用Java合并2个HashMap

Question

我有一个程序需要合并两个HashMap。哈希图的键为a
String，值为Integer。合并的特殊条件是，如果键已在字典中，则Integer需要将其添加到现有值中而不是替换它。这是我到目前为止抛出的代码NullPointerException。

public void addDictionary(HashMap<String, Integer> incomingDictionary) {
        for (String key : incomingDictionary.keySet()) {
            if (totalDictionary.containsKey(key)) {
                Integer newValue = incomingDictionary.get(key) + totalDictionary.get(key);
                totalDictionary.put(key, newValue);
            } else {
                totalDictionary.put(key, incomingDictionary.get(key));
            }
        }
    }

Answer 1

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。