Joiner j = Joiner.on("x").useForNull("y");
    assertThrows(UnsupportedOperationException.class, j::skipNulls);
  }

  public void test_skipNulls_useForNull() {
    Joiner j = Joiner.on("x").skipNulls();
    assertThrows(UnsupportedOperationException.class, () -> j.useForNull("y"));
  }

  public void test_useForNull_twice() {
    Joiner j = Joiner.on("x").useForNull("y");

 *
 * <p>If neither {@link #skipNulls()} nor {@link #useForNull(String)} is specified, the joining
 * methods will throw {@link NullPointerException} if any given element is null.
 *
 * <p><b>Warning: joiner instances are always immutable</b>; a configuration method such as {@code
 * useForNull} has no effect on the instance it is invoked on! You must store and use the new joiner

 *
 * @author Ben Yu
 */
final class Types {

  /** Class#toString without the "class " and "interface " prefixes */
  private static final Joiner COMMA_JOINER = Joiner.on(", ").useForNull("null");

  /** Returns the array type of {@code componentType}. */
  static Type newArrayType(Type componentType) {
    if (componentType instanceof WildcardType) {

              List<Object> factoryArgs = argGroups.get(item.groupNumber).get(item.itemNumber);
              return factory.getName()
                  + "("
                  + Joiner.on(", ").useForNull("null").join(factoryArgs)
                  + ")";
            });
    tester.addEqualityGroup(instance, createInstance(factory, equalArgs));
    for (int i = 0; i < params.size(); i++) {

 *
 * @author Ben Yu
 */
final class Types {

  /** Class#toString without the "class " and "interface " prefixes */
  private static final Joiner COMMA_JOINER = Joiner.on(", ").useForNull("null");

  /** Returns the array type of {@code componentType}. */
  static Type newArrayType(Type componentType) {
    if (componentType instanceof WildcardType) {

              List<Object> factoryArgs = argGroups.get(item.groupNumber).get(item.itemNumber);
              return factory.getName()
                  + "("
                  + Joiner.on(", ").useForNull("null").join(factoryArgs)
                  + ")";
            });
    tester.addEqualityGroup(instance, createInstance(factory, equalArgs));
    for (int i = 0; i < params.size(); i++) {