*
   * @since 8.0
   */
  public static <E> Function<E, E> asFunction(Interner<E> interner) {
    return new InternerFunction<>(checkNotNull(interner));
  }

  private static final class InternerFunction<E> implements Function<E, E> {

    private final Interner<E> interner;

    InternerFunction(Interner<E> interner) {
      this.interner = interner;
    }

    @Override
    public E apply(E input) {

   *
   * @since 8.0
   */
  public static <E> Function<E, E> asFunction(Interner<E> interner) {
    return new InternerFunction<>(checkNotNull(interner));
  }

  private static final class InternerFunction<E> implements Function<E, E> {

    private final Interner<E> interner;

    InternerFunction(Interner<E> interner) {
      this.interner = interner;
    }

    @Override
    public E apply(E input) {

    String canonical = "a";
    String not = new String("a");

    Function<String, String> internerFunction =
        Interners.asFunction(Interners.<String>newStrongInterner());

    assertThat(internerFunction.apply(canonical)).isSameInstanceAs(canonical);
    assertThat(internerFunction.apply(not)).isSameInstanceAs(canonical);
  }

  public void testNullPointerExceptions() {

    String canonical = "a";
    String not = new String("a");

    Function<String, String> internerFunction =
        Interners.asFunction(Interners.<String>newStrongInterner());

    assertThat(internerFunction.apply(canonical)).isSameInstanceAs(canonical);
    assertThat(internerFunction.apply(not)).isSameInstanceAs(canonical);
  }

  public void testNullPointerExceptions() {