private final Object monitor = new Object();

    MockCallback(String expectedValue) {
      this.value = expectedValue;
    }

    MockCallback(Throwable expectedFailure) {
      this.failure = expectedFailure;
    }

    @Override
    public void onSuccess(String result) {
      synchronized (monitor) {
        assertFalse(wasCalled);
        wasCalled = true;

    private final Object monitor = new Object();

    MockCallback(String expectedValue) {
      this.value = expectedValue;
    }

    MockCallback(Throwable expectedFailure) {
      this.failure = expectedFailure;
    }

    @Override
    public void onSuccess(String result) {
      synchronized (monitor) {
        assertFalse(wasCalled);
        wasCalled = true;