Java Data Type - 如何检查字符串中的字符出现

import java.util.HashMap;
import java.util.Map;

public class Main {
  public static void main(String[] args) {
    Map<Character, Integer> counting = new HashMap<Character, Integer>();
    String testcase1 = "this is a test";
    for (char ch : testcase1.toCharArray()) {
      Integer freq = counting.get(ch);
      counting.put(ch, (freq == null) ? 1 : freq + 1);
    }
    System.out.println(counting.size() + " distinct characters:");
    System.out.println(counting);
  }

}