Java Program to Sort an ArrayList

Java built in sort arraylist – Java Program to Sort an ArrayList

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Java built in sort arraylist: In this article we will see how to sort an ArrayList in Java Programming language.

Java Program to Sort an ArrayList

We can sort the ArrayList by using 3 different ways as follows

Method-1: Java Program to Sort an ArrayList by Using Collections.sort() Method

Java built in sort arraylist: An ArrayList can be sorted by using the sort() method of the Collection class.

Parameter – Collection to be sorted

Returns – Sorted Collection in ascending order by default

Approach:

  • Declare an integer arraylist say evenNumber.
  • Add elements into arraylist.
  • Sort it by using inbuilt function Collections.sort(evenNumber);
  • Print the sorted ArrayList.

Program:

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Main 
{
    public static void main(String[] args) 
    {
        //ArrayList of integers declared
        List<Integer> evenNumber = new ArrayList<>();
        //Elements added into ArrayList
        evenNumber.add(8);
        evenNumber.add(14);
        evenNumber.add(20);
        evenNumber.add(6);
        evenNumber.add(4);
        
        System.out.println("ArrayList before sorting : " + evenNumber);
        
        // Sorting an ArrayList using built-In method
        Collections.sort(evenNumber);
        System.out.println("ArrayList after sorting : " + evenNumber);
    }
}

Output:

ArrayList before sorting : [8, 14, 20, 6, 4]
ArrayList after sorting : [4, 6, 8, 14, 20]

Method-2: Java Program to Sort an ArrayList by Using ArrayList.sort() Method

An ArrayList can be sorted by using the sort() method by passing a Comparator.

Parameter –

  1. List to be sorted based on the comparator.
  2. Comparator class instance

Returns – sorted in ascending order by default but does not return anything

Approach:

  • Declare an String arraylist say days.
  • Add elements into arraylist.
  • Sort it by using inbuilt function days.sort( );
  • Print the sorted ArrayList.

Program:

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

public class Main 
{

    public static void main(String[] args)
    {
        //ArrayList of String declared named as 'days'
        List<String> days = new ArrayList<>();
        //elements added
        days.add("Monday");
        days.add("Friday");
        days.add("Sunday");
        days.add("Thursday");
        
        System.out.println("Original Arraylist  : " + days);
        
        // Sort an ArrayList using its sort() method. 
        // Pass a Comparator to the ArrayList.sort() method.
        days.sort(new Comparator<String>() 
        {
            @Override
            public int compare(String day1, String day2) 
            {
                return day1.compareTo(day2);
            }
        });
        
        // sort() method written simply using lambda expression
        days.sort((day1, day2) -> day1.compareTo(day2));
        
        // Following is an even more concise solution
        days.sort(Comparator.naturalOrder());
        System.out.println("Sorted ArrayList: " + days);
    }
}

Output:

Original Arraylist : [Monday, Friday, Sunday, Thursday]
Sorted ArrayList: [Friday, Monday, Sunday, Thursday]

Method-3: Java Program to Sort an ArrayList by Using Custom Comparator

Approach:

  • Here, we’ve defined a UserDetails class with a String variable userName and Integer userID
  • We’ve also added a constructor that initializes the username and userId with getter and setter functions.
  • In the main() method, we’ve created an array list of users.
  • For sorting the list with the given property, we use the list‘s sort()
  • The sort() method takes the list to be sorted and a comparator.

Program:

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

// main class
public class Main 
{
    public static void main(String[] args) 
    {
        // declared a list of UserDetails
        List<UserDetails> users = new ArrayList<>();
        users.add(new UserDetails("Devika", 3));
        users.add(new UserDetails("Akash", 7));
        users.add(new UserDetails("Chitra", 4));
        users.add(new UserDetails("Barun", 1));
        
        System.out.println("Users Registered: " + users);
        
        // Sort users by their user-id
        users.sort((user1, user2) -> 
        {
        return user1.getUserID() - user2.getUserID();
        });
        
        
        // Sorting by using Collections.sort() method by passing the custom Comparator
        
        Collections.sort(users, Comparator.comparing(UserDetails::getUserName));
        System.out.println("Lexicographically Sorted Users List by UserName : " + users);
    }
}

class UserDetails
{
    // Private variables declared
    private String userName;
    private Integer userID;
    
    // constructor of UserDetails class
    public UserDetails(String userName, Integer userID) 
    {
    this.userName = userName;
    this.userID = userID;
    }
    
    // getter and setter methods to get access of private variables
    public String getUserName() 
    {
    return userName;
    }
    
    public void setUserName(String name) 
    {
    this.userName = userName;
    }
    
    public Integer getUserID() 
    {
    return userID;
    }
    
    public void setUserID(Integer age) 
    {
    this.userID = userID;
    }
    
    public String toString() 
    {
        return "{" +
        "User-Name ='" + userName + '\'' +",User-Id=" + userID +'}';
    }
}


Output:

Users Registered: [{User-Name ='Devika',User-Id=3}, {User-Name ='Akash',User-Id=7}, {User-Name ='Chitra',User-Id=4}, {User-Name ='Barun',User-Id=1}]
Lexicographically Sorted Users List by UserName : [{User-Name ='Akash',User-Id=7}, {User-Name ='Barun',User-Id=1}, {User-Name ='Chitra',User-Id=4}, {User-Name ='Devika',User-Id=3}]

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