• Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 275
  • Last Modified:

hasCode for 3 int.

Hi,
I need your advise.
I have 3 integer  a,b,c in myClass
and I want to insert this Class has key in HashMap, then I need to Implment HashCode for this case

What is the  best why to do this ?
I thought about
String s = a + "#" + b + "#" + c;

Ami
Thanks.


 
0
krelman
Asked:
krelman
  • 9
  • 2
  • 2
  • +2
1 Solution
 
zzynxSoftware engineerCommented:
Typically xor is used:  a ^ b ^ c
0
 
CEHJCommented:
hashCode() you could implement as

return a * b * c;
0
 
zzynxSoftware engineerCommented:
So,

public int hashCode() {
   return a ^ b ^ c;
}
0
Independent Software Vendors: We Want Your Opinion

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

 
derkecCommented:
Keep in mind that if you're overriding hashcode like this, you might also want to override .equals(). Two objects that .equals() eachother should have the same hashcode.
0
 
zzynxSoftware engineerCommented:
Definitely true.
(I supposed that as known by the author)
0
 
objectsCommented:
public int hashCode()
{
   return a + b + c;
}

public boolean equals(Object o)
{
   myClass mc = (myClass) o;
   return a==mc.a && b==mc.b && c==mc.b;
}
0
 
zzynxSoftware engineerCommented:
This one is even better:

public int hashCode() {
   return a + (b<<1) + (c<<2);
}

Why?
The goal of the hashCode is to construct a key for the HashMap.
The more unique the key is, the better. (=the faster the lookup will be)

All previous (valid) solutions (^, * or +) give the same result for the following
three different objects:

            a=1, b=2, c=4
       
            a=4, b=2, c=1
and
            a=1, b=4, c=2

while the above produces three different hash codes.
0
 
zzynxSoftware engineerCommented:
>> I need your advise
Hi krelman, what do you think of our advices so far?
0
 
krelmanAuthor Commented:
Hi,
has you write all the previous solution don't have a uniq result.

Are you sure this have a uniq result ? ( I am checking it )

public int hashCode()
{
   return a + (b<<1) + (c<<2);
}
0
 
krelmanAuthor Commented:
Hi,
has you right - all the previous solution don't have a uniq result.

Are you sure this have a uniq result ? ( I am checking it )

public int hashCode()
{
   return a + (b<<1) + (c<<2);
}
0
 
zzynxSoftware engineerCommented:
>>Are you sure this have a uniq result ?
No I'm not. I guess it isn't.
But it shouldn't be unique, it should be "as unique as possible".
If it isn't unique that is *no functional problem*: the item will be found.
No problem.

But it *could* be a performance problem: for one (nearly unique) key, multiple items are found.
And this leads to some extra comparisons.
0
 
zzynxSoftware engineerCommented:
>> I am checking it
Don't look any further

    a=0, b=0, c=4
    a=16, b=0, c=0
    a=0, b=8, c=0

all produce 16 as hash code. (No "real" problem as said before)

But for a,b,c values different of 0, it looks rather difficult to find two sets that produce the same code.
So, I guess you're rather safe with that.
0
 
CEHJCommented:
I haven't done any statistical research on this, but i'd guess that the introduction of primes could be of use, but don't get too hung up on this. As has already been said, complete uniqueness is not required
0
 
zzynxSoftware engineerCommented:
To cope with the 0 problem you could go further and use

public int hashCode() {
   return a + ((b+1)<<1) + ((c+1)<<2);
}

or

public int hashCode() {
   return ((a+1)<<1) + ((b+1)<<2) + ((c+1)<<3);
}

The sky is the limit.
But you have to ask yourself if the (slightly) better performance is worth the (looking for the holy grail-) effort.
0
 
zzynxSoftware engineerCommented:
Thanks for accepting.
That keeps us answering your future questions too.
0

Featured Post

Concerto's Cloud Advisory Services

Want to avoid the missteps to gaining all the benefits of the cloud? Learn more about the different assessment options from our Cloud Advisory team.

  • 9
  • 2
  • 2
  • +2
Tackle projects and never again get stuck behind a technical roadblock.
Join Now