What is the Graph class and createExpensiveGraph() method in the Manual Population example of Caffeine Cache:
Cache<Key, Graph> cache = Caffeine.newBuilder()
.expireAfterWrite(10, TimeUnit.MINUTES)
.maximumSize(10_000)
.build();
// Lookup an entry, or null if not found
Graph graph = cache.getIfPresent(key);
// Lookup and compute an entry if absent, or null if not computable
graph = cache.get(key, k -> createExpensiveGraph(key));
// Insert or update an entry
cache.put(key, graph);
// Remove an entry
cache.invalidate(key);
https://github.com/ben-manes/caffeine/wiki/Population
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