Java: List<List<String>> list = new ArrayList<>();

Question

How I can group all lists in list which contains same exact value.

List<List<String>> list = new ArrayList<>();

This list contains list1 list2 list3 and etc.... I want to find if list1.get(0) is the same as list2.get(0) if no check with list3.get(0) and etc... Later I need to group lists if I find the same values in each. For example if true add list1 and list3 and others which were found to

  List<List<String>> list2 = new ArrayList<>();

Now check with list2.get(0) if equal to others if yes group them and add them to anoter:

   List<List<String>> list3 = new ArrayList<>();

What I achieved so far:

    private static void findEqualLists(List<List<String>> list) {

    int i = 0;
    for (List<String> list1 : list) {
        if (!Collections.disjoint(list1, list.get(i))) {
            System.out.println(list1.get(0)+list.get(i).get(0));
        }else{
            System.out.println(list1.get(0)+list.get(i).get(0));
        }
        i++;
    }
}

Value exmaple:

list = {{2017-02,jhon,car},{2017-03,maria,car},{2017-03, boy, car}, {2017-01,arya, car}, {2017-02, girl, car}}

Output:

Group1:
2017-03,maria,car
2017-03, boy, car

Group2:
2017-02,jhon,car
2017-02, girl, car

Group3
2017-01,arya, car

1. what's the question? 2. what does it mean to "group" lists ? 3. instead of talking about how you're doing something, it's better to discuss what is it you want to achieve. — Nir Alfasi
– Nir Alfasi, Commented Oct 25, 2017 at 16:49
"similar" and "grouping" are not well defined: are you "grouping" by date only? and again - what does "grouping" mean ? — Nir Alfasi
– Nir Alfasi, Commented Oct 25, 2017 at 16:52
"I want to achieve outpoot by giving value" -- Could you translate that, please? :-) — Mörre
– Mörre, Commented Oct 25, 2017 at 16:59

VGR · Accepted Answer · 2017-10-25 17:07:52Z

3

You want Collectors.groupingBy:

private static Map<String, List<List<String>>>
            findEqualLists(List<List<String>> list) {

    return list.stream().collect(Collectors.groupingBy(l -> l.get(0)));
}

The returned Map will use l.get(0) as a unique key, with each corresponding value being a List of only those Lists whose first element matches that key.

answered Oct 25, 2017 at 17:07

VGR

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6 Comments

User1254 Over a year ago

An how I am able to Output returned Map like I showed above?

VGR Over a year ago

@D1's Call the Map’s values() method. You can either print the returned Collection directly, or, if you want curly braces ({ }) instead of brackets ([ ]), you can loop through the Collection and build a String yourself.

User1254 Over a year ago

yes I understand that I can use values() method, but how I can divide into groups and print it seperatly? I mean that one key now has multiply values, do I need to iterate through every key and then to iterate through that key list to get values for that certain key?

VGR Over a year ago

@D1's One key has multiple values, because there are multiple entries in the original List which start with, for example, "2017-03". You don’t need to iterate through keys; you can simply iterate through values(), which is already divided into groups. The whole point of a Map is that each member of values() corresponds to exactly one key.

TheDude Over a year ago

Yes it will be divided, but I don't get it how you gonna know when that group ended and another started to form?

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