doubt in Ch7 of K & B book

[prajal Mehta]

hi,
I have a doubt in question number 7 of SELF TEST of Chapter 7 (Generics and Collections) of K&B book-
Here is the code-

 import java.util.*;
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("Monday");
ToDos t3 = new ToDos("Tuesday");
m.put(t1, "doLaundry");
m.put(t2, "payBills");
m.put(t3, "cleanAttic");
System.out.println(m.size());
}
}
class ToDos{
String day;
ToDos(String d) { day = d; }
public boolean equals(Object o) {
return ((ToDos)o).day == this.day;
}
// public int hashCode() { return 9; }
}   

Options are -

 A. As the code stands it will not compile.
B. As the code stands the output will be 2.
C. As the code stands the output will be 3.
D. If the hashCode() method is uncommented the output will be 2.
E. If the hashCode() method is uncommented the output will be 3.
F. If the hashCode() method is uncommented the code will not compile.   

Correct answer is --
3 C and D are correct. If hashCode() is not overridden then every entry will go into its own
bucket, and the overridden equals() method will have no effect on determining equivalency.
If hashCode() is overridden, then the overridden equals() method will view t1 and t2 as duplicates.
-------------------------------------------------------------------

Now, my questions are
1. Will equals method not at all run if there is no hashCode method in our class?
2. What is the sequence of execution of equals and hashCode methods. I suppose hashCode method runs first..am I right?

[Ram Reddy]

hi prajal,
there are n buckets say 1, 2, 3, 4 ...n

whenever you are trying to insert value in hash table , or hash map first it will get the hashcode using the getHashCode method on that object whcih you are trying to insert.

Then using that bucket number it will find is there any other object which is equals to the current object which you are trying to insert using equals method. If is finds then it will replace the existing object with new object. other wise it will insert new object in that bucket.

now review the question you posted and confirm that answers given by your self.

if you have still doubt you are free to ask.

thanks
rami.