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From "Scott Kilpatrick (JIRA)" <>
Subject [jira] [Commented] (LANG-1323) Type implementations in TypeUtils compute hash code that breaks Object.equals() with Sun's OpenJDK
Date Mon, 24 Apr 2017 14:45:04 GMT


Scott Kilpatrick commented on LANG-1323:

Yeah, that makes sense. I understand that the Java SE doesn't -- _but should!_ -- define how
to implement {{Type.hashCode}}. There seems to be a years-old JDK bug report about this exact

Given the size and importance of the OpenJDK, however, would it not make sense to treat that
as the canonical definition of {{Type.hashCode()}}, and modify the Commons Lang implementations
to adhere to that?

> Type implementations in TypeUtils compute hash code that breaks Object.equals() with
Sun's OpenJDK
> --------------------------------------------------------------------------------------------------
>                 Key: LANG-1323
>                 URL:
>             Project: Commons Lang
>          Issue Type: Bug
>          Components: lang.reflect.*
>    Affects Versions: 3.2, 3.5
>         Environment: Sun OpenJDK
>            Reporter: Scott Kilpatrick
>            Priority: Minor
> {{TypeUtils}} in {{lang.reflect}} provides convenient methods for creating objects of
the interface {{Type}}. Those objects are defined by the following classes:
> * ParameterizedTypeImpl (implements {{ParameterizedType}})
> * WildcardTypeImpl (implements {{WildcardType}})
> * GenericArrayTypeImpl (implements {{GenericArrayType}})
> Similarly, there are corresponding classes, which implement the same interfaces, defined
in one's particular JDK. And it's these latter classes that are instantiated when you get
objects of type {{Type}} via reflection. Let's call these the "internal {{Type}} implementations."
In the case of Sun's OpenJDK, [they are defined|]
in package {{sun.reflect.generics.reflectiveObjects}}.
> Each of the {{TypeUtils}} classes implements {{Object.equals(Object)}} in a general way
that's compatible with the internal {{Type}} implementations. For example, if I access a field
declared with type {{Map<String, Integer>}} and get its generic type, via {{Field.getGenericType()}},
then that will be equal to the {{TypeUtils}} object returned by:
> {code:java}
> TypeUtils.parameterize(Map.class, String.class, Integer.class)
> {code}
> That's what I'd expect, so that's great.
> However, the {{TypeUtils}} classes implement their {{Object.hashCode()}} method in a
_different_ way from the corresponding implementations in Sun OpenJDK implementations. That's
not so surprising, _but it breaks the contract of {{Object.hashCode()}}_:
> bq. 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.
> In other words, the two {{Type}} objects above will both consider themselves {{equals}}
to each other, but they have different hash codes.
> One example of a negative consequence of this problem is a collection class that implements
its equality (to other collections) by checking hash codes of its elements, e.g., Guava's
immutable collections. If you have {{Type}} objects in those collections, with {{TypeUtils}}
{{Type}} objects in {{c1}} and Sun OpenJDK {{Type}} objects in {{c2}}, you will see that {{c1.equals(c2)}}
returns {{false}} -- because their elements don't all have the same hash codes -- even though
those elements are all considered equal.

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