final HttpCacheEntry existingValue = HttpTestUtils.makeCacheEntry();
final HttpCacheEntry updatedValue = HttpTestUtils.makeCacheEntry();
final Element existingElement = new Element(key, new byte[]{});
final HttpCacheUpdateCallback callback = new HttpCacheUpdateCallback(){
@Override
public HttpCacheEntry update(final HttpCacheEntry old){
assertEquals(existingValue, old);
return updatedValue;
}