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POCustom equals/hash when inserting key (Guava Cache)
 

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    <p>Simply, I have to override the way in which the cache chooses the right key because some fields (e.g., timestamp, message id, etc.) shouldn't be considered when retrieving a key. <strong>I cannot modify the actual hash function</strong> of the key object because it is already used to recognize in my code. <br> Is it possibile with Guava Caches? And with a workaround? <br><br> This is my configuration:</p> <pre><code>CacheBuilder.newBuilder().maximumSize(CACHE_SIZE).recordStats(). expireAfterWrite(DEFAULT_AGE, TimeUnit.DAYS).build( new CacheLoader&lt;Request, Response&gt;() { @Override public Response load(Request request) { return request.getResponse(); } }); </code></pre> <p>And this is my hash function (used somewhere else in my code):</p> <pre><code>public int hashCode() { final int prime = 31; int result = 1; result = prime * result + code; result = prime * result + messageID; // &lt;= **this one shouldn't evaluated** result = prime * result + Arrays.hashCode(payload); result = prime * result + (int) (timestamp ^ timestamp &gt;&gt;&gt; 32); // &lt;= **this one shouldn't evaluated** result = prime * result + (type == null ? 0 : type.hashCode()); result = prime * result + version; return result; } </code></pre> <p>Btw, is this kind of cache using its own implementation of hash function (e.g., through introspection) or is it using the default one? <br><br> ** <strong>EDIT:</strong> ** <br> As pointed out in the responses, the best way to achieve this result is a wrapper class.<br> <strong>My solution</strong>:</p> <pre><code>/** * Nested class to store a request as a key in the cache. It is needed to * normalize the variable fields of the normal requests. */ private static final class CachedRequest extends Request { private static CachedRequest fromRequest(Request request) { // set only the fields that cannot change between two same requests // ... } @Override public int hashCode() { HashFunction hashFunction = Hashing.md5(); HashCode hashCode; // ... return hashCode.asInt(); } @Override public boolean equals(Object obj) { // coherent with hashCode() // ... } } </code></pre>
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    1. COi can't imagine it would be a very useful utility if it used its own hash implementation.
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    2. COalso, can you further explain what you mean by "I cannot modify my hash() function because it is already used to recognize "real" equals objects."?
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    3. COIn fact I'm not asking to add this feature, but if someone knows how to deal with this situation.
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