Question

I’m trying to use Guava to simulate a LRU map.

Map<K, V> map = CacheBuilder.newBuilder()
        .maximumSize(maxSize)
        .build() // not using a cache loader
        .asMap();

But when I try do this I get an error.

Type mismatch: cannot convert from ConcurrentMap<Object,Object> to Map<K,V>

However, if I create the Map with a temporary reference to the cache it works fine.

Cache<K, V> cache = CacheBuilder.newBuilder()
        .maximumSize(maxSize)
        .build();
Map<K, V> map = cache.asMap();

Why does this work and the first sample doesn’t?

Answer 1

It’s Java’s generics fault – generic types cannot be inferred in this case. You should add <K, V> to tell Java to treat newly created cache as Cache<K, V>, not Cache<Object, Object>.

private static <K, V> Map<K, V> makeMap(final int maxSize) {
  return CacheBuilder.newBuilder()
      .maximumSize(maxSize)
      .<K, V>build() // not using a cache loader
      .asMap();
}

and then it’ll work:

final Map<Long, String> map = makeMap(10);

See this part of Angelika Langer’s Java Generics FAQs.