将两张不同类型的地图合并为一张地图

Question

I have two map from different type:

class Type3{
  Type1 type1;
  Type2 type2;

// getters and setters...

}


Map<Integer,Type1> map1;
Map<Integer,Type2> map2;

I want to merge them to one map by same key. like his:
Map<Integer,Type3> map3 = // merege map1,map2.

How can I do it?

Answer 1

You can do it with Stream s:

Map<Integer,Type3> output =
    map1.entrySet()
        .stream()
        .collect(Collectors.toMap(Map.Entry::getKey,
                                  e -> new Type3(e.getValue(),map2.get(e.getKey()))));

Assuming Type3 has a constructor that accepts a Type1 instance and a Type2 instance.

Note that the output Map will contain only keys that appear in the first Map ( map1 ).

If both of the input Map s may have keys not present in the other map, it would be better to stream over the union of the key sets of both Map s, in order not to skip any keys:

Set<Integer> allKeys = new HashSet<> (map1.keySet());
allKeys.addAll(map2.keySet());
Map<Integer,Type3> output =
    allKeys.stream()
           .collect(Collectors.toMap(Function.identity(),
                                     key -> new Type3(map1.get(key),map2.get(key))));

Answer 2

If Type1 and Type2 inherits the same Parent, you can re-initialise your Map with the following:

Map<Integer,IType> mapCommon;

IType would be like a dummy class: Define an interface

interface IType {

}
//Program Type1 & Type2 to be like this

class Type1 implements IType{..}

class Type2 implements IType{..}