Get "ClassNotFoundException when using HttpClientBuilder

Old programmer, new to Java. I am trying to run what I think is pretty common example code that is similar in a number of places on the web, HttpClient httpClient = HttpClientBuilder.create().build() throws an exception and I can't figure out why. I'm using HttpClient 4.3.

import java.io.IOException;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.util.EntityUtils;

public class ATest{

   public static void main(String[] args) throws Exception {
      String strURL  = "http://192.9.10.11/cgi-bin/echo";
      String message = "hello world";
      // next line throwsClassNotFoundException, why?
      HttpClient httpClient = HttpClientBuilder.create().build();
      HttpPost   httpPost   = new  HttpPost(strURL);
      httpPost.setEntity(new StringEntity(message));
      HttpResponse response = httpClient.execute(httpPost);
      try {
         System.out.println(response.getStatusLine());
         HttpEntity entity = response.getEntity();
         // do something useful with the response body
         // and ensure it is fully consumed
         EntityUtils.consume(entity);
      } finally {
         response.close();
      }
   }
}