How to merge multiple Maps having the same keys but different values into a single Map

I have n maps of the kind:

HashMap<String,Double> map1;
HashMap<String,Double> map2; ...

What could I do to merge all these maps into a single map?

And I want to achieve this without losing the data.

Example:

map1 has entries: {<X1, Y1>, <X2,Y2>, <X3,Y3>} 

map2 has entries: {<X1, Y6>, <X2, Y7>, <X3,Y8>}

mergedMap: {<X1, Y1+Y6>, <X2,Y2+Y7>, <X3, Y3+Y8>}

I have tried this:

ArrayList<HashMap<String, Double>> listHashMap = new ArrayList<>();
            
HashMap<String,Double> mergedMap = new HashMap<>();
for(HashMap<String,Double> map: listHashMap) {  
    mergedMap.putAll(map);          
}

But I've understood that the values mapped to the same key would be replaced (put), not added.

What could I do to add up each value mapped to the same key?

You can utilize Java 8 method merge() to combine the data of each entry:

List<Map<String, Double>> list = // initializing source list
Map<String, Double> mergedMap = new HashMap<>();

list.forEach(map -> map.forEach((k, v) -> mergedMap.merge(k, v, Double::sum)));

See that code run live at Ideone.com.

Or you can make use of Stream API with combination of built-in collectors groupingBy() and summingDouble():

Map<String, Double> mergedMap = list.stream()
            .flatMap(map -> map.entrySet().stream())
            .collect(Collectors.groupingBy(
                Map.Entry::getKey, Collectors.summingDouble(Map.Entry::getValue)
            ));

Take a look at these tutorials provided by Oracle for more information on lambda expressions and streams.