How to junit cover and enter the if condition of !CollectionUtils.isEmpty(disReadings)?

I am learning junit and trying to junit test the class and I am not able to enter the if !CollectionUtils.isEmpty(disReadings) part of code through the code:

List<DisLRead> disLReadList = new ArrayList<>();
cHlDisList.forEach(dis -> {
List<Double> disReadingsL = disRepository
		.getReadsForDisId(dis.getDisId(), sDate, eDate);
		
if (!CollectionUtils.isEmpty(disReadingsL)) {
	double max = Round.RoundToTwoDecimal(
			disReadingsL.stream().mapToDouble(Double::doubleValue).max().getAsDouble());
	double min = Round.RoundToTwoDecimal(
			disReadingsL.stream().mapToDouble(Double::doubleValue).min().getAsDouble());
	double avg = Round.RoundToTwoDecimal(
			disReadingsL.stream().mapToDouble(Double::doubleValue).average().getAsDouble());
	DisLReading disLReading = new DisLReading();
	disLReading.setDisId(dis.getDisId());
	disLReading.setDisName(dis.getDisName());
	disLReading.setDisZ(dis.getDisZ());
	disLReading.setMax(max);
	disLReading.setMin(min);
	disLReading.setAverage(avg);
	disLReadList.add(disLReading);
}});

Update:

Snippet of my test class:

@Mock
DisRepository disRepository;

@Mock
Round round;
	List<DisLRead> disLReadList = new ArrayList<>();
	List<Double> disReadingsL = new ArrayList<>();
	disReadingsL.add(100.0000);
	disReadingsL.add(200.0000);
	when(disRepository.getReadsForDisId(anyInt(), anyString(), anyString()))
			.thenReturn(disReadingsL);
	assertNotNull(disReadingsL);
	assertEquals(!CollectionUtils.isEmpty(disReadingsL), true);

What am I doing wrong? how to enter the cover this part of code. Thanks in advance!

You have more or less two options either use a mocking framework like mockito to create a mock object for "disRepository" it would look like this:

DisrepositoryClass disrepositoryMock = Mockito.mock(disrepository.class);
when(disrepositoryMock.getReadsForDisId(any(),any(),any()).thenReturn(List.of(Double.of(1d)));

Alternativly you create your own Mock object and provide it.

CollectionUtils.isEmpty() is a static method; therefore, you have to use PowerMockito.

mockStatic(CollectionUtils.class);
given(CollectionUtils.isEmpty(anyList())).willReturn(true);