Java：合并两个哈希图？

Question

I have tried to implement my own merge function but it is getting more and more complicated and have more and more error. I was thinking maybe Java already have this function since it seems like a common development problem.

Imagine I have two HashMaps<String, int> :

 HM1.put(foo,1)
 HM1.put(bar,1)

 HM2.put(foo,1)
 HM2.put(main,1)

When they are merged, the new HM2 will be something like this:

 [foo: 2]
 [bar, 1]
 [main,1]

Answer 1

You can iterate through the second map and update HM1 dynamically:

HM2.entrySet()
   .forEach(entry -> HM1.compute(
            entry.getKey(),
            (key, value) -> value == null ? 
                            entry.getValue() : 
                            entry.getValue() + value));

EDIT :
Better use .merge (suggested by lexicore in this comment ), which avoids unnecessary runs of the compute function for absent keys:

HM2.entrySet()
   .forEach(entry -> HM1.merge(
            entry.getKey(),
            entry.getValue(),
            (key, value) -> entry.getValue()   + value));

Either of the above will update HM1 to have the final result.

System.out.println(HM1);

Outpus:

{bar=1, foo=2, main=1}

Answer 2

Here's a stream/collector-based solution. It turns maps into streams of entries, concatenates them and then collects into a map, using entry keys as keys and summing values as int s.

    Map<String, Integer> HM1 = new HashMap<>();
    Map<String, Integer> HM2 = new HashMap<>();
    HM1.put("foo", 1);
    HM1.put("bar", 1);

    HM2.put("foo", 1);
    HM2.put("main", 1);

    Map<String, Integer> result = Stream
        .concat(HM1.entrySet().stream(), HM2.entrySet().stream())
        .collect(
                Collectors.groupingBy(
                        Map.Entry::getKey,
                        Collectors.summingInt(Map.Entry::getValue)));

Answer 3

hope, this will help you

Set<String> keySet=hashMap1.keySet();

for(String str: keySet){
    int value=hashMap1.get(str);
    if(hashMap2.containsKey(str)){
        hashMap2.put(str, hashMap2.get(str)+value);
    }
    else{
        hashMap2.put(str, value);
    }
}

Answer 4

Map.merge() , as used above, is wrong. Remapping function there should be

(oldvalue, newvalue) -> oldvalue + newvalue

Interestingly enough, the original author mistook 2 * 1 , as implemented by him, for 1 + 1 . Hey, the result is what I want, right? Lesson learnt, 1 is not a good test value.

Answer 5

Another way to accomplish this is by using the toMap Collector , which is defined to take a merge function:

var hm1 = new HashMap<String, Integer>();
hm1.put("foo",1);
hm1.put("bar",2);
var hm2 = new HashMap<String, Integer>();
hm2.put("foo",4);
hm2.put("baz",8);
var v = Stream.of(hm1, hm2)
    .map(Map::entrySet)
    .flatMap(Set::stream)
    .collect(
        Collectors.toMap(
            Map.Entry::getKey,   // key mapper
            Map.Entry::getValue, // value mapper
            Integer::sum         // merge function
        )
    );
System.out.println(v);

Output:

{bar=2, foo=5, baz=8}