android/guava/src/com/google/common/collect/ForwardingNavigableMap.java
protected K standardLastKey() {
    Entry<K, V> entry = lastEntry();
    if (entry == null) {
      throw new NoSuchElementException();
    } else {
      return entry.getKey();
    }
  }
protected K standardLastKey() {
    Entry<K, V> entry = lastEntry();
    if (entry == null) {
      throw new NoSuchElementException();
    } else {
      return entry.getKey();
    }
  }
A sensible definition of {@link #lastKey} in terms of {@code lastEntry}. If you override {@code
lastEntry}, you may wish to override {@code lastKey} to forward to this implementation.