return PRIMITIVE_TO_WRAPPER_TYPE.keySet();
  }

  /**
   * Returns an immutable set of all nine primitive-wrapper types (including {@link Void}).
   *
   * @since 3.0
   */
  public static Set<Class<?>> allWrapperTypes() {
    return WRAPPER_TO_PRIMITIVE_TYPE.keySet();
  }

  /**
   * Returns {@code true} if {@code type} is one of the nine primitive-wrapper types, such as {@link
   * Integer}.
   *

            void.class);

    assertThrows(UnsupportedOperationException.class, () -> primitives.remove(boolean.class));
  }

  public void testAllWrapperTypes() {
    Set<Class<?>> wrappers = Primitives.allWrapperTypes();
    assertThat(wrappers)
        .containsExactly(
            Boolean.class,
            Byte.class,
            Character.class,
            Double.class,
            Float.class,

            void.class);

    assertThrows(UnsupportedOperationException.class, () -> primitives.remove(boolean.class));
  }

  public void testAllWrapperTypes() {
    Set<Class<?>> wrappers = Primitives.allWrapperTypes();
    assertThat(wrappers)
        .containsExactly(
            Boolean.class,
            Byte.class,
            Character.class,
            Double.class,
            Float.class,

      Class<T> type = (Class<T>) runtimeType;
      return of(Primitives.wrap(type));
    }
    return this;
  }

  private boolean isWrapper() {
    return Primitives.allWrapperTypes().contains(runtimeType);
  }

  /**
   * Returns the corresponding primitive type if this is a wrapper type; otherwise returns {@code
   * this} itself. Idempotent.
   *
   * @since 15.0
   */

  public <T extends Integer> void testPrimitiveWrappingAndUnwrapping() {
    for (Class<?> type : Primitives.allPrimitiveTypes()) {
      assertIsPrimitive(TypeToken.of(type));
    }
    for (Class<?> type : Primitives.allWrapperTypes()) {
      assertIsWrapper(TypeToken.of(type));
    }
    assertNotPrimitiveNorWrapper(TypeToken.of(String.class));
    assertNotPrimitiveNorWrapper(TypeToken.of(Object[].class));

  public <T extends Integer> void testPrimitiveWrappingAndUnwrapping() {
    for (Class<?> type : Primitives.allPrimitiveTypes()) {
      assertIsPrimitive(TypeToken.of(type));
    }
    for (Class<?> type : Primitives.allWrapperTypes()) {
      assertIsWrapper(TypeToken.of(type));
    }
    assertNotPrimitiveNorWrapper(TypeToken.of(String.class));
    assertNotPrimitiveNorWrapper(TypeToken.of(Object[].class));