java hashcode return value

Question

Basically i am using the hasCode() from the Objects class to obtain a hash code of some strings.

I want that number to represent a position in the array. Basically a hash table. I havent written code for this yet.

I had in mind:

 int hashNumber = SomeString.hascode(), pos; 
 String array[] = new String[10];

 if (hashNumber > 0)
   pos = hashNumber % array.length
 if (hasNumber < 0 )
   //dont know what to do

I do know for a fact that hashCode can return a negative integer. What to do if its negative integer? i though about adding the array length

  pos = hashNumber + array.length

is this the best way?

thanks in advance

Have you considered just using a HashSet?

Aurand
– Aurand

2013-01-25 17:53:27 +00:00
Commented Jan 25, 2013 at 17:53 — Aurand
– Aurand, Commented Jan 25, 2013 at 17:53

Community · Accepted Answer · 2017-05-23 12:14:14Z

3

If hashNumber is negative, just get the mod of -hashNumber (which will be positive):

if (hashNumber >= 0)
  pos = hashNumber % array.length
else
  pos = -hashNumber % array.length

Or for a single expression that will work for both:

pos = (hashNumber % array.length + array.length) % array.length

See this answer to a question about Java's behavior with mod and negative numbers.

edited May 23, 2017 at 12:14

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answered Jan 25, 2013 at 17:48

Andrew Clark

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7 Comments

Alessandroempire Over a year ago

i just tested it. and its giving me a negative integer. In other words: it does not work.

Andrew Clark Over a year ago

Interesting... I did a quick test in Python and assumed modulo would be implemented the same elsewhere. I would expect -3 % 5 == 2.

Alessandroempire Over a year ago

question: why do you do: %array.length outside the parenthesis? with: (hashNumber % array.length + array.length) it will give you a number between the array length. It seems redundant to me.

Andrew Clark Over a year ago

Consider (9 % 10 + 10), that would become 9 + 10 or 19, so we need the % 10 on the outside to bring it back in range. It is only for negative numbers where the value inside of the parentheses will be in the right range.

Alessandroempire Over a year ago

o yes yes. Well i decided to make an IF, and when its a negative integer: it will calculate the position using: (hashNumber % array.length + array.length) and for what i seen its valid.

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Community · Accepted Answer · 2017-05-23 11:58:30Z

0

http://docs.oracle.com/javase/specs/jls/se5.0/html/expressions.html#15.17.3

the sign of the result equals the sign of the dividend.

See here for more discussion: How does java do modulus calculations with negative numbers?

To be safe, you could simply use pos = Math.abs(hashNumber) % array.length

edited May 23, 2017 at 11:58

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answered Jan 25, 2013 at 19:25

Tom Carchrae

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