CODEWITHSUNDEEP
java
DerrAIner
DerrAIner

Reputation: 59

Order a List in Map java

So i have a Armstrong List = " [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407] "

And i am trying to put it in a Map like this " {1=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9], 3=[153, 370, 371, 407]} "

My Code:

    public static Map<Integer, List<Integer>> groupByLength(List<Integer> list){
        Map<Integer, List<Integer>> map = new TreeMap<Integer, List<Integer>>();
        List<Integer> neu = new ArrayList<Integer>();
        String s = "";
        for(int i = 0; i < list.size(); i++) {
            s = list.get(i) + "";
            if(map.containsKey(s.length())) {
                neu.add(list.get(i));
            }
            else {
                map.put(s.length(), neu);
                neu.clear();
            }
            s = "";
        }
        return map;
    }

My problem is, if i change the list (neu), the list in the map will change too

Output

{1=[370, 371, 407], 3=[370, 371, 407]}

Upvotes: 0

Views: 171

Answers (1)

Stefan Zhelyazkov
Stefan Zhelyazkov

Reputation: 2981

Your map has Key-Value pairs and you need to be adding elements to the list that is the Value. If your map doesn't have the Key-Value pair, then you need to instantiate a new list, add a value to it and put it in the map. The method getOrDefault() is very helpful in this case because it lets you get the Value list or create a new one into which you can add values:

public static Map<Integer, List<Integer>> groupByLength(List<Integer> list){
    Map<Integer, List<Integer>> map = new TreeMap<Integer, List<Integer>>();
    for(Integer el : list) {
        int key = el.toString().length();
        List<Integer> value = map.getOrDefault(key, new ArrayList<Integer>());
        value.add(el);
        map.put(key, value);
    }
    return map;
}

Edit: A "fancier" at first sight but quite clever optimization as suggested by Pshemo. You can find the same example in the documentation for computeIfAbsent()

public static Map<Integer, List<Integer>> groupByLength(List<Integer> list){
    Map<Integer, List<Integer>> map = new TreeMap<Integer, List<Integer>>();
    for(Integer el : list) {
        int key = el.toString().length();
        map.computeIfAbsent(key, k -> new ArrayList<>()).add(el);
    }
    return map;
}

Upvotes: 3

Related Questions