什么是 switch 表達式，它們與 switch 語句有何不同？

Question

作為 Java SE 12 的一部分，引入了switch表達式，並且自 Java SE 14 以來，它們已被標准化。 它們與switch語句有何不同？

Answer 1

switch語句：

與if/else if/else語句不同， switch語句可以有許多可能的執行路徑。 switch適用於原始類型byte 、 short 、 char和int 、它們各自的包裝類型（ Byte 、 Short 、 Character和Integer ）、枚舉類型和String類型¹ 。 if-else語句用於測試基於值或條件范圍的表達式，而switch語句用於測試僅基於單個值的表達式。

演示

enum PaymentStatus {
    UNPAID, PARTPAID, PAID, DISPUTED, UNKNOWN;
}

public class Main {
    public static void main(String[] args) {
        String message = "";
        PaymentStatus paymentStatus = PaymentStatus.PARTPAID;

        switch (paymentStatus) {
        case UNPAID:
            message = "The order has not been paid yet. Please make the minimum/full amount to procced.";
            break;
        case PARTPAID:
            message = "The order is partially paid. Some features will not be available. Please check the brochure for details.";
            break;
        case PAID:
            message = "The order is fully paid. Please choose the desired items from the menu.";
            break;
        default:
            throw new IllegalStateException("Invalid payment status: " + paymentStatus);
        }
        System.out.println(message);
    }
}

Output：

The order is partially paid. Some features will not be available. Please check the brochure for details.

switch表達式：

The switch expression was introduced with Java SE 12. However, it remained as a Preview feature in Java SE 12 and 13 and finally got standardized with Java SE 14. Like any expression , switch expressions evaluate to a single value, and can be used in陳述。 它還引入了“箭頭case ”標簽，無需使用break語句來防止失敗。 從 Java SE 15 開始，支持的數據類型沒有變化（在上面的switch語句部分中提到）。

演示

enum PaymentStatus {
    UNPAID, PARTPAID, PAID, DISPUTED, UNKNOWN;
}

public class Main {
    public static void main(String[] args) {
        PaymentStatus paymentStatus = PaymentStatus.PARTPAID;

        String message = switch (paymentStatus) {
        case UNPAID -> "The order has not been paid yet. Please make the minimum/full amount to procced.";
        case PARTPAID -> "The order is partially paid. Some features will not be available. Please check the brochure for details.";
        case PAID -> "The order is fully paid. Please choose the desired items from the menu.";
        default -> throw new IllegalStateException("Invalid payment status: " + paymentStatus);
        };

        System.out.println(message);
    }
}

Output：

The order is partially paid. Some features will not be available. Please check the brochure for details.

帶有yield的switch表達式：

從 Java SE 13 開始，您可以使用yield語句而不是箭頭運算符 (->) 從switch表達式返回值。

演示

enum PaymentStatus {
    UNPAID, PARTPAID, PAID, DISPUTED, UNKNOWN;
}

public class Main {
    public static void main(String[] args) {
        PaymentStatus paymentStatus = PaymentStatus.PARTPAID;

        String message = switch (paymentStatus) {
        case UNPAID:
            yield "The order has not been paid yet. Please make the minimum/full amount to procced.";
        case PARTPAID:
            yield "The order is partially paid. Some features will not be available. Please check the brochure for details.";
        case PAID:
            yield "The order is fully paid. Please choose the desired items from the menu.";
        default:
            throw new IllegalStateException("Invalid payment status: " + paymentStatus);
        };

        System.out.println(message);
    }
}

Output：

The order is partially paid. Some features will not be available. Please check the brochure for details.

---

¹ JDK 7 添加了對String的支持

Answer 2

不錯的文案。 但我也可以為單個 case 語句添加多個 case 的功能。 下面的例子非常做作（有很多更好的方法可以實現），它對元音、數字、輔音進行了簡單的頻率計數。 和字符串中的其他字符。

int count[] = new int[4];

String s = "829s2bi9jskj*&@)(so2i2ksso";

for (char c : s.toCharArray()) {
      int i = switch (c) {
                case  'a', 'e', 'i', 'o', 'u' -> 0;
                case  '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' -> 1;
                case  'b', 'c', 'd', 'f', 'g', 'h', 'j', 'k', 'l',
                      'm', 'n', 'p', 'q', 'r', 's', 't', 'v', 'w',
                      'x', 'y', 'z' -> 2;
                default -> 3;
      };
      count[i]++;
}
System.out.printf("vowels  - %d%n", count[0]);
System.out.printf("digits  - %d%n", count[1]);
System.out.printf("consonants - %d%n", count[2]);
System.out.printf("other   - %d%n", count[3]);

印刷

vowels  - 4
digits  - 7
consonants - 10
other   - 5

Answer 3

添加到現有答案： yield也可以與->一起使用，其主要目的是允許在單個表達式不足以滿足給定情況時使用塊：

var test = switch (value) {
    case A -> 1;
    case B -> 2;
    case C -> {
        System.err.println("neither A nor B"); // or some calculation
        yield -1;
    }
}

我還要提到JEP-354 ，其中提出並描述了 switch 表達式。
可以在Java 語言規范中找到正式規范。

Answer 4

除了上述答案之外，還可以使用 switch 表達式解決 switch 語句來查看當前的“不足”：

開關語句：

• 問題 1：意外跌落
• 問題 2：變量作用域
• 問題 3：沒有返回值

開關表達式：

• 沒有失敗
• 組合常量
• 屏蔽 scope
• 回報率

例子：

開關語句：

private static void oldSwitchStatement(String month) {

    int i = 0;
    String quarter = "";

    switch (month) {
        case "JAN":
        case "FEB":
        case "MAR":
            i = i + 1;
            quarter = "Q1";
            // break;
        case "APR":
        case "MAY":
        case "JUN":
            i = i + 2;
            quarter = "Q2";
            // break;
        case "JUL":
        case "AUG":
        case "SEP":
            i = i + 3;
            quarter = "Q3";
            // break;
        case "OCT":
        case "NOV":
        case "DEC":
            i = i + 4;
            quarter = "Q4";
        default:
            System.out.println("Unknown case");
    }

    System.out.println("QUARTER: "+ i + " "+ quarter);
}

切換表達式

private static String newSwitchExpressionYield(String month) {

    return switch (month) {
        case "JAN", "FEB", "MAR" -> "Q1";
        case "APR", "MAY", "JUN" -> {
            System.out.println("Using yield.");
            yield "Q2";
        }
        case "JUL", "AUG", "SEP" -> "Q3";
        case "OCT", "NOV", "DEC" -> "Q4";
        default -> "Unknown case";
    };
}

要測試以上通過oldSwitchStatement("APR"); 和newSwitchExpressionYield("APR"); 並查看結果。