合并两张地图的最佳做法是什么？ [英] What is the best practices to merge two maps

问题描述

如何将新地图添加到现有地图。地图具有相同的类型 Map< String，Integer> 。如果新地图中的密钥存在于旧地图中，则应添加值。

How can I add a new map to existing map. The maps have the same type Map<String, Integer>. If the key from new map exists in the old map the values should be added.

Map<String, Integer> oldMap = new TreeMap<>();
Map<String, Integer> newMap = new TreeMap<>();

//Data added

//Now what is the best way to iterate these maps to add the values from both?

推荐答案

通过添加，我假设您要添加整数值，而不是创建 Map< String，List< Integer>> 。

By add, I assume you want to add the integer values, not create a Map<String, List<Integer>>.

在java 7之前，你'我必须迭代@laune显示（+1给他）。否则使用java 8，Map上有一个合并方法。所以你可以这样做：

Before java 7, you'll have to iterate as @laune showed (+1 to him). Otherwise with java 8, there is a merge method on Map. So you could do it like this:

Map<String, Integer> oldMap = new TreeMap<>();
Map<String, Integer> newMap = new TreeMap<>();

oldMap.put("1", 10);
oldMap.put("2", 5);
newMap.put("1", 7);

oldMap.forEach((k, v) -> newMap.merge(k, v, (a, b) -> a + b));

System.out.println(newMap); //{1=17, 2=5}

它的作用是每个键值对，它合并密钥（如果它还没有在 newMap 中，它只是创建一个新的键值对，否则它通过添加来更新现有键的先前值保持两个整数）

What it does is that for each key-value pair, it merges the key (if it's not yet in newMap, it simply creates a new key-value pair, otherwise it updates the previous value hold by the existing key by adding the two Integers)

也许您应该考虑使用 Map< String，Long> 来避免在添加时出现溢出两个整数。

Also maybe you should consider using a Map<String, Long> to avoid overflow when adding two integers.

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