How to sort ArrayList in Natural and Custom Order in Java - Example Tutorial

Sorting ArrayList in Java is a common task for Java developers and we have touched it while discussing in my last article 10 Examples of  ArrayList in Java and again when we discussed comparator and comparable in Java. In order to sort an ArrayList, we need to use the Collections utility class which contains an overloaded sort() method for sorting different collections and supports different comparators in Java. In this article, we will see how to sort ArrayList in the natural order of elements and then sorting ArrayList in Java with a comparator.

The java.util.Collections class provides a reverse Comparator that can be used to sort the array in decreasing order. You can obtain this Comparator by calling the Collections.reverseOrder() method.

Sorting ArrayList in Java with natural Order

In Order to Sort Java ArrayList on the natural order of elements, the object stored in ArrayList must implement a Comparable interface in Java and should override the compareTo() method as per their natural order. 

In our example of natural order sorting in ArrayList, we have implemented compareTo of smartphones and sorted them based on brands. So an Apple smartphone comes before Nokia smartphones. 

Once your object is OK just store them in Java ArrayList and pass that list to the Collections.sort() method, which will sort the list in the natural order of objects. 

See the bottom of this Java tutorial for a complete code example of Sorting Java ArrayList in Natural Order. If you love books, you can also check out these core Java courses to learn more about sorting ArrayList. One of the best online courses and tutorials to learn core Java. 

Sorting Java ArrayList with custom Order

To sort an ArrayList in Java on Custom order we need to supply an external Comparator along with ArrayList to Collections.sort(List, Comparator) method. compare() method will define how the sorting of objects will take place in ArrayList.

In our example of custom order sorting of Java ArrayList, we have created a PriceComparator which sorts objects based on their price. So you can get the cheapest or expensive smartphone stored in ArrayList. 

By the way, Sorting an ArrayList has become even easier in Java with a lambda expression, method reference, and various utility methods like comparing() and thenComparing() methods added on the Comparable and Comparator class. See this tutorial gets a feel of sorting ArrayList in Java 8.

Now, let's see below for a full code example of ArrayList Sorting in the custom order in Java:

Java Program to sort an ArrayList in Custom Order using Comparator

Here is a complete code example of sorting an ArrayList in java on both natural and custom orders by using a custom comparator.

package test;

import java.util.ArrayList;

import java.util.Collections;

import java.util.Comparator;

public class ArrayListSortingExample {

   private static class SmartPhone implements Comparable {

        private String brand;

        private String model;

        private int price;

        public SmartPhone(String brand, String model, int price){

            this.brand = brand;

            this.model = model;

            this.price = price;

        }

      

        @Override

        public int compareTo(SmartPhone sp) {

            return this.brand.compareTo(sp.brand);

        }

        @Override

        public String toString() {

            return "SmartPhone{" + "brand=" + brand + ", model=" + model + ", price=" + price + '}';

        }

      

    }

  

    private static class PriceComparator implements Comparator{

        @Override

        public int compare(SmartPhone sp1, SmartPhone sp2) {

            return (sp1.price < sp2.price ) ? -1: (sp1.price > sp2.price) ? 1:0 ;

        }

      

    }

    public static void main(String... args) {

      

        //creating objects for arraylist sorting example

        SmartPhone apple = new SmartPhone("Apple", "IPhone4S",1000);

        SmartPhone nokia = new SmartPhone("Nokia", "Lumia 800",600);

        SmartPhone samsung = new SmartPhone("Samsung", "Galaxy Ace",800);

        SmartPhone lg = new SmartPhone("LG", "Optimus",500);

      

        //creating Arraylist for sorting example

        ArrayList smartPhones = new ArrayList();

      

        //storing objects into ArrayList for sorting

        smartPhones.add(apple);

        smartPhones.add(nokia);

        smartPhones.add(samsung);

        smartPhones.add(lg);

      

        //Sorting Arraylist in Java on natural order of object

        Collections.sort(smartPhones);

      

        //print sorted arraylist on natural order

        System.out.println(smartPhones);

      

        //Sorting Arraylist in Java on custom order defined by Comparator

        Collections.sort(smartPhones,new PriceComparator());

      

        //print sorted arraylist on custom order

        System.out.println(smartPhones);

    

    }

}

Output:

[SmartPhone{brand=Apple, model=IPhone4S, price=1000}, SmartPhone{brand=LG, model=Optimus, price=500}, SmartPhone{brand=Nokia, model=Lumia 800, price=600}, SmartPhone{brand=Samsung, model=Galaxy Ace, price=800}]

[SmartPhone{brand=LG, model=Optimus, price=500}, SmartPhone{brand=Nokia, model=Lumia 800, price=600}, SmartPhone{brand=Samsung, model=Galaxy Ace, price=800}, SmartPhone{brand=Apple, model=IPhone4S, price=1000}]

How to sort ArrayList of String in Descending Order in Java?

ArrayList can also be sorted in descending or reverse order by using Collections.reverseOrder() and Collection.reverseOrder(Comparator cmp). 

Former method will sort in reverse order of natural ordering while later method will sort in the reverse order of specified comparator as shown in following example of sorting ArrayList into reverse order :

//sorting ArrayList in descending or reverse order in Java
List<String> unsortedList = Arrays.asList("abc", "bcd", "ade", "cde");
Collections.sort(unsortedList, Collections.reverseOrder());

System.out.println("Arraylist in descending order: " + unsortedList);


Output:
ArrayList before sorting in reverse order: [abc, bcd, ade, cde]
ArrayList in descending order: [cde, bcd, ade, abc]

By the way, if you are sorting an ArrayList of objects then you can also sort them on multiple fields as shown in that example where we have sorted ArrayList of Book objects by Author and Title. This way all the books of same author will come first. 

How to sort ArrayList of String in Case insensitive Order

ArrayList of String can also be sorted with case-insensitive comparison. String class defines a convenient case insensitive comparator which can be accessed directly like String.CASE_INSENSITIVE_ORDER. 

If you pass this Comparator to ArrayList.sort() which contains String then those will be sorted accordingly. 

Here is an example of sorting ArrayList in a case insensitive order:

//sorting ArrayList on case insensitive order of String

unsortedList = Arrays.asList("abc", "bcd", "ABC", "BCD");

System.out.println("ArrayList before case insensitive sort: " + unsortedList);

             

Collections.sort(unsortedList, String.CASE_INSENSITIVE_ORDER);

System.out.println("ArrayList after case insensitive sort: " + unsortedList);

Output:

ArrayList before case insensitive sort: [abc, bcd, ABC, BCD]

ArrayList after case insensitive sort: [abc, ABC, bcd, BCD]

That’s all about how to sort ArrayList in Java based on the natural order of Object and any custom order by using Custom Comparator. This is a useful technique to know when it comes to comparing and sorting objects in Java. Comparator gives you a lot of power and fine grained control over sorting. 

Java 8 enhancements on Comparator API has made this even more popular and by using those features you can easily creator Comparator which are much more readable and reuseable. I have explained the new way to sort List of object in Java here, you can also check that article now that you are familiar with custom orders and Comparators in Java. 

Let me know if you face any issues while running Sorting ArrayList Example code in Java and I would be glad to help you.

Thanks

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