@REM
@REM   JAVA_HOME           (Optional) Points to a Java installation.
@REM   MAVEN_BATCH_ECHO    (Optional) Set to 'on' to enable the echoing of the batch commands.
@REM   MAVEN_BATCH_PAUSE   (Optional) set to 'on' to wait for a key stroke before ending.
@REM   MAVEN_OPTS          (Optional) Java runtime options used when Maven is executed.
@REM   MAVEN_SKIP_RC       (Optional) Flag to disable loading of mavenrc files.

			Optional:    true,
			Type:        "path",
			Sensitive:   true,
		},
		config.HelpKV{
			Key:         ClientCertKey,
			Description: `client cert key for mTLS authentication` + defaultHelpPostfix(ClientCertKey),
			Optional:    true,
			Type:        "path",
			Sensitive:   true,
		},
		config.HelpKV{
			Key:         config.Comment,
			Description: config.DefaultComment,
			Optional:    true,

 * under the License.
 */
package org.apache.maven.graph;

import java.io.File;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Optional;
import java.util.Set;
import java.util.stream.Collectors;

import org.apache.maven.MavenExecutionException;
import org.apache.maven.execution.MavenExecutionRequest;
import org.apache.maven.execution.ProjectActivation;

   *
   * <p><b>Java 9 users:</b> use {@code optional.stream()} instead.
   */
  @Beta
  @InlineMe(replacement = "optional.stream()")
  @InlineMeValidationDisabled("Java 9+ API only")
  public static <T> Stream<T> stream(java.util.Optional<T> optional) {
    return optional.isPresent() ? Stream.of(optional.get()) : Stream.empty();
  }

  /**
   * If a value is present in {@code optional}, returns a stream containing only that element,

     * @param active Should the project be activated?
     * @param optional Can the build continue if the project does not exist?
     */
    public void addProjectActivation(String selector, boolean active, boolean optional) {
        final ActivationSettings settings = ActivationSettings.of(active, optional);
        this.activations.add(new ProjectActivationSettings(selector, settings));
    }

- 代わりに ✨ **`Union[SomeType, None]` を使いましょう** ✨。

どちらも等価で内部的には同一ですが、「optional（任意）」という語が値が任意だと誤解させやすく、実際の意味は「`None` を取り得る」であり、任意ではなく依然として必須である場合でもそうです。そのため `Optional` より `Union` を勧めます。

`Union[SomeType, None]` の方が意味がより明確だと思います。

これは用語や名前付けの話に過ぎませんが、その言葉があなたやチームメイトのコードの捉え方に影響します。

例として次の関数を見てみましょう:

```python
from typing import Optional


def say_hi(name: Optional[str]):
    print(f"Hey {name}!")
```

			Description: `name for the site e.g. "cal-rack0"`,
			Optional:    true,
		},
		HelpKV{
			Key:         RegionKey,
			Type:        "string",
			Description: `name of the location of the server e.g. "us-west-1"`,
			Optional:    true,
		},
		HelpKV{
			Key:         Comment,
			Type:        "sentence",
			Description: DefaultComment,
			Optional:    true,
		},
	}

	RegionHelp = HelpKVS{
		HelpKV{

			Optional:    true,
			Type:        "csv",
		},
		config.HelpKV{
			Key:         AllowEncrypted,
			Description: `enable 'encryption' along with compression`,
			Optional:    true,
			Type:        "on|off",
		},
		config.HelpKV{
			Key:         config.Comment,
			Description: config.DefaultComment,
			Optional:    true,
			Type:        "sentence",
		},
	}

   * @throws NullPointerException if any element in the stream is {@code null}.
   * @return {@code Optional.of(onlyElement)} if the stream has exactly one element (must not be
   *     {@code null}) and returns {@code Optional.empty()} if it has none.
   */
  @SuppressWarnings("unchecked")
  public static <T> Collector<T, ?, Optional<T>> toOptional() {
    return (Collector) TO_OPTIONAL;
  }

            @Nonnull
            @Override
            public Optional<Source> getInstallationToolchainsSource() {
                return Optional.ofNullable(installationToolchainsSource);
            }

            @Nonnull
            @Override
            public Optional<Source> getUserToolchainsSource() {
                return Optional.ofNullable(userToolchainsSource);
            }

            @Override