{"id":24727,"date":"2022-12-05T07:53:20","date_gmt":"2022-12-05T02:23:20","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-how-can-i-create-a-map-of-my-nested-list-objects-in-java\/"},"modified":"2022-12-05T07:53:20","modified_gmt":"2022-12-05T02:23:20","slug":"solved-how-can-i-create-a-map-of-my-nested-list-objects-in-java","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-how-can-i-create-a-map-of-my-nested-list-objects-in-java\/","title":{"rendered":"[Solved] How can I create a Map of my nested List objects in java?"},"content":{"rendered":"

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First, it would be impossible to create a map<\/em> of parentId to each childId if there is a list<\/em> of children in each node because in general case a parentId<\/code> key may have multiple childId<\/code> values and this conflict has to be resolved somehow.<\/p>\n

So, either a Map<Integer, List<Integer>><\/code> where List<Integer><\/code> contains ids of all children, or a List<List<Integer>><\/code> (where the inner List<Integer><\/code> contains a pair of parentId<\/code> and childId<\/code>) may be created.<\/p>\n

And finally, it needs to be defined whether the result contains roots<\/em> or leafs<\/em>.
\nThe difference may be described as: roots<\/em> have their parentId = null<\/code> and non-null childId<\/code>, leafs<\/em> always have non-null parentId<\/code> while childId<\/code> may be null<\/code>.<\/p>\n

Let’s assume the input list of nodes is defined like this:<\/p>\n

List<XNode> list = Arrays.asList(\n    new XNode(1, Arrays.asList(new XNode(11, Arrays.asList(new XNode(111), new XNode(112))), new XNode(12), new XNode(13))),\n    new XNode(2),\n    new XNode(3, Arrays.asList(new XNode(31, Arrays.asList(new XNode(311)))))\n);\n<\/code><\/pre>\n
Then the map may be created as follows:<\/p>\n
Map<Integer, List<Integer>> map = list.stream()\n    .flatMap(Main::flatMapEntry)\n    .collect(Collectors.toMap(\n        Map.Entry::getKey, Map.Entry::getValue, (a, b) -> a, LinkedHashMap::new\n    ));  \nSystem.out.println(\"map example: \" + map);\n<\/code><\/pre>\n
with a recursive method flatMapEntry<\/code>:<\/p>\n
static Stream<Map.Entry<Integer, List<Integer>>> flatMapEntry(XNode node) {\n    return node.getChildren().isEmpty()\n        ? Stream.of(Map.entry(node.getId(), Collections.emptyList()))\n        : Stream.concat(\n            Stream.of(Map.entry(\n                node.getId(), \n                node.getChildren().stream()\n                    .map(XNode::getId)\n                    .collect(Collectors.toList())\n            )),\n            node.getChildren().stream().flatMap(Main::flatMapEntry)\n        );\n}\n<\/code><\/pre>\n
Output for the given input (10 keys, 13 values):<\/p>\n
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map example: {1=[11, 12, 13], 11=[111, 112], 111=[], 112=[], 12=[], 13=[], 2=[], 3=[31], 31=[311], 311=[]}<\/p>\n<\/blockquote>\n
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Examples of building the lists of pairs:<\/p>\n