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From "Troy Howard (JIRA)" <j...@apache.org>
Subject [Lucene.Net] [jira] [Commented] (LUCENENET-437) Port Contrib.Shingle from Java
Date Mon, 18 Jul 2011 22:54:58 GMT

    [ https://issues.apache.org/jira/browse/LUCENENET-437?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13067370#comment-13067370
] 

Troy Howard commented on LUCENENET-437:
---------------------------------------

>From the docs for List.hashCode():

"Returns the hash code value for this list. The hash code of a list is defined to be the result
of the following calculation:
  hashCode = 1;
  Iterator i = list.iterator();
  while (i.hasNext()) {
      Object obj = i.next();
      hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
  }
 
This ensures that list1.equals(list2) implies that list1.hashCode()==list2.hashCode() for
any two lists, list1 and list2, as required by the general contract of Object.hashCode."

The contract intended for Object.hashCode() specifically includes that object1.equals(object2)
== object1.hashCode().equals(object2.hashCode()). 

This is stated here:

http://download.oracle.com/javase/1.4.2/docs/api/java/lang/Object.html#hashCode()

Quoted:

public int hashCode()

Returns a hash code value for the object. This method is supported for the benefit of hashtables
such as those provided by java.util.Hashtable.

The general contract of hashCode is:

- Whenever it is invoked on the same object more than once during an execution of a Java application,
the hashCode method must consistently return the same integer, provided no information used
in equals comparisons on the object is modified. This integer need not remain consistent from
one execution of an application to another execution of the same application.
- 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.
- 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.

As much as is reasonably practical, the hashCode method defined by class Object does return
distinct integers for distinct objects. (This is typically implemented by converting the internal
address of the object into an integer, but this implementation technique is not required by
the JavaTM programming language.)"

Again, this is Java's code contract, which was in some cases, improperly implemented in the
.NET CLR.

> Port Contrib.Shingle from Java
> ------------------------------
>
>                 Key: LUCENENET-437
>                 URL: https://issues.apache.org/jira/browse/LUCENENET-437
>             Project: Lucene.Net
>          Issue Type: Task
>          Components: Lucene.Net Contrib, Lucene.Net Test
>    Affects Versions: Lucene.Net 2.9.4, Lucene.Net 2.9.4g
>            Reporter: Troy Howard
>            Assignee: Troy Howard
>            Priority: Minor
>             Fix For: Lucene.Net 2.9.4, Lucene.Net 2.9.4g
>
>
> Port Contrib.Shingle from Java

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