You want to process the contents of a string one character at a time.

Use a for loop and the String’s charAt( ) method.

A string’s charAt( ) method retrieves a given character by index number (starting at zero) from within the String object. To process all the characters in a String, one after another, use a for loop ranging from zero to String.length( )-1. Here we process all the characters in a String:

// StrCharAt.java
String a = "A quick bronze fox leapt a lazy bovine";
for (int i=0; i < a.length(  ); i++)
    System.out.println("Char " + i + " is " + a.charAt(i));

A checksum is a numeric quantity representing and confirming the contents of a file. If you transmit the checksum of a file separately from the contents, a recipient can checksum the file -- assuming the algorithm is known -- and verify that the file was received intact. Example 3-3 shows the simplest possible checksum, computed just by adding the numeric value of each character together. It should produce the value “1248” if the input is “an apple a day”. Note that on files, it will not include the values of the newline characters; to fix this, retrieve System.getProperty("line.separator"); and add its character value(s) into the sum at the end of each line. Or, give up on line mode and read the file a character at a time.

/** CheckSum one file, given an open BufferedReader. */
    public int process(BufferedReader is) {
        int sum = 0;
        try {
            String inputLine;

            while ((inputLine = is.readLine(  )) != null) {
                int i;
                for (i=0; i<inputLine.length(  ); i++) {
                    sum += inputLine.charAt(i);
                }
            }
            is.close(  );
        } catch (IOException e) {
            System.out.println("IOException: " + e);
        } finally {
            return sum;
        }
}