Re: [collections] Question about CollectionUtils semantics

[Greg Zoller]

Peter...

Ok, fair enough. Point well taken. If I expect hash-related data structures to
work I need to complete the implementation of my objects. But... are you
advocating that a programmer have intimate knowledge of an API's implementation
in order to get the "expected" semantics? The fact remains that there is a behavior
difference between the commons and java APIs, for whatever reason. Neither documentation set stresses the importance of implementing #hashCode
(and at least in my experience this is not something I would commonly do
for every object I create). At the very least this should be documented heavily
to reset users' expectations or the semantics changed.

Regards,
Greg

Peter Ko�ek wrote:

----- Original Message -----
From: "Greg Zoller"
Subject: Re: Question about CollectionUtils semantics

I think the reason your example works is that you're using String-typed
variables...

No. That's not really the reason why.

I have another example that uses another object:

public class Hey {
private String id;
public Hey( String id ) { this.id = id; }
public boolean equals( Object obj ) {
Hey otherHey = (Hey) obj;
if( otherHey.id.equals( this.id ) )
return true;
return false;
}
public String toString() { return id; }
}

Hey - where is your #hashCode implementation? You forgot to fulfill the contract
mentioned in
http://java.sun.com/j2se/1.4/docs/api/java/lang/Object.html#hashCode() . Years
before the first Java programmer forgot to fulfill the contract, programmers of
other languages did the same:
http://wiki.cs.uiuc.edu/VisualWorks/Not+implementing+%23hash+while+overriding+%3
D - and still there's no end in sight. If you didn't know this problem at all,
you should have a look into the book "Effective Java" by Joshua Bloch.

Incomplete object implementations which override #equals but don't override
#hashCode cannot be used within HashSets and HashMaps. The hashing algorithm
will fail then.

The CollectionUtils#intersection implementation relies on the use of HashMaps.
The Collection#retainAll doesn't use hashing, so it can't fail. But if you try
to use incomplete object implementations, your code will run into large problems
sooner or later, because hashing is used so often.

Regards,
Greg

-peter


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