Sorting List of List by inner list's index in Java? (Java Equivalent of Python's 'sorted()')

Question

In Java, I have a List of List that looks something like the following:

[ ["Kelly", "3.0"], ["Jeff", "2.0"], ["Mark", "1.0]" ]

How do I sort the list by a certain index? In this case, index of 1

Ex)

Given:   [ ["Kelly", "3.0"], ["Jeff", "2.0"], ["Mark", "1.0"] ]
Desired: [ ["Mark", "1.0"], ["Jeff", "2.0"], ["Kelly", "3.0"] ]

What I'm looking for is something very similar to the 'sorted()' method in Python (the exact solution I'm looking for in Python shown here: https://www.kite.com/python/answers/how-to-sort-a-list-of-lists-by-an-index-of-each-inner-list-in-python and here: sort list of lists by specific index of inner list) but I'm looking for a solution in Java.

Also as a bonus, how would I sort these in descending order? What about instances of same values (i.e. there's multiple '3.0' values)

You can use a custom comparator that compares the second element of each list to determine sort order. It's simply a matter of flipping the variables around to achieve descending order. — bliss
– bliss, Commented Mar 21, 2021 at 7:45

Nowhere Man · Accepted Answer · 2021-03-21 08:35:50Z

Since Java 8 there is method List::sort accepting a custom Comparator which can be built using Comparator.comparing.

So, the input list of lists may be concisely sorted as follows:

List<List<String>> data = Arrays.asList(
    Arrays.asList("Kelly", "3.0"),
    Arrays.asList("Mark", "1.0"),
    Arrays.asList("Jeff", "2.0")
);

// sorting by index 1
data.sort(Comparator.comparing(x -> x.get(1))); 
// [[Mark, 1.0], [Jeff, 2.0], [Kelly, 3.0]]

// sorting by index 1 in reverse order using Collections.reverseOrder
data.sort(Comparator.comparing(x -> x.get(1), Collections.reverseOrder()));
// [[Kelly, 3.0], [Jeff, 2.0], [Mark, 1.0]]

// sorting by index 0 in reverse order using Comparator.reversed()
// here type of object being compared needs to be specified
data.sort(Comparator.comparing((List<String> x) -> x.get(0)).reversed());
// [[Mark, 1.0], [Kelly, 3.0], [Jeff, 2.0]]

score 2 · Accepted Answer · 2021-03-21 08:22:40Z

2

Try this.

List<List<String>> list = Arrays.asList(
    Arrays.asList("Jhon", "3.0"), Arrays.asList("Kelly", "3.0"),
    Arrays.asList("Jeff", "2.0"), Arrays.asList("Mark", "1.0"));
//  sort the list by index of 1
list.sort(Comparator.comparing(x -> x.get(1)));
System.out.println(list);
//  sort the list by index of 1 descending order
list.sort(Collections.reverseOrder(Comparator.comparing(x -> x.get(1))));
System.out.println(list);

output:

[[Mark, 1.0], [Jeff, 2.0], [Jhon, 3.0], [Kelly, 3.0]]
[[Jhon, 3.0], [Kelly, 3.0], [Jeff, 2.0], [Mark, 1.0]]

It is stable sort in both cases.

edited Mar 21, 2021 at 8:22

answered Mar 21, 2021 at 8:08

user4910279

Comments

mmarion · Accepted Answer · 2021-03-21 07:46:47Z

0

Solved my own question literally 2 minutes after posting. Always have to love when that happens lol:

    for (int i=0; i < listToReverse.size(); i++) {
        for (int j=0; j < (listToReverse.size()-i-1); j++) {
            if ((int)Double.parseDouble(listToReverse.get(j).get(1)) < (int)Double.parseDouble(listToReverse.get(j+1).get(1))) {
                ArrayList<String> temp = listToReverse.get(j);
                listToReverse.set(j, asList.get(j+1));
                listToReverse.set(j+1, temp);
            }
        }
    }
//This implementation sorts in descending order. Switch the '<' for a '>' to sort in ascending order

Thank you to @bliss for also pointing out a custom comparator could be used here. Probably much prettier than the iterative way

answered Mar 21, 2021 at 7:46

mmarion

1,10513 silver badges32 bronze badges

1 Comment

bliss Over a year ago

Applying a custom comparator is as simple as listToReverse.sort((list1, list2) -> list1.get(1).compareTo(list2.get(1))); and it utilizes a far more efficient library sorting algorithm.

Collectives™ on Stack Overflow

Sorting List of List by inner list's index in Java? (Java Equivalent of Python's 'sorted()')

3 Answers 3

Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related