.error(throwable -> { /* handle error */ });
```

---

## Important Notes

### Thread Safety
- **Suggester**: Thread-safe for queries
- **SuggestIndexer**: Thread-safe for indexing
- **SuggestSettings**: NOT thread-safe for modifications

### Performance
- Use batch operations for indexing (100-500 items)
- Don't call `refresh()` too frequently

   * Returns an immutable map containing the same entries as {@code map}, sorted by the natural
   * ordering of the keys.
   *
   * <p>Despite the method name, this method attempts to avoid actually copying the data when it is
   * safe to do so. The exact circumstances under which a copy will or will not be performed are
   * undocumented and subject to change.
   *

 * <p>
 * <strong>Thread-Safety:</strong> This class is <strong>NOT thread-safe</strong>.
 * It extends {@link LinkedHashMap} without synchronization. If multiple threads access
 * an instance concurrently, and at least one thread modifies the map structurally,
 * it must be synchronized externally.
 * </p>
 * <p>
 * For thread-safe usage, wrap with {@link java.util.Collections#synchronizedMap(Map)}:
 * </p>
 * <pre>

   * cast:
   *
   * <pre>
   *   {@code @SuppressWarnings("unchecked") // guaranteed by invokeAll contract}
   *   {@code List<ListenableFuture<T>> futures = (List) executor.invokeAll(tasks);}
   * </pre>
   *
   * @return A list of {@code ListenableFuture} instances representing the tasks, in the same

  boolean isEmpty() {
    // The casts are safe because of the has*Bound() checks.
    return (hasUpperBound() && tooLow(uncheckedCastNullableTToT(getUpperEndpoint())))
        || (hasLowerBound() && tooHigh(uncheckedCastNullableTToT(getLowerEndpoint())));
  }

  boolean tooLow(@ParametricNullness T t) {
    if (!hasLowerBound()) {
      return false;
    }
    // The cast is safe because of the hasLowerBound() check.

  boolean isEmpty() {
    // The casts are safe because of the has*Bound() checks.
    return (hasUpperBound() && tooLow(uncheckedCastNullableTToT(getUpperEndpoint())))
        || (hasLowerBound() && tooHigh(uncheckedCastNullableTToT(getLowerEndpoint())));
  }

  boolean tooLow(@ParametricNullness T t) {
    if (!hasLowerBound()) {
      return false;
    }
    // The cast is safe because of the hasLowerBound() check.

 * useForNull} has no effect on the instance it is invoked on! You must store and use the new joiner
 * instance returned by the method. This makes joiners thread-safe, and safe to store as {@code
 * static final} constants.
 *
 * {@snippet :
 * // Bad! Do not do this!
 * Joiner joiner = Joiner.on(',');
 * joiner.skipNulls(); // does nothing!
 * return joiner.join("wrong", null, "wrong");

  private transient volatile @Nullable CacheEntry<K, V> cacheEntry2;

  MapRetrievalCache(Map<K, V> backingMap) {
    super(backingMap);
  }

  @SuppressWarnings("unchecked") // Safe because we only cast if key is found in map.
  @Override
  @Nullable V get(Object key) {
    checkNotNull(key);
    V value = getIfCached(key);
    if (value != null) {
      return value;
    }

      bufferSize = 1;
    } else if (bufferSize < k) {
      buffer[bufferSize++] = elem;
      // uncheckedCastNullableTToT is safe because bufferSize > 0.
      if (comparator.compare(elem, uncheckedCastNullableTToT(threshold)) > 0) {
        threshold = elem;
      }
      // uncheckedCastNullableTToT is safe because bufferSize > 0.
    } else if (comparator.compare(elem, uncheckedCastNullableTToT(threshold)) < 0) {

    Escaper escaper = Escapers.builder().setUnsafeReplacement("X").build();
    assertThat(escaper.escape("\0\uFFFF")).isEqualTo("\0\uFFFF");
  }

  public void testBuilderRetainsState() {
    // Setting a safe range and unsafe replacement works as expected.
    Escapers.Builder builder = Escapers.builder();
    builder.setSafeRange('a', 'z');
    builder.setUnsafeReplacement("X");