Merge different type maps into one in Java 8

Question

There are two maps

• <Integer,String> map1 which is <ID,Question>
• <Integer,String> map2 which is <ID,Answer>

I want to merge them into a single map <String,String> resultMap which is <Question,Answer> such that the Key in this map is the value from map1(Question) and value in resultMap is value from map2(Answer) and this is based on the same ID.

I can do this easily in java 6 as seen in below code.

for(Map.Entry<Integer,String> entry:map1.entrySet()){
    qaMap.put(entry.getValue(),map2.get(entry.getKey()));
}

But I want to write this in Java 8 using streams and lambdas. How to do that?

Answer 1

Well assuming your keys(IDs) are the same in both the maps we can do something like

Map<String,String> map = map1.keySet().stream()
    .collect(Collectors.toMap(map1::get, map2::get));

• In the above statement, map1.keySet().stream() will get you a stream of IDs.
• Then collect(Collectors.toMap(map1::get, map2::get) will create a Map from the stream of IDs with key as map1.get(id) (i.e. your question) and value as map2.get(id) (i.e. your answer) for each id.

Answer 2

@above solution is elegant But someone should give alternative solution which should beginner to understand . So I could go with foreach and lambda. You can iterate through Each map1 and add to the qaMap.

Map<Integer,String> map1 = new HashMap<>();
Map<Integer,String> map2 = new HashMap<>();
Map<String,String> qaMap = new HashMap<>();
map1.put(1,"what is your age");
map2.put(1,"25");

map1.forEach((k,v)->qaMap.put(v,map2.get(k)));
System.out.println(qaMap.size());