foo.java

package scratch;

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

public class StartsWithPerf {

  public static void main(String[] args) {
    try {
      new StartsWithPerf().go();
    } catch (Exception e) {
      e.printStackTrace();
    }
  }

  public void go() throws Exception {

    Map<String, String> map = new HashMap();

    int numApps = 1000;
    int numTopicsPerApp = 10000;

    for (int i = 0; i < numApps; i++) {

      for (int j = 0; j < numTopicsPerApp; j++) {

        String key = String.format("tenant1_app%d_topic%d", i, j);

        map.put(key, key);

      }
    }

    for (int i = 0; i < 1000; i++) {

      long start = System.currentTimeMillis();
      int count = 0;
      for (Map.Entry<String, String> entry : map.entrySet()) {
        String key = entry.getKey();
        if (key.startsWith("tenant1_app23_")) {
          count++;
        }
      }

      long end = System.currentTimeMillis();

      System.out.println("Count is " + count + " duration " + (end - start));
    }
  }
}