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