How to sort an ArrayList?

Question

I have a List of doubles in java and I want to sort ArrayList in descending order.

Input ArrayList is as below:

List testList = new Arr         


        

Answer 1

In JAVA 8 its much easy now.

List<String> alphaNumbers = Arrays.asList("one", "two", "three", "four");
List<String> alphaNumbersUpperCase = alphaNumbers.stream()
    .map(String::toUpperCase)
    .sorted()
    .collect(Collectors.toList());
System.out.println(alphaNumbersUpperCase); // [FOUR, ONE, THREE, TWO]

-- For reverse use this

.sorted(Comparator.reverseOrder())

Answer 2

If you have to sort object based on its id in the ArrayList , then use java8 stream.

 List<Person> personList = new ArrayList<>();

    List<Person> personListSorted =
                personList.stream()
                  .sorted(Comparator.comparing(Person::getPersonId))
                  .collect(Collectors.toList());

Answer 3

Descending:

Collections.sort(mArrayList, new Comparator<CustomData>() {
    @Override
    public int compare(CustomData lhs, CustomData rhs) {
        // -1 - less than, 1 - greater than, 0 - equal, all inversed for descending
        return lhs.customInt > rhs.customInt ? -1 : (lhs.customInt < rhs.customInt) ? 1 : 0;
    }
});

Answer 4

You can do like this:

List<String> yourList = new ArrayList<String>();
Collections.sort(yourList, Collections.reverseOrder());

Collection has a default Comparator that can help you with that.

Also, if you want to use some Java 8 new features, you can do like that:

List<String> yourList = new ArrayList<String>();
yourList = yourList.stream().sorted(Collections.reverseOrder()).collect(Collectors.toList());

Answer 5

if you are using Java SE 8, then this might be of help.

//create a comparator object using a Lambda expression
Comparator<Double> compareDouble = (d1, d2) -> d1.compareTo(d2);

//Sort the Collection in this case 'testList' in reverse order
Collections.sort(testList, Collections.reverseOrder(compareDouble));

//print the sorted list using method reference only applicable in SE 8
testList.forEach(System.out::println);

Answer 6

For your example, this will do the magic in Java 8

List<Double> testList = new ArrayList();
testList.sort(Comparator.naturalOrder());

But if you want to sort by some of the fields of the object you are sorting, you can do it easily by:

testList.sort(Comparator.comparing(ClassName::getFieldName));

or

 testList.sort(Comparator.comparing(ClassName::getFieldName).reversed());

or

 testList.stream().sorted(Comparator.comparing(ClassName::getFieldName).reversed()).collect(Collectors.toList());

Sources: https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html