Custom equals/hash when inserting key (Guava Cache)

Question

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. I cannot modify the actual hash function of the key object because it is already used to recognize in my code.
Is it possibile with Guava Caches? And with a workaround?

This is my configuration:

CacheBuilder.newBuilder().maximumSize(CACHE_SIZE).recordStats().
    expireAfterWrite(DEFAULT_AGE, TimeUnit.DAYS).build(
    new CacheLoader<Request, Response>() {
        @Override
        public Response load(Request request) { 
            return request.getResponse();
        }
    });

And this is my hash function (used somewhere else in my code):

public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + code;
    result = prime * result + messageID; // <= **this one shouldn't evaluated**
    result = prime * result + Arrays.hashCode(payload);
    result = prime * result + (int) (timestamp ^ timestamp >>> 32); // <= **this one shouldn't evaluated**
    result = prime * result + (type == null ? 0 : type.hashCode());
    result = prime * result + version;
    return result;
}

Btw, is this kind of cache using its own implementation of hash function (e.g., through introspection) or is it using the default one?

** EDIT: **
As pointed out in the responses, the best way to achieve this result is a wrapper class.
My solution:

/**
 * 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()
            // ...
    }
}

Answer 1

You could simply wrap your Request objects into CachedRequest objects, where CachedRequest would implement hashCode() and equals() based on the desired fields, and provide access to the wrapped Request.

Answer 2

I'm pretty sure, it's not possible with Guava. There are some legitimate cases for using a custom Equivalence, but they say they're too rare to be handled by the CacheBuilder and MapMaker.¹ There's even com.google.common.base.Equivalence, but it gets used only internally (see also here and here).

You need to make you own key out of the fields you want to use for the lookup, or wrap your Request in another object defining equals and hashCode the way you want.

¹The only case when something different from the default equals/hashCode gets used is with weakKeys (softKeys exists no more), and then it's the ==/System.identityHashCode combo. In no case you can choose the equivalence freely.