如果您的代码不能保证
incomingDictionary会在到达此方法之前将其初始化，则您将必须执行null检查，没有出路
public void addDictionary(HashMap<String, Integer> incomingDictionary) { if (incomingDictionary == null) { return; // or throw runtime exception } if (totalDictionary == null) { return;// or throw runtime exception } if (totalDictionary.isEmpty()) { totalDictionary.putAll(incomingDictionary); } else { for (Entry<String, Integer> incomingIter : incomingDictionary.entrySet()) { String incomingKey = incomingIter.getKey(); Integer incomingValue = incomingIter.getValue(); Integer totalValue = totalDictionary.get(incomingKey); // If total dictionary contains null for the incoming key it is // as good as replacing it with incoming value. Integer sum = (totalValue == null ? incomingValue : incomingValue == null ? totalValue : totalValue + incomingValue ); totalDictionary.put(incomingKey, sum); } }}考虑到HashMap允许将null作为值在代码中容易发生NPE的另一个位置是
Integer newValue = incomingDictionary.get(key) + totalDictionary.get(key);
如果这两个都不为空，则将获得NPE。
