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Creating a map from nested lists

Suppose there are 3 classes:

class Level1 {
    int idLevel1;
    List<Level2> level2list;    
}

class Level2 {
    int idLevel2;
    List<Level3> level3list;    
}   

class Level3 {  
    int idLevel3;
    String name;
}   

Suppose there is a List of Level1 objects called initial state

List<Level1> initialList = new ArrayList<>();

I want to create a map from initialList where:

 - Key: is idLevel1
 - Value: is list of all idLevel3 , corresponding to idLevel1

I am able to achieve this using for loops, but I want to achieve this in a more elegant way using Java 8 features (streams and the functions). I tried using Collectors.toMap() also tried grouping but I am unable to get the desired map.

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Answer

By corresponding to idLevel1 I made the assumption that you wanted a list of all idlevel3 that were in the chain for the a particular idLevel1

So there could be a list of level3 ids for some level1 id and a different list of level3 ids for a different level1 id.

Based on that, this is what I came up with.

        Map<Integer, List<Integer>> map = initialList
                .stream()
                .collect(Collectors
                        .toMap(lv1 -> lv1.idLevel1,
                                lv1 -> lv1.level2list
                                        .stream()
                        .flatMap(lv2 -> lv2.level3list
                                .stream())
                        .map(lv3 -> lv3.idLevel3)
                        .collect(Collectors.toList())));
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