With Guava, you can define a simple in-memory cache with

import static java.util.concurrent.TimeUnit.DAYS;
import com.google.common.cache.Cache;
import com.google.common.cache.CacheBuilder;

Cache<K, V> cache =
    CacheBuilder.newBuilder()
        .maximumSize(100000)
        .expireAfterAccess(7, DAYS)
        .build();

With this you can use .put(K, V) to load values, and .getIfPresent(K), which returns null if the key isn't present. Sometimes it's more convenient to use get(K key, Callable<? extends V> valueLoader), where the valueLoader is called on a cache miss, and populates the cache and gives you what a cache hit would have given you. The old Java 7 way of doing this was really ugly:

cache.get(key, new Callable() {
    @Override
    public V call() {
        return calculatedValue(key);
    }
});

Don't write ugly code. With Java 8 Lambdas, just do this:

cache.get(key, () -> {
    calculatedValue(key);
}