Counting letter frequencies

Sometimes an analysis of a file's contents is helpful. A Java method can be used to read in a file, count letters, and display the results.

Let us first consider the example text file. It contains some patterns of letters like 2 "x" characters and one uppercase Z.

This program uses BufferedReader and HashMap. With BufferedReader, it reads in a file line-by-line. We then loop over the line and call charAt to access characters.

Next We test to see if we have a space—we avoid hashing spaces. We then put the char in the HashMap.

Info We call getOrDefault to get an existing value from the HashMap or 0. Then we put one plus that value.

Finally We use a for-loop on the result of keySet(). We display all characters found and their frequencies in the file.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.HashMap;

public class Program {
    public static void main(String[] args) throws IOException {

        HashMap<Integer, Integer> hash = new HashMap<>();

        // Load this text file.
        BufferedReader reader = new BufferedReader(new FileReader(
                "C:\\programs\\file.txt"));

        while (true) {
            String line = reader.readLine();
            if (line == null) {
                break;
            }
            for (int i = 0; i < line.length(); i++) {
                char c = line.charAt(i);
                if (c != ' ') {
                // Increment existing value in HashMap.
                // ... Start with zero if no key exists.
                int value = hash.getOrDefault((int) c, 0);
                hash.put((int) c, value + 1);
            }
        }
    }

    // Close object.
    reader.close();

    // Display values of all keys.
    for (int key : hash.keySet()) {
        System.out.println((char) key + ": " + hash.get(key));
    }
}
}aaaa
bbbbb
aaaa
bbbbb
aaaa bbbbb
CCcc
xx
y y y y y
Za: 12
b: 15
C: 2
c: 2
x: 2
y: 5
Z: 1

For performance, an array of all possible char values can be used. We can use a char as an index by casting it to an int. Then we increment each char index as it is encountered.

With some inspection, we can see that the results of the program are correct. This kind of program will need to be modified to suit your exact goals. But the general approach is durable.

Often in programs we can use a HashMap to count how often an element occurs. It is possible to increment the value on each encountered key.