dummy Java Clock mock to timetravel in unit tests

MockClock.java

import java.time.Clock;
import java.time.Duration;
import java.time.Instant;
import java.time.ZoneId;
import java.time.temporal.ChronoUnit;

/** Clock used so we can shift time in tests. */
public class MockClock extends Clock {

  private final Instant instant;
  private Duration shift;

  public MockClock() {
    this(Instant.now().truncatedTo(ChronoUnit.HOURS));
  }

  public MockClock(Instant base) {
    this.instant = base;
    this.shift = Duration.ZERO;
  }

  public synchronized void shiftAbsolute(Duration shift) {
    assert shift != null;
    this.shift = shift;
  }

  public synchronized void shiftRelative(Duration shift) {
    assert shift != null;
    this.shift = this.shift.plus(shift);
  }

  @Override
  public ZoneId getZone() {
    return ZoneId.systemDefault();
  }

  @Override
  public Clock withZone(ZoneId zone) {
    return new MockClock(instant);
  }

  @Override
  public Instant instant() {
    return instant.plus(shift);
  }
}

MyTest.java

class MyTest {

  private MockClock clock;
  private MyObjectThatUseClock other;

  @BeforeEach
  void setUp() {
    clock = new MockClock();
    other = new MyObjectThatUseClock(clock);
  }

  @Test
  void should_work() {
    assertThat(other.getState()).isEqualTo(State.INITIALIZING);

    clock.shiftAbsolute(Duration.ofSeconds(9));
    other.recomputeState();
    assertThat(other.getState()).isEqualTo(State.INITIALIZING);

    clock.shiftAbsolute(Duration.ofSeconds(11));
    other.recomputeState();
    assertThat(other.getState()).isEqualTo(State.PROCESSING);
  }
}