@ParametricNullness
    @SuppressWarnings("unchecked") // Cast from Supplier<Void> to Supplier<T> is always valid
    public T get() {
      // Because Supplier is read-heavy, we use the "double-checked locking" pattern.
      if (delegate != SUCCESSFULLY_COMPUTED) {
        synchronized (this) {
          if (delegate != SUCCESSFULLY_COMPUTED) {
            T t = delegate.get();
            value = t;

 * override one or more methods to modify the behavior of the backing map as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p><b>Warning:</b> The methods of {@code ForwardingNavigableMap} forward <i>indiscriminately</i>
 * to the methods of the delegate. For example, overriding {@link #put} alone <i>will not</i> change

    checkNotNull(t);
    /*
     * TODO(cpovirk): Consider optimizing iterator() to catch IOOBE instead of doing bounds checks.
     *
     * TODO(cpovirk): Consider the UnsignedBytes pattern if it performs faster and doesn't cause
     * AOSP grief.
     */
    return new AbstractList<StackTraceElement>() {
      /*
       * The following requireNonNull calls are safe because we use jlaStackTrace() only if

    checkNotNull(t);
    /*
     * TODO(cpovirk): Consider optimizing iterator() to catch IOOBE instead of doing bounds checks.
     *
     * TODO(cpovirk): Consider the UnsignedBytes pattern if it performs faster and doesn't cause
     * AOSP grief.
     */
    return new AbstractList<StackTraceElement>() {
      /*
       * The following requireNonNull calls are safe because we use jlaStackTrace() only if

   * for more important information about the Java 8+ transition.
   */
  public static Function<Object, String> toStringFunction() {
    return ToStringFunction.INSTANCE;
  }

  // enum singleton pattern
  private enum ToStringFunction implements Function<Object, String> {
    INSTANCE;

    @Override
    public String apply(Object o) {
      checkNotNull(o); // eager for GWT.
      return o.toString();
    }

   *
   * The coding pattern above is heavily used in java.util, e.g. in ArrayList. There is a
   * RangeCheckMicroBenchmark in the JDK that was used to test this.
   *
   * But the methods in this class want to throw different exceptions, depending on the args, so it
   * appears that this pattern is not directly applicable. But we can use the ridiculous, devious

 * Running on Java 6, exceptions that cannot be thrown must be logged rather than being added to the
 * thrown exception as a suppressed exception.
 *
 * <p>This class is intended to be used in the following pattern:
 *
 * <pre>{@code
 * Closer closer = Closer.create();
 * try {
 *   InputStream in = closer.register(openInputStream());
 *   OutputStream out = closer.register(openOutputStream());
 *   // do stuff

 * override one or more methods to modify the behavior of the backing map as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p><b>Warning:</b> The methods of {@code ForwardingNavigableMap} forward <i>indiscriminately</i>
 * to the methods of the delegate. For example, overriding {@link #put} alone <i>will not</i> change

 *       approaches to this</a> that should be considered first)
 * </ul>
 *
 * <p>A common alternative to using this class is to find or create a suitable <a
 * href="http://en.wikipedia.org/wiki/Null_Object_pattern">null object</a> for the type in question.
 *
 * <p>This class is not intended as a direct analogue of any existing "option" or "maybe" construct
 * from other programming environments, though it may bear some similarities.
 *

        // Pattern uses Object (identity) equality, so we have to reach
        // inside to compare individual fields.
        return Objects.equal(pattern.pattern(), that.pattern.pattern())
            && pattern.flags() == that.pattern.flags();
      }
      return false;
    }

    @Override
    public String toString() {
      String patternString =