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

Answer 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())

Answer 2

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}