1) Define your class City, as usual.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace CityCompare
{
public class City
{
//fields
private string name;
private string state;
//properties
public string Name
{
get { return name; }
set { name = value; }
}
public string State
{
get { return state; }
set { state = value; }
}
//constructors
public City() { } //null constructor
public City(string _name, string _state)
{
this.name = _name;
this.state = _state;
}
//override toString method
public override string ToString()
{
return name + ", " + state;
}
}
}
2) We need IComparable to our class to provide for a simple sorting capability
We need to provide a simple sorting capability to the code. We make use of the interface IComparable. To do that, we need to tell the class to implement the interface IComparable. Just add a column (:) and the interface name next to the class name.
public class City : IComparable
3) IComparable Sorting capability - The CompareTo function
Now, we need to implement IComparable CompareTo function to provide the default sort order. We add the following code to our class:
//IComparable.CompareTo needs to be implemented
//to provide default sort order. Make it public
public int CompareTo(Object obj)
{
City c = (City)obj; //cast Object type to City object
return string.Compare(this.name, c.name);
}
4) Writing your tester class with an ArrayList
In this tutorial, I used the Form1 class as the tester class. The tester class will populate the Arraylist with values.
A listbox is made available on the form so that the cities are listed there.
Also, a button that shows "Simple Sort" is available and uses the ArrayList.Sort() method and does not require any additional implementation.
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Collections; //add this line to use ArrayList
namespace CityCompare
{
public partial class Form1 : Form
{
//fields
private ArrayList cities = new ArrayList();
//properties
public ArrayList Cities
{
get { return cities; }
set { cities = value; }
}
//constructors
public Form1()
{
InitializeComponent();
}
//methods
//Populate the listbox
public void PopulateListBox()
{
//clear listbox
lisCitiesList.Items.Clear();
//Populate the listbox
foreach (Object item in cities)
{
lisCitiesList.Items.Add(item.ToString());
}
}
//Populate the ArrayList with cities and states
public void PopulateArrayList()
{
cities.Add(new City("Sydney", "New South Wales"));
cities.Add(new City("Albury", "New South Wales"));
cities.Add(new City("Armindale", "New South Wales"));
cities.Add(new City("Bathurst", "New South Wales"));
cities.Add(new City("Blue Mountains", "New South Wales"));
cities.Add(new City("Palmerston", "Northern Territory"));
cities.Add(new City("Darwin", "Northern Territory"));
cities.Add(new City("Melbourne", "Victoria"));
cities.Add(new City("Perth", "Western Australia"));
cities.Add(new City("Albany", "Western Australia"));
cities.Add(new City("Canning", "Western Australia"));
cities.Add(new City("Gosnells", "Western Australia"));
cities.Add(new City("Hobart", "Tasmania"));
cities.Add(new City("Hobart", "AnotherState"));
}
//events
private void Form1_Load(object sender, EventArgs e)
{
//Populate cities arraylist
PopulateArrayList();
//Populate listbox
PopulateListBox();
}
private void btnNormalSort_Click(object sender, EventArgs e)
{
cities.Sort();
PopulateListBox(); //refresh list
}
}
}
5) Sort list of cities first by state name, then by city name, without modifying the implementation of the City class
As Alan stated,
you cannot redefine the CompareTo method. Let's stick to the requirement and define a CityComparer class that implements the Comparator<City> interface.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections; //add this line to use interface IComparer
namespace CityCompare
{ //STEP 2
public class CityComparer: IComparer //we use this IComparer as it gives
//additional comparison mechanisms
{
//we need to implement the interface member Compare(obj,obj)
//from the interface IComparer
public int Compare(Object a, Object b) //make it public
{
City c1 = (City)a; //cast Object to City
City c2 = (City)b; //cast Object to City
return String.Compare(c1.State, c2.State); //State is a string,
//hence we use String.Compare
}
//StateSort method
//sorts the ArrayList elements by state
public static IComparer StateSort()
{
return (IComparer)new CityComparer();
}
}
}
6) Call the method StateSort to sort by state in the Form1 class.
private void btnCityComparer_Click(object sender, EventArgs e)
{
cities.Sort(CityComparer.StateSort()); //STEP 3
PopulateListBox(); //refresh list
}
All done! Suggestions and comments are welcome.
