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Confused with Set equality

 
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import java.util.*;
class CollTest1
{

public static void main(String[] args)
{
SetElem k1 = new SetElem(1);
SetElem k2 = new SetElem(1);
Set s = new HashSet();
System.out.println(k1.equals(k2)); // #1 true
System.out.println(s.add(k1)); // #2 true
System.out.println(s.add(k2)); // #3 true
System.out.println(s.add(new String("raju"))); //true
System.out.println(s.add(new String("raju"))); //false
System.out.println(s.size());
}
}
class SetElem
{
int i;
public SetElem(int i){
this.i = i;
}
public boolean equals(Object o){
return true;
// return ((SetElem)o).i == this.i;
}
public int hasCode(){
return 1;
// return i;
}
};
how #3 return true in above example? please explain.
 
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You don't have any hashCode method. If you think you have one, check its spelling carefully.
 
Venkata Saraswathi
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Hi, You are right
 
Venkata Saraswathi
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Yes, you are right.
One more question regarding this. Please calrify

import java.util.*;
class CollTest1
{

public static void main(String[] args)
{
SetElem k1 = new SetElem(1);
SetElem k2 = new SetElem(2);
Set s = new HashSet();
s.add(k1);
s.add(k2);
System.out.println(s.size()); // #1 print 2
k2.i = 1;
System.out.println(s.size()); // #2 print 2
s.remove(k2);
System.out.println(s.size()); // #3 print 1

}
}
class SetElem
{
int i;
public SetElem(int i){
this.i = i;
}
public boolean equals(Object o){
return ((SetElem)o).i == this.i;
}
public int hashCode(){
return i;
}
};

How line 3 prints value 1?, whenever an object is added to a set, does set will create a copy of a object and always make reference to that?
 
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Venkata Saraswathi wrote: whenever an object is added to a set, does set will create a copy of a object and always make reference to that?



First, use code tag to post your Code. Coming to your Question . Java Class method argument always pass by value[copy of a object ] .
 
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seetharaman venkatasamy wrote:Java Class method argument always pass by value[copy of a object ] .


I tend to disagree. Pass a value doesn't mean "copy an object". For primitives it copies them, but for class types, it only copies the references.

But now I have a problem with the Set/HashSet. If I have two different SetElem and I add them to the Set, it all works fine. But If I change the instance value of the one SetElem to the same as the other (#1), shouldn't we have duplicates in the set after the equals method? Doesn't look like, the size is still 2 (#2). Even if I remove it (#3) and add it again (#4), all works fine and we have two elements in the set (#5).
Where is my mistake?

Thanks
Bob

 
Seetharaman Venkatasamy
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Bob Wheeler wrote:
I tend to disagree. Pass a value doesn't mean "copy an object". For primitives it copies them, but for class types, it only copies the references.


Agree. Apologies

Bob Wheeler wrote:



you removed k2. then you added . then where the duplicate came?
 
Christophe Verré
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A HashSet is using a HashMap to store its values. Look at what the java.util.Map tells us : "great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on a such a map."

The golden rule is : values used in equals/hashCode in objects used in collections should be immutable.
 
Bob Wheeler
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Christophe Verré wrote:A HashSet is using a HashMap to store its values. Look at what the java.util.Map tells us : "great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on a such a map."

The golden rule is : values used in equals/hashCode in objects used in collections should be immutable.


Thanks for the reply. Really interesting.
But there is still a confusing part for me. If we change the instance variable BEFORE we call the remove-method (#1), the set doesn't recognize the element as
as duplicate. But, if we change the instance variable after the call of the remove-method it does (#2).
Why? Because of this?

The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map


Yes, I guess so. I should stop talking to myself

Thanks Christophe

cheers
Bob

 
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But there is still a confusing part for me. If we change the instance variable BEFORE we call the remove-method (#1), the set doesn't recognize the element as
as duplicate. But, if we change the instance variable after the call of the remove-method it does (#2).
Why? Because of this?



Basically, when "we change the instance variable BEFORE we call the remove method", we corrupted the set. We were lucky that removed even worked -- as the object could have been in the wrong bucket.

Henry
 
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You may find How to Write an Equality Method in Java an interesting read.
 
Bob Wheeler
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Thank you guys. Got it now. Great link by the way.

cheers
Bob
 
Venkata Saraswathi
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oh! great link. What a concept it is.....

Clarified lot of doubts with this link.

Thanks,
-- Venkata
 
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