In Mock Objects in Unit Tests Lu Jian writes about two tools (EasyMock and Mocquer) which generate mock objects. Here’s my most frequently used mock object.

Disclaimer: I know one could argue whether this is a ‘real’ mock object or more of a mock object facade bastard thingy.

I use Clock everywhere I would call System.currentTimeMillis() or call Calendar.get(...). This gives me an easy way to fast-forward in time without sleeps during unit tests.

public class Clock {


// some methods left out for clarity


  private static boolean debugMode = false;


  /**
   * dummy time, default value of 0 produced some strange errors in some tests
   */
  private static long lMillis = 12341234;
  private static int hour;


  private Clock() {
    // no instances needed
  }
  /**
   * facade for System.currentTimeMillis()
   */
  public static long currentTimeMillis() {
    if (debugMode) {
      return lMillis;
    }
    return System.currentTimeMillis();
  }


  /**
   * facade for Calendar.get(Calendar.HOUR_OF_DAY)
   */
  public static int getHour() {
    if (debugMode) {
      return hour;
    }
    final Calendar c = Calendar.getInstance();
    return c.get(Calendar.HOUR_OF_DAY);
  }


  public static void setDebugMode(boolean b) {
    debugMode = b;
  }


  private static void checkDebug() {
    if (!debugMode) {
      throw new IllegalStateException("You cannot set or change time now. Turn on debug mode first");
    }
  }


  public static void setCurrentTimeMillis(final long l) {
    checkDebug();
    lMillis = l;
  }


  public static void addCurrentTimeMillis(final long l) {
    checkDebug();
    lMillis += l;
  }


  public static void setHour(final int i) {
    checkDebug();
    if (i < 0 || i > 23) {
      throw new IllegalArgumentException("hour '" + i + "'");
    }
    hour = i;
  }
}