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<td class="text-paragraph-pattern-container mobile-resize-text " style="padding: 0px; border-collapse: collapse; padding: 0 0 10px 0"> <p style="margin: 10px 0 0 0; margin-top: 0">Thanks for the quick reply!</p> <p style="margin: 10px 0 0 0">What I wanted to say is that the validation relies on the <tt>hashCode()</tt> and <tt>hashCode()</tt> (at least in our use case) only works if the instance is already valid. So in the use case I mentioned the validation only works if the instance is already valid, defeating the purpose of validation.</p> <p style="margin: 10px 0 0 0">JSON deserialization is working fine. When we receive a JSON string, we can create an instance of our immutable class without problems. The thing is that the deserialized instance may not follow the class invariant (since it was not created through the builder that would ensure it). Then we need validation to ensure that the deserialized instance follows indeed the class invariant (or preconditions/assumptions if you prefer).</p> <p style="margin: 10px 0 0 0">Take the following (made up) example where <tt>Son</tt> is our automatically generated immutable class and <tt>SonBuilder</tt> the automatically generated builder. We cannot (for the most part) change their code.</p>
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<pre class="code-java" style="margin: 10px 0 0 0; margin-top: 0; max-height: 30em; overflow: auto; white-space: pre-wrap; word-wrap: normal">
<span class="code-keyword" style="color: #000091">public</span> <span class="code-keyword" style="color: #000091">final</span> class Son {
@NotNull
<span class="code-keyword" style="color: #000091">private</span> Father father;
<span class="code-keyword" style="color: #000091">public</span> Father getFather() {
<span class="code-keyword" style="color: #000091">return</span> father;
}
@Override
<span class="code-keyword" style="color: #000091">public</span> <span class="code-object" style="color: #910091">int</span> hashCode() {
<span class="code-object" style="color: #910091">int</span> h = 31;
<span class="code-keyword" style="color: #000091">return</span> h * father.hashCode(); <span class="code-comment" style="color: #808080">// May <span class="code-keyword" style="color: #000091; color: #808080">throw</span> NullPointerException <span class="code-keyword" style="color: #000091; color: #808080">if</span> class invariant is violated.
</span> }
@Override
<span class="code-keyword" style="color: #000091">public</span> <span class="code-object" style="color: #910091">boolean</span> equals(<span class="code-object" style="color: #910091">Object</span> o) {
<span class="code-comment" style="color: #808080">// ...
</span> }
}
<span class="code-keyword" style="color: #000091">public</span> <span class="code-keyword" style="color: #000091">final</span> class SonBuilder {
<span class="code-comment" style="color: #808080">// ...
</span> <span class="code-comment" style="color: #808080">// Methods <span class="code-keyword" style="color: #000091; color: #808080">for</span> setting a father and other helper methods.
</span> <span class="code-comment" style="color: #808080">// ...
</span>
<span class="code-keyword" style="color: #000091">public</span> Son build() {
Son son = <span class="code-keyword" style="color: #000091">new</span> Son();
<span class="code-comment" style="color: #808080">// Set fields of son ...
</span> checkInvariant(son); <span class="code-comment" style="color: #808080">// Ensures the class invariant (i.e. father != <span class="code-keyword" style="color: #000091; color: #808080">null</span>).
</span> <span class="code-keyword" style="color: #000091">return</span> son;
}
}
</pre>
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</div> <p style="margin: 10px 0 0 0">If we use the builder, we can be sure that the resulting <tt>Son</tt> instance is valid. However if someone sends a JSON (e.g. as part of a web service request), the deserialized instance may be invalid since deserialization does not use the builder. If the JSON does not contain <tt>father</tt>, the instance is deserialized just fine, but the field <tt>father</tt> is set to <tt>null</tt>. Nevertheless the web service has to ensure that it is dealing with correct instances. Hence at this point we need the additional bean validation. But since the field is <tt>null</tt> <tt>hashCode()</tt> will throw and we cannot properly validate the instance.</p> <p style="margin: 10px 0 0 0">Changing the <tt>hashCode()</tt> to circumvent the class invariant might be possible in some cases, but not in others. We think it's a not so good idea in our use case, since this could mask problems and errors in our automatically generated builders. In other words we like that <tt>hashCode()</tt> fails if the instance does not follow its invariant as this gives us the opportunity to discover errors.</p> <p style="margin: 10px 0 0 0">I think that the behavior of the validation is currently conceptually wrong because it requires immunity to invalid inner state from some methods (from <tt>hashCode()</tt> at least). But it's because of invalid inner state that we use validation: to be sure that we can safely use all the methods of the class.</p> <p style="margin: 10px 0 0 0">I hope I succeeded to clarify the context a bit more and thanks again!</p> </td>
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