@ElementTypesAreNonnullByDefault
public abstract class ImmutableMap<K, V> implements Map<K, V>, Serializable {

  /**
   * Returns a {@link Collector} that accumulates elements into an {@code ImmutableMap} whose keys
   * and values are the result of applying the provided mapping functions to the input elements.
   * Entries appear in the result {@code ImmutableMap} in encounter order.
   *