HashSet 中意外的空键，第一个被添加而不是其余的

Question

我正在学习Activiti 7，我画了一个BPMN图如下：当 highlight1 UserTask 已完成但 highlight2 UserTask 仍未决时，我运行以下代码突出显示已完成的流程元素。

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());
    }
  }

请看这段代码：

      // 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());
      });

本应在 highSequenceSet 中得到 2 个元素，但得到了 3 个，出乎意料 null。控制台打印的日志是：

为什么第一个 null 添加到 HashSet 而其余的没有？

Answer 1

Why is the first null added to the HashSet but not the rest?

HashSet实现了Set接口，不允许重复值。

HashSet 中意外的空键，第一个被添加而不是其余的

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

hashset

activiti