Sometimes we want to sort a List based on some property. Suppose we have a List of UserModel and we want to sort List by firstName then we can do that easily by using Collections.sort() and Comparator interface.

Example: In the below example I'm sorting userList by firstName.

1. First you nee to create a Model class on which you want to apply sorting.

UserModel.java

public class UserModel
{
    private int userId;
    private String firstName;
    private String lastName;
    private String email;
   
    public int getUserId()
    {
        return userId;
    }

    public void setUserId(int userId)
    {
        this.userId = userId;
    }

    public String getFirstName()
    {
        return firstName;
    }

    public void setFirstName(String firstName)
    {
        this.firstName = firstName;
    }

    public String getLastName()
    {
        return lastName;
    }

    public void setLastName(String lastName)
    {
        this.lastName = lastName;
    }

    public String getEmail()
    {
        return email;
    }

    public void setEmail(String email)
    {
        this.email = email;
    }
}

2. Now create Comparator class as below

    /**
     * Comparator class to compare users by firstName
     */
    public static class UserModelComparator implements Comparator<UserModel> {
        @Override
        public int compare(UserModel o1, UserModel o2)
        {
            return (int) (o1.getFirstName().toLowerCase().compareTo(o2.getFirstName().toLowerCase()));
        }
    }

3. Now create sort function and pass List into that function as below

    public static List<UserModel> getSortedListByFirstName(List<UserModel> userModels)
    {
        Collections.sort(userModels, new UserModelComparator());
        return userModels;
    }

the above function will return sorted list by firstName.

Hope this will help you :)