How to Sort ArrayList in Java

When you create an ArrayList in Java, elements are stored in the order they are inserted. By default, iterating over the list will display items in this insertion order. However, in real-world applications you often need to sort an ArrayList in Java, whether alphabetically, numerically, or by custom rules such as dates or objects.

In this tutorial, we’ll explore how to sort ArrayList in Java using different approaches available in Java. We’ll cover sorting Strings, Integers, and Dates in both ascending and descending order.

Refer How to sort arraylist of custom objects in Java to see how to sort an ArrayList of custom objects.

Table of contents

Options for sorting a List
Sorting ArrayList of strings using sort() method of List
Using Collections.sort() method to sort ArrayList
Sorting an ArrayList of strings in descending order
Sorting ArrayList in descending order using reverseOrder method
Sorting ArrayList in descending order using custom Comparator
Sorting Java ArrayList using sorted method of the Java Stream

Options for sorting a List

Java provides multiple ways to sort a list:

Using List.sort() (Java 8 and later)

The sort() method was introduced as a default interface method in the List interface. It allows you to sort directly without using external utilities.
```
list.sort(Comparator.naturalOrder());   // Ascending order
list.sort(Comparator.reverseOrder());   // Descending order
```
Method signature
default void sort(Comparator<? super E> c)- If the comparator is null, elements must implement Comparable, and natural ordering will be applied.
Using Collections.sort() method

The traditional way to sort an ArrayList is via the Collections utility class. There are two overloaded versions of sort method and according to Java docs their description is-
- public static <T extends Comparable<? super T>> void sort(List<T> list)- Sorts the specified list into ascending order, according to the natural ordering of its elements. All elements in the list must implement the Comparable interface. Furthermore, all elements in the list must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the list).
- public static <T> void sort(List<T> list, Comparator<? super T> c)- Sorts the specified list according to the order induced by the specified comparator. All elements in the list must be mutually comparable using the specified comparator (that is, c.compare(e1, e2)must not throw a ClassCastException for any elements e1 and e2 in the list).
Usage-
```
Collections.sort(list); // Natural ascending order
Collections.sort(list, Comparator.reverseOrder()); // Custom order
```
Using Java Stream API
Using sorted method of the Java Stream API which provides a functional approach to sorting, especially useful when you want a sorted view without modifying the original list. There are two overloaded variants of the sorted method.
- sorted()- Returns a stream consisting of the elements of this stream, sorted according to natural order.
- sorted(Comparator<? super T> comparator)- Returns a stream consisting of the elements of this stream, sorted according to the provided Comparator.

Sorting ArrayList of strings using sort() method of List

public class SortListDemo {
  public static void main(String[] args) {
    List<String> cityList = new ArrayList<>();
    cityList.add("Delhi");
    cityList.add("Mumbai");
    cityList.add("Bangalore");
    cityList.add("Chennai");
    cityList.add("Kolkata");
    cityList.add("Mumbai");
    // Passing null so natural ordering is used
    cityList.sort(null);
    System.out.println("List sorted using natural ordering" + cityList);
    
    // Using naturalOrder method to sort in natural order
    cityList.sort(Comparator.naturalOrder());
    System.out.println("List sorted using natural ordering" + cityList);
    
    // Using reverseOrder method to impose reverse of natural ordering
    cityList.sort(Comparator.reverseOrder());
    System.out.println("List sorted in reverse" + cityList);
  }
}

Output

List sorted using natural ordering[Bangalore, Chennai, Delhi, Kolkata, Mumbai, Mumbai]
List sorted using natural ordering[Bangalore, Chennai, Delhi, Kolkata, Mumbai, Mumbai]
List sorted in reverse[Mumbai, Mumbai, Kolkata, Delhi, Chennai, Bangalore]

Using Collections.sort() method to sort ArrayList

To sort an ArrayList of strings according to the natural ordering of its elements we can use the first of the two sort methods.

Collections.sort(List<T> list)

public class SortListDemo {
  public static void main(String[] args) {
    List<String> cityList = Arrays.asList("Delhi","Mumbai","Bangalore","Chennai","Kolkata","Mumbai");
    // sorting the list
    Collections.sort(cityList);
    System.out.println("List sorted using natural ordering" + cityList);
  }
}

Output

List sorted using natural ordering[Bangalore, Chennai, Delhi, Kolkata, Mumbai, Mumbai]

As you can see you just need to pass the ArrayList to sort method. Collections.sort will work in this case because String class implements Comparable interface and provides implementation for the method compareTo(String anotherString).

Same way Integer class or Date class also implements Comparable interface so list of integers (or dates) can also be sorted in natural order by using sort() method of the Collections class or by using sort() method of the List interface. In fact Java docs give a list of all the classes that implements comparable interface thus can be used with the sort method to sort the elements in the natural order.

Classes Implementing Comparable

Following is the list of classes that already implement Comparable interface in Java. Thus the ArrayList storing obejcts of any of these classes can be sorted in its natural ordering by passing the list to sort() method.

Class	Natural Ordering
Byte	Signed numerical
Character	Unsigned numerical
Long	Signed numerical
Integer	Signed numerical
Short	Signed numerical
Double	Signed numerical
Float	Signed numerical
BigInteger	Signed numerical
BigDecimal	Signed numerical
Boolean	Boolean.FALSE < Boolean.TRUE
File	System-dependent lexicographic on path name
String	Lexicographic
Date	Chronological
CollationKey	Locale-specific lexicographic

Sorting an ArrayList of strings in descending order

Collections.sort() method always sorts ArrayList of strings in ascending order. For sorting an ArrayList in descending order you need to use the second sort method which takes two parameters. First is the list that has to be sorted and second a comparator class that can be used to allow precise control over the sort order.
For sorting an ArrayList in descending order there are two options,

Use method reverseOrder() provided by Collections class itself
General form and description
```
public static <T> Comparator<T> reverseOrder()
```
Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface.
Using a custom comparator.

Sorting ArrayList in descending order using reverseOrder method

public class SortListDemo {
  public static void main(String[] args) {
    List<String> cityList = Arrays.asList("Delhi","Mumbai","Bangalore","Chennai","Kolkata","Mumbai");
     // sorting the list in descending order
    Collections.sort(cityList, Collections.reverseOrder());
    System.out.println("List sorted in reverses order- " + cityList);
  }
}

Output

List sorted in reverses order- [Mumbai, Mumbai, Kolkata, Delhi, Chennai, Bangalore]

Sorting ArrayList in descending order using custom Comparator

Internally reverseOrder method calls a Comparator class to sort the list in reverse order. We can do it ourselves too by writing our own comparator class.

public class SortListDemo {
  public static void main(String[] args) {
    List<String> cityList = Arrays.asList("Delhi","Mumbai","Bangalore","Chennai","Kolkata","Mumbai");
     // sorting the list in descending order
    Collections.sort(cityList, (String a, String b)->  b.compareTo(a));
    System.out.println("List sorted in reverses order- " + cityList);
  }
}

Output

List sorted in reverses order- [Mumbai, Mumbai, Kolkata, Delhi, Chennai, Bangalore]

Note that Comparator is implemented as a lambda expression here.

Sorting Java ArrayList using sorted method of the Java Stream

public class SortListDemo {
  public static void main(String[] args) {
    List<String> cityList = Arrays.asList("Delhi","Mumbai","Bangalore","Chennai","Kolkata","Mumbai");
    List<String> tempList = cityList.stream().sorted().collect(Collectors.toList());
    System.out.println("List sorted in natural order- " + tempList);
    tempList = cityList.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
    System.out.println("List sorted in reverse order- " + tempList);
  }
}

Output

List sorted in natural order- [Bangalore, Chennai, Delhi, Kolkata, Mumbai, Mumbai]
List sorted in reverse order- [Mumbai, Mumbai, Kolkata, Delhi, Chennai, Bangalore]

That's all for this topic How to Sort ArrayList in Java. If you have any doubt or any suggestions to make please drop a comment. Thanks!

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