How to create a for loop to display alphabet occurence count from file

So basically i'm trying to write a code that will count the amount of times a letter appears in a text file.
I've got most of the code down, and as far I know it should work. However, I can not figure out how to display
the count, there is a precision error, and i'm guessing it has to do with the part where i try to use a char where or an int should be or vice versa. I am reusing code that I wrote for a different project that used a char array,
this one uses an int array to do it, which explains the precision error, but i do not know how to convert the code to work for an int array. i've tried switching int to char, but that doesn't work.

import java.io.*;
import java.util.*;
import java.io.IOException;

public class SchardinA02 {
 
 public static void main(String [] args) {

	int[] counter = new int[26];
	char c = 0;
	int buf = -1;
	Reader in = null;
	
	try {
	
		System.out.println("Count of each letter found in " + "input.txt" + ":");
		System.out.println("");
		
		in = new FileReader("input.txt");
			while((buf = in.read()) > -1) {
				c = (char) buf;
				c = Character.toLowerCase(c);
				
				if (Character.isLetter(c)) {
					counter [c -'a']++;
			   }
		   }
	   //-------------------------------------------------------------
	 System.out.println("Count of each letter found in input.txt: " );
	 
	 Arrays.sort(counter);
	 
	  for (int addUp = 0; addUp < counter.length; addUp++) {
           char ch = counter[addUp];
           int count = 0;
         
			for (int i = 0; i < counter.length; i++) {
              if (ch == counter[i])
                  count++;
             }//first nested for loop
               boolean flag = false;
         for (int j = addUp - 1; j >= 0; j--) {
              if (ch == counter[j])
                  flag = true;
             }//second nested for loop
              if (!flag) {
              System.out.println(ch + ":" + count);
              }//if statement

		 }//Main for loop
	 
		
    System.out.println("Total letters found: "+ counter.length );
		//------------------------------------------------------

		
	}catch (IOException e) {
     System.out.println("Could not read from file: " + e.getMessage());
	}
	
 }

}