Collections question.

[Alex Serna]

Hi everyone,

I have read that in maps you need to override equals when you create your own class to use it as a key. I didn't do it and still it lets me insert elements as you can see in the following code:

 package com.pruebas;
import java.util.*;
public class MapEQ {

    public static void main(String[] args) {
        Map<ToDos, String> m=new HashMap<ToDos, String>();
        ToDos t1=new ToDos("Monday");
        ToDos t2=new ToDos("Tuesday");
        ToDos t3=new ToDos("Friday");
        m.put(t1, "gotoGym");
        m.put(t2, "compra");
        m.put(t3,"fiesta");
        System.out.println(m.size());
    }
}
class ToDos{
    String task;
    ToDos(String t){task=t;}
}  

I don't have much experience working with collections. I have read the collection's chapter in K & B book, but still I don't feel I confident with it. Does anyone know about a book or a website where collections are explained in more detail???

[Ankit Garg]

You were able to add elements to the Map but you won't be able to get elements out of it correctly. Look at this code

ToDos t1=new ToDos("Monday");
ToDos t2=new ToDos("Tuesday");
ToDos t3=new ToDos("Friday");
m.put(t1, "gotoGym");
m.put(t2, "compra");
m.put(t3,"fiesta");
//Now I want to get the ToDos of Monday with a new Monday object
ToDos find = new ToDos("Monday");
String value = m.get(find);    //returns null

Although the ToDos object find that I created is meaningfully equivalent to t1, but I can't get the element out of the map using find as hashCode and equals were not overridden...

[Valentin Ivanov]

hi Alex,

it is working that way because your class ToDos inherit equals() method from Object class. But in that way once you lose the reference(address) to the object, used as a key, you can't retreive that object because equals() method compares the bitpattern of the variables

[Alex Serna]

Thank you very much, I understand it now. So the default equals from Obejct compares the references, but what about Object's hashcode, what's its behaviour?

[Seetharaman Venkatasamy]

http://www.javapractices.com/topic/TopicAction.do?Id=28