How to sort an ArrayList in Java

In this post, you will learn how to sort an ArrayList using the Java programming language.

ArrayList in Java

The ArrayList class implements the List interface. It uses a dynamic array to store the duplicate elements of different data types. ArrayList is a part of the collection framework and is present in the java.util package. It is like an array, but there is no size limit.

ArrayList is a dynamic array, and we do not have to specify the size while creating it. We can add or remove elements at any time. ArrayList is not synchronized, which means multiple threads can work on ArrayList at the same time. It provides some useful methods like add(), addAll(), capacity(), clear(), clone(), elements(), equals(), size(), set(), remove(), and trimToSize().

Java sort an ArrayList using the Collections.sort() Method

Java collections class provides the Collections.sort(List) method to sort the linked list, array, queue, and other data structures. In this post, we use this method to sort an ArrayList in ascending order. It takes the elements of ArrayList as parameters and sorts them in ascending order, and then returns an ArrayList that is sorted in ascending order.

Syntax -

Collections.sort(ArrayList);

Example 1-

// Java program to Sort ArrayList in Ascending Order  
// using sort() method of collection class

// Import the Java util class
import java.util.*;   
public class SortArrayList{
    
public static void main(String args[]){ 
    
    // Creating object of ArrayList class called list
    ArrayList list = new ArrayList();
    
    // Adding elements to the ArrayList   
    list.add(55);   
    list.add(10);   
    list.add(62);   
    list.add(42);   
    list.add(86);  
    
    // Displaying the unsorted ArrayList   
    System.out.println("Before Sorting: "+ list); 
    
    // Sorting ArrayList in Ascending Order   
    Collections.sort(list);   
    
    // Displaying the sorted ArrayList   
    System.out.println("After Sorting: "+ list);  
    }   
}  

Output of the above code:

Before Sorting: [55, 10, 62, 42, 86]
After Sorting: [10, 42, 55, 62, 86]

Example 2-

// Java program to Sort ArrayList in Ascending Order  
// using sort() method of collection class

// Import the Java util class
import java.util.*;   
public class SortArrayList{
    
public static void main(String args[]){ 
    
    // Creating object of ArrayList class called bicycle
    ArrayList<String> bicycle = new ArrayList<String>();
    
    // Adding elements to the ArrayList   
    bicycle.add("Decathlon");   
    bicycle.add("Firefox");   
    bicycle.add("Montra");   
    bicycle.add("Hercules");   
    bicycle.add("Cradiac");   
    bicycle.add("Avon");   
    bicycle.add("Hero");  
    
    // Displaying the unsorted ArrayList   
    System.out.println("Before Sorting: "+ bicycle); 
    
    // Sorting ArrayList in Ascending Order   
    Collections.sort(bicycle);   
    
    // Displaying the sorted ArrayList   
    System.out.println("After Sorting: "+ bicycle);  
    }   
}  

Output of the above code:

Before Sorting: [Decathlon, Firefox, Montra, Hercules, Cradiac, Avon, Hero]
After Sorting: [Avon, Cradiac, Decathlon, Firefox, Hercules, Hero, Montra]

Java sort an ArrayList using Comparator attribute

In the given Java program, we use the sort() method to sort the list of elements by values by passing a customized comparator. It sorts the list elements by comparing the ASCII values of the elements.

Syntax-

sort(ArrayList list, Comparator <T>)

It sorts the elements of the list according to the order included by the comparator.

Example-

// Java program to Sort ArrayList in Ascending Order  
// using sort() method 

// Import the Java util class
import java.util.*;   
public class SortArrayList{
    
public static void main(String args[]){ 
    
    // Creating object of ArrayList class called list
    ArrayList list = new ArrayList();
    
    // Adding elements to the ArrayList   
    list.add(89);   
    list.add(20);   
    list.add(64);   
    list.add(98);   
    list.add(39);  
    
    // Displaying the unsorted ArrayList   
    System.out.println("Before Sorting: "+ list); 
    
    // Sorting ArrayList in Ascending Order   
    list.sort(Comparator.naturalOrder()); 
    
    // Displaying the sorted ArrayList   
    System.out.println("After Sorting: "+ list);  
    }   
}  

Output of the above code:

Before Sorting: [89, 20, 64, 98, 39]
After Sorting: [20, 39, 64, 89, 98]

In the above code, the naturalOrder() method of Comparator Interface returns a comparator that use to compare Comparable objects in natural order.

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