* </ul>
   *
   * A specific warning about locking: Code that executes user-supplied tasks, such as {@code
   * ListenableFuture} listeners, should take care not to do so while holding a lock. Additionally,
   * as a further line of defense, prefer not to perform any locking inside a task that will be run

    MemoizingSupplier(Supplier<T> delegate) {
      this.delegate = checkNotNull(delegate);
    }

    @Override
    @ParametricNullness
    public T get() {
      // A 2-field variant of Double Checked Locking.
      if (!initialized) {
        synchronized (this) {
          if (!initialized) {
            T t = delegate.get();
            value = t;
            initialized = true;
            return t;

    assertEquals(4, list.lastIndexOf("b"));
  }

  public void testCopyOfDefensiveCopy() {
    // Depending on JDK version, either toArray() or toArray(T[]) may be called... use this class
    // rather than mocking to ensure that one of those methods is called.
    class TestArrayList<E> extends ArrayList<E> {
      boolean toArrayCalled = false;

      @Override
      public Object[] toArray() {
        toArrayCalled = true;

    MemoizingSupplier(Supplier<T> delegate) {
      this.delegate = checkNotNull(delegate);
    }

    @Override
    @ParametricNullness
    public T get() {
      // A 2-field variant of Double Checked Locking.
      if (!initialized) {
        synchronized (this) {
          if (!initialized) {
            T t = delegate.get();
            value = t;
            initialized = true;
            return t;

      return "Ordering.arbitrary()";
    }

    /*
     * We need to be able to mock identityHashCode() calls for tests, because it
     * can take 1-10 seconds to find colliding objects. Mocking frameworks that
     * can do magic to mock static method calls still can't do so for a system
     * class, so we need the indirection. In production, Hotspot should still