Sorting an ArrayList<ArrayList<Integer>> by the third element in the ArrayList<Integer>

Question

I'm having difficulty finding a way to sort an ArrayList<ArrayList<Integer>> by the third element in the ArrayList<Integer>. For example, given

ArrayList<ArrayList<Integer>> = [[5,4,0],[3,2,1],[3,3,3],[2,2,2]]

I want to sort it so that the ArrayList<Integer>s are in ascending order by the third element giving the output:

ArrayList<ArrayList<Integer>> = [[5,4,0],[3,2,1],[2,2,2],[3,3,3]]

I have utilized Collections in sorting an ArrayList before but this one is a bit trickier.

The java code below may also help with understanding the problem I am having.

public static void main(String[] args){
   ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();

   Random rand = new Random();
   for(int i=0; i<3; i++){
       ArrayList<Integer> components = new ArrayList<Integer>();
       components.addAll(Arrays.asList(rand.nextInt(10),rand.nextInt(10),rand.nextInt(10)));
       list.add(components);
   }
   System.out.println(list);
   sort(list);

}

public static void sort(ArrayList<ArrayList<Integer>> list){
    for(int i=0; i<list.size(); i++){
        //Here is where I want to sort the list by the element retrieved from the println below
        System.out.println(list.get(i).get(2));
    }
}

Any help is appreciated. Thanks for your time.

are the third elements unique? or they may get repeated?

Sal-laS
– Sal-laS

2017-07-30 05:55:50 +00:00
Commented Jul 30, 2017 at 5:55 — Sal-laS
– Sal-laS, Commented Jul 30, 2017 at 5:55
they are not unique, and there may be multiple ones

Jamin
– Jamin

2017-07-30 05:56:15 +00:00
Commented Jul 30, 2017 at 5:56 — Jamin
– Jamin, Commented Jul 30, 2017 at 5:56
list.sort(Comparator.comparing(l -> l.get(2)));

shmosel
– shmosel

2017-07-30 06:04:59 +00:00
Commented Jul 30, 2017 at 6:04 — shmosel
– shmosel, Commented Jul 30, 2017 at 6:04
thank you shmosel works as intended

Jamin
– Jamin

2017-07-30 06:21:55 +00:00
Commented Jul 30, 2017 at 6:21 — Jamin
– Jamin, Commented Jul 30, 2017 at 6:21

Elliott Frisch · Accepted Answer · 2017-07-30 06:07:33Z

3

You might use Arrays.asList to construct your List<List<Integer>> (please program to the interface) and assuming you are using Java 8+, you can write a Comparator with a lambda. Like,

public static void main(String[] args) {
    List<List<Integer>> al = Arrays.asList( //
            Arrays.asList(5, 4, 0), Arrays.asList(3, 2, 1), //
            Arrays.asList(3, 3, 3), Arrays.asList(2, 2, 2));
    Collections.sort(al, (a, b) -> a.get(2).compareTo(b.get(2)));
    // Collections.sort(al, (a, b) -> Integer.compare(a.get(2), b.get(2)));
    System.out.println(al);
}

Which outputs

[[5, 4, 0], [3, 2, 1], [2, 2, 2], [3, 3, 3]]

edited Jul 30, 2017 at 6:07

answered Jul 30, 2017 at 5:58

Elliott Frisch

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2 Comments

Sal-laS Over a year ago

but can you explain how it works based on third element?

Elliott Frisch Over a year ago

@Salman In the lambda expression (a, b) -> Integer.compare(a.get(2), b.get(2)) we are comparing the third element of a with the third element of b (the first is 0, the second is 1).

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