java：从数组列表中加入所有哈希图

Question

I have an aray list of hashmaps and I'd like to merge these hashmaps into a single one (keys should be unique). How would I have to do it? Can anybody give me a hint?

Answer 1

one way to do it would be to create a new instance of HashMap ubermap , iterate over the ArrayList<HashMap> and call putAll() method of ubermap, one map at a time. A smart optimization would be to give a large initial capacity to ubermap in question so you would avoid many rehash calls.

Answer 2

You can iterate trought Map.Entry entries from your hash maps. You can get entries using entrySet() method. Code should look like this:

public static <X,Y> Map<X,Y> test(Collection<Map<X,Y>> maps){
    HashMap<X,Y> result = new HashMap<X,Y>();
    for (Map<X,Y> singleMap:maps){
        for(Map.Entry<X,Y> entry:singleMap.entrySet()){
            result.put(entry.getKey(),entry.getValue());
        }
    }
    return result;
}

UPD : Many users wisely advised to use putAll method, but I forgot about it. So it will be better to use this code:

   public static <X,Y> Map<X,Y> test(Collection<Map<X,Y>> maps){
        HashMap<X,Y> result = new HashMap<X,Y>();
        for (Map<X,Y> singleMap:maps){
            result.putAll(singleMap);
        }
        return result;
    }

Answer 3

您应该看看Map.putAll（）函数

Answer 4

Map has method putAll(Map m) . Iterate over your list and do putAll on a result map for every entry.

Answer 5

您可以使用for-each循环，并使用HashMap.putAll（）从列表中的一个哈希图中一一添加地图。

Answer 6

遍历list并使用putAll()将所有地图添加到生成的地图中

Answer 7

Here is the way:

1. Create a new (big) hashmap that will contain the merged key-value pairs

2. Iterate through your list

3. For each item of your list, iterate through the hashmap in question

4. For each value of the hashmap, add the pair in the (big) hashmap