CacheBuilderFactory.java 文件源码

/**
 * Sets.cartesianProduct doesn't allow sets that contain null, but we want null to mean
 * "don't call the associated CacheBuilder method" - that is, get the default CacheBuilder
 * behavior. This method wraps the elements in the input sets (which may contain null) as
 * Optionals, calls Sets.cartesianProduct with those, then transforms the result to unwrap
 * the Optionals.
 */
private Iterable<List<Object>> buildCartesianProduct(Set<?>... sets) {
  List<Set<Optional<?>>> optionalSets = Lists.newArrayListWithExpectedSize(sets.length);
  for (Set<?> set : sets) {
    Set<Optional<?>> optionalSet =
        Sets.newLinkedHashSet(Iterables.transform(set, NULLABLE_TO_OPTIONAL));
    optionalSets.add(optionalSet);
  }
  Set<List<Optional<?>>> cartesianProduct = Sets.cartesianProduct(optionalSets);
  return Iterables.transform(cartesianProduct,
      new Function<List<Optional<?>>, List<Object>>() {
        @Override public List<Object> apply(List<Optional<?>> objs) {
          return Lists.transform(objs, OPTIONAL_TO_NULLABLE);
        }
      });
}