> On June 25, 2013, 11:27 p.m., Abraham Elmahrek wrote:
> > Overall, looks good. I am mostly unsure about the use of 31 as the only
> > prime number separating hash code values.
>
> Jarek Cecho wrote:
> Hi Abe,
> thank you very much for the review, appreciated! All the hashCode methods
> were generated by my IDE (IntelliJ IDEA) including the magical constant 31. I
> assume that this is best practice, however please correct me if I'm wrong.
>
> Abraham Elmahrek wrote:
> The number 31 seems arbitrary. My understanding is that hash codes with
> multiple instance members involved require some level of separation. By
> consistently using the number 31, there is a small chance that there will be
> some overlap between the hash codes produced by the members. It should
> increase the chance of a collision... the amount it increases it by is likely
> small though if the members are strings. If the members are Boolean, the
> overlap is guaranteed... IE: class A: member1 = Boolean, member1 = Boolean,
> hashCode = func { result = 0; if member1 != null, then result =
> member1.hashCode(); if member2 != null, then result = 31*result +
> member2.hashCode(); }. NOTE: Boolean in java has two values for its hash code.
>
> Venkat Ranganathan wrote:
> I think the basic idea is to have an hash code that is somewhat similar
> to member1*31^(n-1) + member2*31^(n-2) + ... + member[n] using int arithmetic
> (where member1 is the hashcode of member object 1 for example computed
> similarly). But it is done with integer arithmetic. I have seen other
> numbers used, like say 37, but may be that has better dispersion but still
> will be an arbitrary number, right?
>
> Abraham Elmahrek wrote:
> Venkat,
> That seems right. It's just the implementation is a bit skeptical given
> null values in members effectively zero out an iteration providing more
> opportunity for collisions. IE the scenario I've stated above.
>
> Anyways, I don't think it matters too much. If you guys are fine with it.
> I am too.
I see what you are saying. But is it not that initializing result to a
non-zero value is what is needed here. That is while computing hash code,
initialize result to, say 1.
result = 1
result = result * 31 + member1
result = result * 31 + member2
Thanks
- Venkat
-----------------------------------------------------------
This is an automatically generated e-mail. To reply, visit:
https://reviews.apache.org/r/12089/#review22380
-----------------------------------------------------------
On June 25, 2013, 9:42 p.m., Jarek Cecho wrote:
>
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/12089/
> -----------------------------------------------------------
>
> (Updated June 25, 2013, 9:42 p.m.)
>
>
> Review request for Sqoop.
>
>
> Bugs: SQOOP-1101
> https://issues.apache.org/jira/browse/SQOOP-1101
>
>
> Repository: sqoop-sqoop2
>
>
> Description
> -------
>
> This part of SQOOP-1073 is adding classes representing the data types
> proposed in SQOOP-515.
>
>
> Diffs
> -----
>
> common/pom.xml 2921800
> common/src/main/java/org/apache/sqoop/schema/Schema.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/SchemaError.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/AbstractComplexType.java
> PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/AbstractDateTime.java
> PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/AbstractNumber.java
> PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/AbstractString.java
> PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Array.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Binary.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Bit.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Column.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Date.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/DateTime.java
> PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Decimal.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Enum.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/FixedPoint.java
> PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/FloatingPoint.java
> PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Map.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Set.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Text.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Time.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Type.java PRE-CREATION
> common/src/main/java/org/apache/sqoop/schema/type/Unsupported.java
> PRE-CREATION
>
> Diff: https://reviews.apache.org/r/12089/diff/
>
>
> Testing
> -------
>
>
> Thanks,
>
> Jarek Cecho
>
>
