convert list of object into a map of <String,Map<String,Integer>> java8 streams

Question

hello I have a list of objects my object have three fields

class MyObject{
   String x;
   String y;
   int  z;
   //getters n setters
}

I need to convert this list into a Map<String,Map<String,Integer>>

that is like this: {x1:{y1:z1,y2:z2,y3:z3},x2{y4:z4,y5:z5}} format I want to do this in Java 8 which I think I am relatively new to it.

I have tried the following :

Map<String,Map<String,Integer>> map=list.stream().
                collect(Collectors.
                        groupingBy(MyObject::getX,list.stream().
                                collect(Collectors.groupingBy(MyObject::getY,
                                        Collectors.summingInt(MyObject::getZ)))));

this does not even compile. help is much appreciated

Please provide a sample input, is not clear how you get all those y's and z's out of a single x — Óscar López
– Óscar López, Commented Jul 31, 2016 at 17:48
You shouldn't re-use your stream like that, nor should you need too. I don't think you problem is a good match for Streams, as I think you are trying to take the first two elements and map them to MyObject.x and the third and forth elements and map them using the second elements x value. It might be possible with a custom collector, but you are manipulating a lot of state — Gavin
– Gavin, Commented Jul 31, 2016 at 17:48

Eran · Accepted Answer · 2016-07-31 17:53:17Z

7

You can do it by chaining two groupingBy Collectors and one summingInt Collector:

Map<String,Map<String,Integer>> map = 
    list.stream()
        .collect(Collectors.
                    groupingBy(MyObject::getX,
                               Collectors.groupingBy(MyObject::getY,
                                                     Collectors.summingInt(MyObject::getZ))));

I hope I got the logic you wanted right.

When adding to the input List the following :

list.add(new MyObject("A","B",10));
list.add(new MyObject("A","C",5));
list.add(new MyObject("A","B",15));
list.add(new MyObject("B","C",10));
list.add(new MyObject("A","C",12));

You get an output Map of :

{A={B=25, C=17}, B={C=10}}

answered Jul 31, 2016 at 17:53

Eran

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