How to modify key in hashmap (Java)?

Question

I want to change the key in a hashmap. I am making a hashmap from another hashmap.

I am basically getting an id and returning a name.

So basically what I am getting is:

'BOS': 300

But I want to get:

'Boston':300

private Map<MetricName, Map<String, Integer>> getMetric(String regionId, Map<String, String> locationMap){
        Map<MetricName, Map<String, Integer>> metricTargetsMap = analyzeMetaService
                .getMetricTargetsForRegion(regionId);
        Map<MetricName, Map<String, Integer>> metricTargetsMapModified = new HashMap<MetricName, Map<String, Integer>>();
        metricTargetsMap.forEach((metricName,targetMap)-> {
                    HashMap<String, Integer> modifiedMap = new HashMap<String, Integer>();
                    targetMap.forEach((location, targetValue) -> modifiedMap.put(locationMap.get(location), targetValue));
            metricTargetsMapModified.put(metricName, modifiedMap);
                }
        );
 return metricTargetsMapModified;
}

Please add sample input data and corresponding expected output — Thorbjørn Ravn Andersen, Nov 16 '20 at 19:57
_Why_ would you need to modify the key in the hash map? Generally it is considered [**bad** practice](https://stackoverflow.com/questions/7842049/are-mutable-hashmap-keys-a-dangerous-practice) to use mutable keys because it may have undesirable side effect. If you mean `renaming` the key, you should remove the old key and put the the value with the new key. — Nowhere Man, Nov 16 '20 at 20:04
@AlexRudenko I am basically making a hashmap from another hashmap. — , Nov 16 '20 at 20:13
@Mike, I meant removing old key and putting the same value with the updated key to keep the map consistent. — Nowhere Man, Nov 16 '20 at 20:13
@AlexRudenko please see my changes and maybe it makes more sense — , Nov 16 '20 at 20:14
It still doesn't make sense. Is the code you provided doing what you want? Your creating a new map, that is populated with maps that are copies of the original map, Do you want the copies to have new keys? — matt, Nov 16 '20 at 20:59

score 0 · Accepted Answer · answered Nov 16 '20 at 20:51

This can be implemented by re-mapping keys in the existing map and re-collecting new map:

private Map<MetricName, Map<String, Integer>> getMetric(String regionId, Map<String, String> locationMap) {
    Map<MetricName, Map<String, Integer>> metricTargetsMap = analyzeMetaService.getMetricTargetsForRegion(regionId);
    
    return metricTargetsMap
            .entrySet()
            .stream()   // stream of Map.Entry<MetricName, Map<String, Integer>>
            .map(top -> Map.entry(
                    top.getKey(),  // MetricName
                    top.getValue().entrySet()
                                  .stream()  // stream for inner map Map.Entry<String, Integer>
                                  .collect(Collectors.toMap(
                                      e -> locationMap.get(e.getKey()), // remapped key
                                      e -> e.getValue(),  // existing value
                                      (v1, v2) -> v1)  // merge function to resolve possible conflicts
                                  )
            ))
            .collect(Collectors.toMap(
                    Map.Entry::getKey,  // MetricName
                    Map.Entry::getValue // updated map <String, Integer>
            ));
}

score 0 · Answer 2 · answered Nov 16 '20 at 23:27

You don't change the key. You (a) remove the item using the old key, and (b) insert the item under the new key.

Alternatively, if as in your case you are making a new map, it's basically

  entry = oldMap.get(oldKey);
  newKey = ….whatever...;
  newMap.put(newKey, entry);

Under the covers, some function on the key is used as the mechanism to locate the entry in the map. So if you were able to change the key, the "some function" would no longer take you to the place where the entry should have been found.

How to modify key in hashmap (Java)?

2 Answers2