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POlimit map key and value types - more complicated
 

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    <p>I have a more complicated issue (than question 'Java map with values limited by key's type parameter' question) for mapping key and value type in a Map. Here it is:</p> <pre><code>interface AnnotatedFieldValidator&lt;A extends Annotation&gt; { void validate(Field f, A annotation, Object target); Class&lt;A&gt; getSupportedAnnotationClass(); } </code></pre> <p>Now, I want to store validators in a map, so that I can write the following method:</p> <pre><code>validate(Object o) { Field[] fields = getAllFields(o.getClass()); for (Field field: fields) { for (Annotation a: field.getAnnotations()) { AnnotatedFieldValidator validator = validators.get(a); if (validator != null) { validator.validate(field, a, target); } } } } </code></pre> <p>(type parameters are omitted here, since I do not have the solution). I also want to be able to register my validators:</p> <pre><code>public void addValidator(AnnotatedFieldValidator&lt;? extends Annotation&gt; v) { validators.put(v.getSupportedAnnotatedClass(), v); } </code></pre> <p>With this (only) public modifier method, I can ensure the map contains entries for which the key (annotation class) matches the validator's supported annotation class.</p> <p>Here is a try:</p> <p>I declare the validators Map like this:</p> <pre><code>private Map&lt;Class&lt;? extends Annotation&gt;, AnnotatedFieldValidator&lt;? extends Annotation&gt;&gt; validators; </code></pre> <p>I'm aware I cannot properly link the key and value (link is assumed OK due to only access through <code>addValidator()</code>), so I tried a cast:</p> <pre><code>for (Annotation a: field.getAnnotations()) { AnnotatedFieldValidator&lt;? extends Annotation&gt; validator = validators.get(a); if (validator != null) { validator.validate(field, validator.getSupportedAnnotationClass().cast(a), target); } } </code></pre> <p>But this does not work: <code>The method validate(Field, capture#8-of ?, Object) in the type AnnotatedFieldValidator&lt;capture#8-of ?&gt; is not applicable for the arguments (Field, capture#9-of ?, Object)</code>.</p> <p>I can't figure out why this does not work: the <code>AnnotatedFieldValidator</code> has a single type parameter (A), which is used both as the return type of <code>getSupportedAnnotationClass()</code> and as a parameter of <code>validate()</code>; thus, when casting the annotation to supportedAnnotationClass, I should be able to pass it as the parameter to <code>validate()</code>. Why is the result of <code>getSupportedAnnotationClass()</code> considered a different type than the parameter of <code>validate()</code>?</p> <p>I can solve the <code>validate()</code> method by removing wildcards in the validators declaration and <code>validate()</code> method, but then, of course, <code>addValidator()</code> doesn't compile.</p>
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