Using Comparator to order ArrayList Java

Question

I have to order Appointments by date and time. I have an ArrayList of Appointments and have tried to create a comparator to compare their dates and times. I am trying to use the Collections.sort method, passing it the ArrayList of Appointments and the AppointmentComparator I have created. When compiling I get a "No suitable method for sort." Here's a link to the full error message generated by the compiler : http://prntscr.com/7y4qb

Comparator:

public class AppointmentComparator implements Comparator<Appointment>
{
public int compare(Appointment a, Appointment b)
{
    if (a.getDay() < b.getDay())
        return -1;

    if (a.getDay() == b.getDay())
    {
        if (a.getStart() < b.getStart())
            return -1;
        if (a.getStart() > b.getStart())
            return 1;
        return 0;
    }

    return 1;
}

Line with syntax error:

Collections.sort(book, new AppointmentComparator());

variable book is an ArrayList of Appointments. ArrayList<Appointment>

AppointmentBook class:

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

public class AppointmentBook
{
private ArrayList<Appointment> book;

public AppointmentBook()
{
    book = new ArrayList<Appointment>();
}

public void addAppointment(Appointment appt)
{
    book.add(appt);
    Collections.sort(book, new AppointmentComparator());
}

public String printAppointments(int day)
{
    String list = "";

    for (int i = 0; i < book.size(); i++)
    {
        if (book.get(i).getDay() == day)
        {
            list = list + "Appointment description: " + book.get(i).getDescription() + "\n" + "Date of Appointment: " +
            book.get(i).getDay() + "\n" + "Time: " + book.get(i).getStart() + " - " + book.get(i).getEnd() + "\n" + "\n";
        }
    }

    return list;
}

Appointment class:

public class Appointment
{
private String desc;
private int day; //in format mmddyyyy
private int start; //in format hhmm
private int end; //in format hhmm

public Appointment(String description, int aptDay, int startTime, int endTime)
{
    desc = description;
    day = aptDay;
    start = startTime;
    end = endTime;
}

public String getDescription()
{
    return desc;
}

public int getDay()
{
    return day;
}

public int getStart()
{
    return start;
}

public int getEnd()
{
    return end;
}

}

Solution 1

From the error message it looks like you forgot to declare your comparator as implementing the interface:

public class AppointmentComparator implements Comparator<Appointment> {}

It needs to have the implements part, not just contain the method.

Solution 2

Also we can use inner class for some cases:

public int indexOfLargest(ArrayList<QuakeEntry> Data) {
    Comparator<QuakeEntry> cmtr = new Comparator<QuakeEntry>() {
        @Override
        public int compare(QuakeEntry t, QuakeEntry t1) {
            if (t.getMagnitude() < t1.getMagnitude())
                return -1;
            if (t.getMagnitude() == t1.getMagnitude()) 
                return 1;
            if (t1.getMagnitude() > t1.getMagnitude())
                return 0;
        return 1;
        }
    };

    QuakeEntry max = Collections.max(Data, cmtr);
    int maxIndex = Data.indexOf(max);
    //----------------------------------------------------------------------
    System.out.println("//---------------------------------------------------");
    System.out.println("ArrayList sorted by Magnitude using inner class with Comparator");
    System.out.println("//---------------------------------------------------");
    Collections.sort(Data, cmtr);
    for (QuakeEntry qe : Data) {
        System.out.println(qe);
    }


    return maxIndex;
}

code for all classes: https://github.com/Evegen55/Java_DukeEdu_Coursera_2/blob/master/Earthquakes_Programming%20and%20Interfaces/src/earthquakes_programming_and_interfaces/QuakeEntry.java

Solution 3

You need to cast your new AppointmentComparator

Collections.sort(book, new (Comparator)AppointmentComparator());