Sort List of string arrays by first element

Question

I want to sort a list of string arrays by first element in each array element of same list, in reverse order, so 2, 1, 0

Here's what i tried so far:

List<String[]> array = new ArrayList<>();

String[] arr1 = {"0", "1/1"};
String[] arr2 = {"1", "1/2"};
String[] arr3 = {"2", "1/4"};

array.add(arr1);
array.add(arr2);
array.add(arr3);

Comparator<String[]> byFirstElement = 
    (String[] array1, String[] array2) -> Integer.parseInt(array1[0]) - 
                                           Integer.parseInt(array2[0]);


List<String[]> result = array.stream()
        .sorted(array,byFirstElement) // error here
        .collect(Collectors.toList());

The problem is that at sorted line i have an error highlighted, saying: "sorted(java.util.List, java.util.Comparator

It should be .sorted(byFirstElement). Remove the array parameter. — GriffeyDog
– GriffeyDog, Commented Jan 27, 2020 at 14:34

ernest_k · Accepted Answer · 2020-01-27 14:48:36Z

1

Stream.sorted() takes a comparator (in addition to the overload that takes no argument). So all you need is ...sorted(byFirstElement)... (the stream sorts its elements)

Note that your comparison logic won't sort in descending order, so you either need to change it to

Comparator<String[]> byFirstElement = 
    (array1, array2) -> Integer.parseInt(array2[0]) - Integer.parseInt(array1[0]);
                        //reversed

or reverse it when calling sorted():

....sorted(byFirstElement.reversed())

edited Jan 27, 2020 at 14:48

answered Jan 27, 2020 at 14:31

ernest_k

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score 0 · Accepted Answer · 2021-01-13 20:47:44Z

0

You can simplify your code with the Comparator.comparing method as follows:

List<String[]> list = List.of(
        new String[]{"0", "1/1"},
        new String[]{"1", "1/2"},
        new String[]{"2", "1/4"});

List<String[]> sorted = list.stream()
        .sorted(Comparator.comparing(
                arr -> Integer.parseInt(arr[0]), Comparator.reverseOrder()))
        .collect(Collectors.toList());

// output
sorted.stream().map(Arrays::toString).forEach(System.out::println);

Output:

[2, 1/4]
[1, 1/2]
[0, 1/1]

^{See also:

• How to sort by a field of class with its own comparator?

• Sorting 2D array of strings in alphabetical order}

edited Jan 13, 2021 at 20:47

answered Jan 13, 2021 at 20:34

user14940971

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