An unexpected null key in HashSet, the first be added not the rest - hashset

Question

I'm learning Activiti 7, I drew a BPMN diagram as below:
When the highlight1 UserTask has been completed but the highlight2 UserTask is still pending, I ran the following code to highlight the completed flow element.
private AjaxResponse highlightHistoricProcess(#RequestParam("instanceId") String instanceId,
#AuthenticationPrincipal UserInfo userInfo) {
try {
// Get the instance from the history table
HistoricProcessInstance instance = historyService
.createHistoricProcessInstanceQuery().processInstanceId(instanceId).singleResult();
BpmnModel bpmnModel = repositoryService.getBpmnModel(instance.getProcessDefinitionId());
Process process = bpmnModel.getProcesses().get(0);
// Get all process elements, including sequences, events, activities, etc.
Collection<FlowElement> flowElements = process.getFlowElements();
Map<String, String> sequenceFlowMap = Maps.newHashMap();
flowElements.forEach(e -> {
if (e instanceof SequenceFlow) {
SequenceFlow sequenceFlow = (SequenceFlow) e;
String sourceRef = sequenceFlow.getSourceRef();
String targetRef = sequenceFlow.getTargetRef();
sequenceFlowMap.put(sourceRef + targetRef, sequenceFlow.getId());
}
});
// Get all historical Activities, i.e. those that have been executed and those that are currently being executed
List<HistoricActivityInstance> actList = historyService.createHistoricActivityInstanceQuery()
.processInstanceId(instanceId)
.list();
// Each history Activity is combined two by two
Set<String> actPairSet = new HashSet<>();
for (HistoricActivityInstance actA : actList) {
for (HistoricActivityInstance actB : actList) {
if (actA != actB) {
actPairSet.add(actA.getActivityId() + actB.getActivityId());
}
}
}
// Highlight Link ID
Set<String> highSequenceSet = Sets.newHashSet();
actPairSet.forEach(actPair -> {
logger.info("actPair:{}, seq:{}", actPair, sequenceFlowMap.get(actPair));
highSequenceSet.add(sequenceFlowMap.get(actPair));
logger.info("{}",highSequenceSet.toString());
});
// Get the completed Activity
List<HistoricActivityInstance> finishedActList = historyService
.createHistoricActivityInstanceQuery()
.processInstanceId(instanceId)
.finished()
.list();
// Highlight the completed Activity
Set<String> highActSet = Sets.newHashSet();
finishedActList.forEach(point -> highActSet.add(point.getActivityId()));
// Get the pending highlighted node, i.e. the currently executing node
List<HistoricActivityInstance> unfinishedActList = historyService
.createHistoricActivityInstanceQuery()
.processInstanceId(instanceId)
.unfinished()
.list();
Set<String> unfinishedPointSet = Sets.newHashSet();
unfinishedActList.forEach(point -> unfinishedPointSet.add(point.getActivityId()));
...
return AjaxResponse.ajax(ResponseCode.SUCCESS.getCode(),
ResponseCode.SUCCESS.getDesc(),
null);
} catch (Exception e) {
e.printStackTrace();
return AjaxResponse.ajax(ResponseCode.ERROR.getCode(),
"highlight failure",
e.toString());
}
}
Please see this piece of code:
// Highlight Link ID
Set<String> highSequenceSet = Sets.newHashSet();
actPairSet.forEach(actPair -> {
logger.info("actPair:{}, seq:{}", actPair, sequenceFlowMap.get(actPair));
highSequenceSet.add(sequenceFlowMap.get(actPair));
logger.info("{}",highSequenceSet.toString());
});
It was expected to get 2 elements in the highSequenceSet, but it got 3, with a unexpected null.
The log printed in the console was:
Why is the first null added to the HashSet but not the rest?

Answer 1

Why is the first null added to the HashSet but not the rest?
HashSet implements the Set interface, duplicate values are not allowed.