Tukukkk

I've an object,

class Object2{

     String name;

     String id;

     Map<String, String> customData;

}



class Object1{

     List<Object2> obj1List;

}

I want to convert this customData Map in the List of object1 into one single map, I am ok with over-writing the values if the key already exists.

Here's a way with lambdas and Java 8:

Map<String, String> map = new LinkedHashMap<>();

object1List.forEach(o1 -> 

        o1.getObject2List().forEach(o2 -> map.putAll(o2.getCustomData())));

Use flatMap and toMap as follows:

List<Object1> source = ...

Map<String, String> result = 

     source.stream()

           .flatMap(e -> e.getObj1List().stream()

                               .flatMap(a -> a.getCustomData().entrySet().stream()))

           .collect(toMap(Map.Entry::getKey, Map.Entry::getValue, (l, r) -> r));

or if you're dealing with a single Object1 object:

Object1 myObj = ...

Map<String, String> result = 

      myObj.getObj1List()

           .stream()

           .flatMap(a -> a.getCustomData().entrySet().stream())

           .collect(toMap(Map.Entry::getKey, Map.Entry::getValue, (l, r) -> r));

Alternatively, you can perform Stream.flatMap and then use Map.putAll as

List<Object1> object1s = new ArrayList<>(); // initialise as you would

Map<String, String> finalCustomData = new LinkedHashMap<>();

object1s.stream() // Stream<Object1>

        .flatMap(o1 -> o1.getObj1List().stream()) // Stream<Object2>

        .map(Object2::getCustomData) // Stream<Map<String, String>>

        .forEach(finalCustomData::putAll);