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Contravariance

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(New page: Everything one would ever like to know about Covariance and Contravariance is available at wikipedia. Here is just a ...)
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Everything one would ever like to know about [[Covariance]] and [[Contravariance]] is available at [[wikipedia:Covariance_and_contravariance_(computer_science)|wikipedia]]. Here is just a small example how it does not work in [[Java]].
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Everything one would ever like to know about [[Covariance]] and [[Contravariance]] is available at [[wikipedia:Covariance_and_contravariance_(computer_science)|wikipedia]]. Here is just a small example how it does not work in [[Java]] (the argumentation is the same as in case of [[Covariance]]). First version would like to be fixed:
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[[TBD]]
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<source lang="java" snippet="variance.contravariance.v1"/>
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However when one tries to change the parameter type:
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<source lang="java" snippet="variance.contravariance.v2"/>
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Existing code would compile against the new library. However because the rules for looking up the right method are different between [[JavaC]] and [[JVM]] code like following will have problems:
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<source lang="java" snippet="variance.contravariance.test"/>
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If compiled against version 1.0 and executed against version 2.0, it will throw linkage errors:
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<source lang="bash">
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Mixing should not work: Compiling with version 1.0 and running against version 2.0 fails.
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Exception in thread "main" java.lang.NoSuchMethodError: api.Contravariance.isPositive(Ljava/lang/Integer;)Z
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at test.ContravarianceTest.main(Unknown Source)
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Java Result: 1
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</source>
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More info at [[Covariance]]. For possible trick around this behavior see [[Erasure]].

Revision as of 17:33, 19 October 2011

Everything one would ever like to know about Covariance and Contravariance is available at wikipedia. Here is just a small example how it does not work in Java (the argumentation is the same as in case of Covariance). First version would like to be fixed:

Code from Contravariance.java:
See the whole file.

public static boolean isPositive(Integer aNumber) {
    return aNumber > 0;
}
 

However when one tries to change the parameter type:

Code from Contravariance.java:
See the whole file.

public static boolean isPositive(Number aNumber) {
    return aNumber.doubleValue() > 0.0d;
}
 

Existing code would compile against the new library. However because the rules for looking up the right method are different between JavaC and JVM code like following will have problems:

Code from ContravarianceTest.java:
See the whole file.

public static void main(String[] args) {
    boolean positive = Contravariance.isPositive(10);
    boolean negative = Contravariance.isPositive(-5);
    assert positive : "10 is positive";
    assert !negative : "-5 is negative";
}
 

If compiled against version 1.0 and executed against version 2.0, it will throw linkage errors: 

Mixing should not work: Compiling with version 1.0 and running against version 2.0 fails.
Exception in thread "main" java.lang.NoSuchMethodError: api.Contravariance.isPositive(Ljava/lang/Integer;)Z
	at test.ContravarianceTest.main(Unknown Source)
Java Result: 1

More info at Covariance. For possible trick around this behavior see Erasure.

Retrieved from "http://wiki.apidesign.org/wiki/Contravariance"
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