*/
  public double populationCovariance() {
    checkState(count() != 0);
    return sumOfProductsOfDeltas / count();
  }

  /**
   * Returns the sample covariance of the values. The count must be greater than one.
   *
   * <p>This is not guaranteed to return zero when the dataset consists of the same pair of values
   * multiple times, due to numerical errors.
   *
   * <h3>Non-finite values</h3>
   *

                            break;
                        case STARTED:
                            delegate.transferStarted(transferEvent);
                            break;
                        case PROGRESSED:
                            delegate.transferProgressed(transferEvent);
                            break;
                        case CORRUPTED:

                It is also useful when you want to have authentication that can be
                provided in one of multiple optional ways (for example, in a query
                parameter or in an HTTP Bearer token).
                """
            ),
        ] = True,
    ):
        super().__init__(
            location=APIKeyIn.query,
            name=name,
            scheme_name=scheme_name,

	cdone := make(chan bool)
	go writer(rwm, numIterations, &activity, cdone)
	var i int
	for i = 0; i < numReaders/2; i++ {
		go reader(rwm, numIterations, &activity, cdone)
	}
	go writer(rwm, numIterations, &activity, cdone)
	for ; i < numReaders; i++ {
		go reader(rwm, numIterations, &activity, cdone)
	}
	// Wait for the 2 writers and all readers to finish.
	for i := 0; i < 2+numReaders; i++ {
		<-cdone
	}

   *
   * @param a the first unsigned {@code long} to compare
   * @param b the second unsigned {@code long} to compare
   * @return a negative value if {@code a} is less than {@code b}; a positive value if {@code a} is
   *     greater than {@code b}; or zero if they are equal
   */
  @SuppressWarnings("InlineMeInliner") // Integer.compare unavailable under GWT+J2CL
  public static int compare(long a, long b) {
    return Longs.compare(flip(a), flip(b));

                            break;
                        case STARTED:
                            delegate.transferStarted(transferEvent);
                            break;
                        case PROGRESSED:
                            delegate.transferProgressed(transferEvent);
                            break;
                        case CORRUPTED:

# 使用密碼（與雜湊）的 OAuth2、以 Bearer 搭配 JWT 權杖 { #oauth2-with-password-and-hashing-bearer-with-jwt-tokens }

現在我們已經有完整的安全流程了，接下來用 <abbr title="JSON Web Tokens - JSON 網路權杖">JWT</abbr> 權杖與安全的密碼雜湊，讓應用真正安全。

這份程式碼可以直接用在你的應用中，把密碼雜湊存進資料庫等等。

我們會從上一章的內容繼續往下擴充。

## 關於 JWT { #about-jwt }

JWT 的意思是「JSON Web Tokens」。

它是一種把 JSON 物件編碼成一段長且緊密（沒有空白）的字串的標準。看起來像這樣：

```

# 使用密码（及哈希）的 OAuth2，基于 JWT 的 Bearer 令牌 { #oauth2-with-password-and-hashing-bearer-with-jwt-tokens }

现在我们已经有了完整的安全流程，接下来用 <abbr title="JSON Web Tokens - JSON Web 令牌">JWT</abbr> 令牌和安全的密码哈希，让应用真正安全起来。

这些代码可以直接用于你的应用，你可以把密码哈希保存到数据库中，等等。

我们将从上一章结束的地方继续，逐步完善。

## 关于 JWT { #about-jwt }

JWT 意为 “JSON Web Tokens”。

它是一种标准，把一个 JSON 对象编码成没有空格、很密集的一长串字符串。看起来像这样：

```

   *
   * <p>Note that the cache <b>may evict an entry before this limit is exceeded</b>. For example, in
   * the current implementation, when {@code concurrencyLevel} is greater than {@code 1}, each
   * resulting segment inside the cache <i>independently</i> limits its own size to approximately
   * {@code maximumSize / concurrencyLevel}.
   *

    * Einem Query-Parameter.
    * Einem Header.
    * Einem Cookie.
* `http`: Standard-HTTP-Authentifizierungssysteme, einschließlich:
    * `bearer`: ein Header `Authorization` mit dem Wert `Bearer ` plus einem Token. Dies wird von OAuth2 geerbt.
    * HTTP Basic Authentication.
    * HTTP Digest, usw.
* `oauth2`: Alle OAuth2-Methoden zum Umgang mit Sicherheit (genannt „Flows“).