*     the same lambda function as passed into {@link #using(Function)}.
 *     <p>This class is scheduled to be removed in October 2019.
 */
// TODO(b/68134636): Remove by 2019-10
@Deprecated
@Beta
@GwtCompatible
@ElementTypesAreNonnullByDefault
public abstract class TreeTraverser<T> {

  /**
   * Returns a tree traverser that uses the given function to navigate from a node to its children.

sequenceDiagram

participant code as Code
participant function as say_hi()
participant execute as Execute function

    rect rgba(0, 255, 0, .1)
        code ->> function: say_hi(name="Camila")
        function ->> execute: 执行函数代码
        execute ->> code: 返回结果
    end

    rect rgba(0, 255, 255, .1)
        code ->> function: say_hi(name="Camila")
        function ->> code: 返回存储的结果
    end

        </ul>
        <script>
        var ws = null;
            function connect(event) {
                var itemId = document.getElementById("itemId")
                var token = document.getElementById("token")
                ws = new WebSocket("ws://localhost:8000/items/" + itemId.value + "/ws?token=" + token.value);
                ws.onmessage = function(event) {
                    var messages = document.getElementById('messages')

        </ul>
        <script>
        var ws = null;
            function connect(event) {
                var itemId = document.getElementById("itemId")
                var token = document.getElementById("token")
                ws = new WebSocket("ws://localhost:8000/items/" + itemId.value + "/ws?token=" + token.value);
                ws.onmessage = function(event) {
                    var messages = document.getElementById('messages')

# Background Tasks - `BackgroundTasks`

You can declare a parameter in a *path operation function* or dependency function with the type `BackgroundTasks`, and then you can use it to schedule the execution of background tasks after the response is sent.

You can import it directly from `fastapi`:

```python
from fastapi import BackgroundTasks
```

  public T orNull() {
    return null;
  }

  @Override
  public Set<T> asSet() {
    return Collections.emptySet();
  }

  @Override
  public <V> Optional<V> transform(Function<? super T, V> function) {
    checkNotNull(function);
    return Optional.absent();
  }

  @Override
  public boolean equals(@CheckForNull Object object) {
    return object == this;
  }

  @Override
  public int hashCode() {

        </ul>
        <script>
        var ws = null;
            function connect(event) {
                var itemId = document.getElementById("itemId")
                var token = document.getElementById("token")
                ws = new WebSocket("ws://localhost:8000/items/" + itemId.value + "/ws?token=" + token.value);
                ws.onmessage = function(event) {
                    var messages = document.getElementById('messages')

        </ul>
        <script>
        var ws = null;
            function connect(event) {
                var itemId = document.getElementById("itemId")
                var token = document.getElementById("token")
                ws = new WebSocket("ws://localhost:8000/items/" + itemId.value + "/ws?token=" + token.value);
                ws.onmessage = function(event) {
                    var messages = document.getElementById('messages')

 * **Route**: address, proxy, socketAddress
 * **TlsVersion**: javaName

#### Renamed Functions

* **Headers.of()**: for symmetry with `listOf()`, `setOf()`, etc., we’ve replaced
  `Headers.of(String...)` with `headersOf(vararg String)`.

#### Extension Functions

We’ve migrated from static functions to extension functions where we think they fit.

| Java                                | Kotlin                          |

import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
import java.util.Spliterator;
import java.util.Spliterator.OfPrimitive;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Tester for {@code Spliterator} implementations.
 *
 * @since NEXT (but since 21.0 in the JRE flavor)