Design for testability: example of using wrappers around static method calls

ClockExample.java

public class ClockExample {

    public class Clock {
        public long now() {
            return System.currentTimeMillis();
        }
    }

    public class TestClock extends Clock {
        DateTime current = DateTime.now();

        @Override
        public long now() {
            return current.getMillis();
        }

        public void addMinutes(int minutes) {
            current = current.plusMinutes(minutes);
        }

        public void setTimeTo(DateTime now) {
            current = now;
        }

    }

    public class Alarm {
        Clock clock;
        private final DateTime ringAt;

        public Alarm(DateTime ringAt, Clock clock) {
            this.ringAt = ringAt;
            this.clock = clock;
        }

        public boolean isRinging() {
            return ringAt.isBefore(clock.now());
        }
    }

    @Test
    public void alarmRingsWhenItsTime() throws Exception {
        DateTime alarmTime = new DateTime(2013, 10, 10, 14, 23, 0);
        TestClock clock = new TestClock();
        clock.setTimeTo(alarmTime.minusMinutes(1));

        Alarm alarm = new Alarm(alarmTime, clock);
        assertThat(alarm.isRinging(), is(false));
        clock.addMinutes(3);
        assertThat(alarm.isRinging(), is(true));
    }
}