Understanding and Preventing NullPointerException in Java Backend Development

Introduction

Hello everyone, I'm the team leader and I will share a real case to deepen understanding of NullPointerException.

A newly hired intermediate Java developer was assigned a simple iteration: match third‑party data with internal channel settings and batch insert the results.

After deployment the matching logic threw an NPE, which is a basic mistake for a mid‑level developer.

Incident Reproduction

1. Pseudocode

The following pseudocode reproduces the issue (note: not the exact production code).

// backend configured channel
String channelNo = channelDao.getOne().getChannelNo();
// third‑party data
List<ThirdData> thirdDataList = httpClientUtils.getThirdDatas(DateUtils.today());
// filter matching
thirdDataList.stream().filter(o -> channelNo.equals(o.getChannelNo())).collect(Collectors.toList());
// batch insert
thirdDataDao.saveAll(thirdDataList);

2. Analysis and Solutions

The four lines hide three potential NPEs.

3. First line analysis

If channelDao.getOne() returns null , calling getChannelNo() triggers an NPE.

Solution: defensive programming – check for null before use.

// if channelNo is required, return early
Channel channel = channelDao.getOne();
if (channel == null) {
return;
}

Alternative: use ternary operator to supply an empty string.

String channelNo = channelDao.getOne() == null ? "" : channelDao.getOne().getChannelNo();

Or use Optional to provide a default.

String channelNo = Optional.ofNullable(channelDao.getOne()).orElse("");

4. Second line analysis

If thirdDataList is null , invoking stream() causes an NPE.

Solution: check collection emptiness before processing.

// recommended defensive check
if (CollectionUtils.isEmpty(thirdDataList)) {
return;
}

Another option (less recommended) is to wrap the logic in an if block.

if (CollectionUtils.isNotEmpty(thirdDataList)) {
// execute subsequent logic
}

5. Third line analysis

If channelNo is null , the expression channelNo.equals(o.getChannelNo()) throws an NPE.

Fixes include:

Explicit null check: channelNo != null && channelNo.equals(o.getChannelNo())

Use Objects.equals(channelNo, o.getChannelNo()) which handles null safely.

Leverage utility libraries such as StringUtils or StrUtil .

Conclusion

The article also recommends the SonarLint plugin (and its server counterpart SonarQube) for real‑time detection of issues like NPEs in IDEA.