Count all Characters
by Isai Damier, Android Engineer @ Google

/*********************************************************************
 * Author: Isai Damier
 * Title: Count all Characters
 * Project: geekviewpoint
 * Package: algorithms
 *
 * Statement:
 *   Given a text, assume ASCII, count the frequency of each character.
 *
 * Time Complexity: O(n)
 * 
 * Technical Details:
 * 
 *  We could easily solve this problem using a HashMap with character
 *  keys and integer values.
 * 
 *     Map<Character,Integer> map = new HashMap<Character,Integer>();
 *     Integer freq;
 *     for(char c : text.toCharArray()){
 *       freq = map.get(a);
 *       map.put(a, null == freq? 1 : freq + 1);
 *     }
 *     return map;
 * 
 *  But that's kind of a heavy solution to such a special problem.
 *  The set of characters in a human alphabet is very small, e.g. 255
 *  in extended ASCII. Therefore, it makes more sense to simply use
 *  an array with 255 elements where the indices represent characters
 *  and the values represent frequencies. The caller could then cast
 *  the indices to char.
 ********************************************************************/ 
 public int[] countAllChars(String text) {
  final int ALL_CHARS = 255;
  int[] freqency = new int[ALL_CHARS];
  for (char c : text.toCharArray()) {
    freqency[(int) c]++;
  }
  return freqency;
}

package algorithms;

import org.junit.Test;
import static org.junit.Assert.*;

public class StringsTest {

 /**
   * Test of countAllChars method, of class Strings.
   * Text from King Lear: Albany's coda after the deaths
   */
  @Test
  public void testCountAllChars() {
    System.out.println("countAllChars");
    String text = "The weight of this sad time we must obey, "
            + "Speak what we feel, not what we ought to say. "
            + "The oldest have borne most. We that are young "
            + "Shall never see so much, nor live so long";
    Strings instance = new Strings();
    int[][] expResult = {
      {32, 36}, {44, 3}, {46, 2}, {83, 2}, {84, 2}, {87, 1}, {97, 9},
      {98, 2}, {99, 1}, {100, 2}, {101, 21}, {102, 2}, {103, 4}, {104, 11},
      {105, 4}, {107, 1}, {108, 6}, {109, 4}, {110, 6}, {111, 13}, {112, 1},
      {114, 4}, {115, 9}, {116, 13}, {117, 4}, {118, 3}, {119, 6}, {121, 3}};

    int[] result = instance.countAllChars(text);
    int ndx = 0;
    for (int i = 0; i < result.length && ndx < expResult.length; i++) {
      if (i == expResult[ndx][0]) {
        if (result[i] != expResult[ndx++][1]) {
          fail("miscount");
        }
      }
    }
  }
}