Doubt in Collection

[Lata Bagga]

Hello Friends,

I have doubt in the following code:

  
class Hello {
public static void main(String [] args) {
Map<Object, Object> m = new HashMap<Object, Object>();
Dog d1 = new Dog("clover");
m.put(d1, "Dog key");
d1.name = "magnolia"; // line
System.out.println(m.get(d1)); // line 2
d1.name = "clover";// line 3
System.out.println(m.get(new Dog("clover"))); // line 4
d1.name = "arthur";// line 5
System.out.println(m.get(new Dog("clover")));//line 6
}
}
class Dog {
public Dog(String n) { name = n; }
public String name;
public boolean equals(Object o) {
if((o instanceof Dog) &&
(((Dog)o).name == name)) {
return true;
} else {
return false;
}
}
public int hashCode() {return name.length(); }
}


  

The output comes as:
null
Dog key
null

I am not able to understand why the line 6 is giving o/p as null.
My understanding for above code is:

This thing I understood that when d1.name is set to "magnolia"in line 1, then line 2 prints null beacuse the d1 object saved earlier in the map has name as "clover" . So the hashcode will be 8 but earlier it was 6.

But when line 5 changes d1.name as "arthur", but line 6 is printing the
dog object whose name is "clover" which was saved into the map, so the hashcode() will be 6 and equals should also satisfy, then why it is not returning "Dog key" instead of null.
I think I am not getting the logic of line 1 - line 6. Can anyone clarify my doubt?

Regards,
Lata

[Deepak Chopra]

Ok, Let me explain you..Think hash map is a Guardian with two guards..!!
When you put an "Dog Key" in Map..you tell the first Guard(HashMap)..Allow any Object to enter whose Hash value is 6...!!
you tell second Guard(equals) that give a copy of "Dog key" to any DOG whose name is "clover"...!! I hoep till now everything will be clear to you..!!

After that you ask the second guard..I dont like "clover"..so allow everyone whose name is "magnolia"
Now First you send DOG "magnolia"..It goes to first guard..but as its Hash value is 8..so guard doesnt allow the DOG..!! Because it require 6 for authentication..!!

After that you ask the second guard..I dont like "magnolia"..so allow everyone whose name is "clover"..

Now you send DOG "clover"..it has value 6..so first guard allows it..again it has clover name so second guard also..so it bring with "DOG KEY"..in its mouth..!!

After that you ask the second guard..I dont like "clover"..so allow everyone whose name is "arthur"..!!
Now you send DOG "clover"..it has value 6..so first guard allows it..but second Guard wants "arthur"..so it doesn't allow it..hence result is null

[marc weber]

Originally posted by Lata Bagga:
... But when line 5 changes d1.name as "arthur", but line 6 is printing the dog object whose name is "clover" which was saved into the map, so the hashcode() will be 6 and equals should also satisfy...

Take a closer look at "equals should also satisfy."