code: protected Entry<K, V> put(@NonNull K key, @NonNull V v) {
          Entry<K, V> newEntry = new Entry<>(key, v);
          mSize++;
          if (mEnd == null) {
              mStart = newEntry;
              mEnd = mStart;
              return newEntry;
          }
  
          mEnd.mNext = newEntry;
          newEntry.mPrevious = mEnd;
          mEnd = newEntry;
          return newEntry;
  
      }
docs: Put.
  
  @param key the key
  @param v the v
  @return the entry