*
   * @since 21.0
   */
  public static class InternerBuilder {
    private final MapMaker mapMaker = new MapMaker();
    private boolean strong = true;

    private InternerBuilder() {}

    /**
     * Instructs the {@link InternerBuilder} to build a strong interner.
     *
     * @see Interners#newStrongInterner()
     */
    public InternerBuilder strong() {
      this.strong = true;
      return this;
    }

   *
   * @since 21.0
   */
  public static class InternerBuilder {
    private final MapMaker mapMaker = new MapMaker();
    private boolean strong = true;

    private InternerBuilder() {}

    /**
     * Instructs the {@link InternerBuilder} to build a strong interner.
     *
     * @see Interners#newStrongInterner()
     */
    public InternerBuilder strong() {
      this.strong = true;
      return this;
    }

  }

  public void testConcurrencyLevel_zero() {
    Interners.InternerBuilder builder = Interners.newBuilder();
    assertThrows(IllegalArgumentException.class, () -> builder.concurrencyLevel(0));
  }

  public void testConcurrencyLevel_negative() {
    Interners.InternerBuilder builder = Interners.newBuilder();
    assertThrows(IllegalArgumentException.class, () -> builder.concurrencyLevel(-42));

  }

  public void testConcurrencyLevel_zero() {
    Interners.InternerBuilder builder = Interners.newBuilder();
    assertThrows(IllegalArgumentException.class, () -> builder.concurrencyLevel(0));
  }

  public void testConcurrencyLevel_negative() {
    Interners.InternerBuilder builder = Interners.newBuilder();
    assertThrows(IllegalArgumentException.class, () -> builder.concurrencyLevel(-42));