@CheckForNull
    protected char[] escape(char c) {
      return c < replaceLength ? replacements[c] : null;
    }
  }

  // Replacement mappings.
  private final Map<Character, String> map;

  // The highest index we've seen so far.
  private int max = -1;

  /** Construct a new sparse array builder. */
  public CharEscaperBuilder() {
    this.map = new HashMap<>();
  }