Character Frequency

/*
 * Problem : Process one line of text.
 * Input   : One line of text.
 * Output  : One line per letter.
 * Approach: Use a counter table.
 * Time    : O(n)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String s) {
        // Step 1: A table of 26 counters, one for each letter.
        int[] counts = new int[26];

        // Step 2: Look at every character. Count it if it is a letter.
        for (char c : s.toLowerCase().toCharArray()) {
            if (c >= 'a' && c <= 'z') {
                counts[c - 'a']++;
            }
        }

        // Step 3: Make one line for each letter that has a count above 0.
        java.util.List<String> lines = new java.util.ArrayList<>();
        for (int i = 0; i < 26; i++) {
            if (counts[i] > 0) {
                lines.add((char) ('a' + i) + " " + counts[i]);
            }
        }
        return lines.toArray(new String[0]);
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.hasNextLine() ? scanner.nextLine() : "";
        System.out.println(String.join("\n", solve(s)));
    }
}