hashcode,equals()

[Shiva Mohan]

Consider the following class:

    
public class GoodOne
{
int theval;
public int hashCode()
{
return theval%3;
}
public boolean equals(Object obj)
{
// 1 insert code here.
}
}
 

Which of the following options may be insterted at //1?

a return true;


b return theval%3 == 0? true :false;


c return theval%2 == 0? true :false;


d return ( (int)Math.random())*10%3 == 0? true :false;


e return false;

Answer given is option b.But i am having trouble on finding the
correct answer on each option.

suppose if i created,
GoodOne s1=new GoodOne();
GoodOne s2=new GoodOne();

s1.theVal=2
s2.theVal=3

option 1:
if the eqauls() method gives result true means the required
hashcode contract is s1.hashCode()==s2.hashCode() but the getting values are
2==0 not equlal.so this option is false

option 2:
s1.equlas(s2) gives false result when i apply the value s1.theVal=2,so there is no
hashcode requiremnet is the contract which is satisfied.

s2.equlas(s1)
s2.theVal=3 when i apply this value,the equals() method returns true means the required
hashcode contract is s1.hashCode()==s2.hashCode().but the getting values are
2==0 not equlal.so i am getting false here

source:enthuware

[Chandra Bhatt]

Hi Shiva Mohan,

See the private messages I have sent to you!


Thanks,

[Deepak Jain]

Here is the contract from Java doc
Which of the following options may be insterted at //1?
1. If two objects are equal according to the equals(Object) method,
then calling the hashCode method on each of the two objects must
produce the same integer result.

2. It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

So #1 is must. If two objects are equals hashCode must be equal

Options:
a)

return true;
If this is inserted in equals() then any two objects are always equal
acc to equals method, But now their hashCodes will be different, So this
does not satisfy the contract.
Ex: g1 = new GoodOne();
g2 = new GoodOne();
g1.theVal = 1;
g2.theVal = 2;
So with above option g1.equals(g2) is true but
g1.hashCode()!=g2.hashCode() because 1!=2 false
So contract failed, So this option is incorrect.

b) return theval%3 == 0? true :false;
This option is correct, Becuase in hashCode() the hashCode of two objects
will be same if theVal is divisble by 3, so now just take the reverse
of this in equls() method, If two objects with theVal that is divisible by
3 then they are equal.
g1.theVal = 1;
g2.theVal = 2;
Here g1.equals(g2) is false and g1.hashCode()!=g2.hashCode() but this is not
mandatory, Unequal objects can have same hashCode, this will cause iefficient hashTables.

So this option is correct.

c) return theval%2 == 0? true :false;
THis is straight forward incorrect becuase g1.equals(g2) is true only
if theVal is divisible by 2 but then the hashCode() will be unequal whihc
does not satisfy contract.

d return ( (int)Math.random())*10%3 == 0? true :false;
This is incorrect.
Hope thie clears the confusion , I am confused now
Just remember the first statement of contract.
Thanks
Deepak

[Manfred Klug]

Hi Shiva,

the only equals() method, which doesn't violate the contract, is e.

[Deepak Jain]

Hi,
I did not see e option,
I think both b and e are correct, Please correct me if am wrong.
Contract is:
1) if a.equals(b) == true then a.hashCode() == b.hashCode() is MUST
2) if a.hashCode!=b.hashCode() then a.equals(n) == false is MUST
Thanks
Deepak

[Manfred Klug]

Originally posted by Deepak Jain:
b) return theval%3 == 0? true :false;
This option is correct, Becuase in hashCode() the hashCode of two objects
will be same if theVal is divisble by 3, so now just take the reverse
of this in equls() method, If two objects with theVal that is divisible by
3 then they are equal.
g1.theVal = 1;
g2.theVal = 2;
Here g1.equals(g2) is false and g1.hashCode()!=g2.hashCode() but this is not
mandatory, Unequal objects can have same hashCode, this will cause iefficient hashTables.

So this option is correct.

You should use the rules from equals() too. This method violates
g1.equals(g2) == g2.equals(g1)

[Manfred Klug]

Originally posted by Deepak Jain:
I think both b and e are correct, Please correct me if am wrong.

Try it with different values for option b.

[Deepak Jain]

Thanks Manfred, Yes option #b fails reflexive property of equals().
Your correct, option e is correct.
Thanks
Deepak

[Chandra Bhatt]

Hi everybody,

public int hashCode() {
    return theVal%3;
}
public boolean equals(Object o) {
    return theval%3 == 0? true :false; 
}

Although the equals method does not abide to symmetric contract of the
hashCode method, but as I think it is ok to be used in the context of
key in the Map.

equals will only be called once hashCode is equal for two object and
ofcourse it is 0 for both so it does not matter whether you are doing
like this.theValue %3 or ((GoodOne)o).theValue%3 because both will
return 0.

I agree that to compare two objects this equals method is not suitable
anyway, because it violates the symmetric property that says

For given two object references x and y if x.equals(y) =true then
y.equals(x) must return true;

I would appreciate any comment on this. Thanks in advance.

Thanks,
[ July 26, 2007: Message edited by: Chandra Bhatt ]

[Manfred Klug]

Originally posted by Chandra Bhatt:
I agree that to compare two objects this equals method is not suitable
anyway, because it violates the symmetric property

And as result it will violate the hashCode() contract sooner or later.

Try it with
g1.theVal = 0;
g2.theVal = 2;

g1.equals(g2) == true and g1.hashCode() != g2.hashCode()

[Chandra Bhatt]

[Manfred]
And as result it will violate the hashCode() contract sooner or later.

Try it with
g1.theVal = 0;
g2.theVal = 2;

g1.equals(g2) == true and g1.hashCode() != g2.hashCode()


I am talking in the context of using the object of this class as key in the
Map. If hashCode() mismatches, equals won't be used for object retrieval.
And it hashCode is matched and it is 0, two objects are equal as the
equals() method says so.

What value you have given, the objects are not equal with respect to what
hashCode() and equals() say.

Thanks,

[Shiva Mohan]

Thank you guys for working in this problem.Many Many
thanks to you chandra.Sorry for entering after a long time.

Chandra!

when entering 2,3 values for hashcode first
how would you say,hashcodes is matched anf the result is 1.
Still that part i don't get.please help.

when entering 3,6 values for hashcode ,it will defnetely return 0.
but what about other possibilities checkng.How would anyone know
apply only 3 multiplies for this question for finding result.

[ July 27, 2007: Message edited by: Shiva Mohan ]
[ July 27, 2007: Message edited by: Shiva Mohan ]

[Chandra Bhatt]

Shiv Mohan

GoodOne g1 = new GoodOne();
GoodOne g2 = new GoodOne();
g1.theValue =3;
g2.theValue =2;
 
g1.equals(g2); true
g2.equals(g1); false

In this example, if you see I first talked about simply equals() method
use that is not used in the Map context (not as a key in the Map). So hashCode() is of no use there.
I just tried to show you how symmetric contract is violated there.

In case theValue is 3 and 2 for two objects, hashCode is not matched and
the comparison fails in the first round (here I am talking about Map
context).

Thanks,

[Manfred Klug]

Originally posted by Chandra Bhatt:
What value you have given, the objects are not equal with respect to what
hashCode() and equals() say.

They are equal according to the equals() method.

public boolean equals(Object o) {
    return theval%3 == 0? true :false; 
}

(0 % 3) == 0 -> true