe smallest is " + smallest

+ "\nThe number of students with scores of 90-100 (A) " + gradeA

+ "\nThe number of students with scores of 80-89 (B) " + gradeB

+ "\nThe number of students with scores of 70-79 (C) " + gradeC

+ "\nThe number of students with scores of 60-69 (D) " + gradeD

+ "\nThe number of students with scores below 60 (F) " + gradeF);

}

}