guava/src/com/google/common/collect/ImmutableMap.java
public static <K, V> ImmutableMap<K, V> of(K k1, V v1) {
    return ImmutableBiMap.of(k1, v1);
  }
public static <K, V> ImmutableMap<K, V> of(K k1, V v1) {
    return ImmutableBiMap.of(k1, v1);
  }
Returns an immutable map containing a single entry. This map behaves and performs comparably to
{@link Collections#singletonMap} but will not accept a null key or value. It is preferable
mainly for consistency and maintainability of your code.