什么是 switch 表达式，它们与 switch 语句有何不同？

Question

作为 Java SE 12 的一部分，引入了 switch expressions，并且自 Java SE 14 起，它们已被标准化。它们与switch 语句有何不同？

Answer 1

switch 声明：

与if/else if/else 语句不同，switch 语句可以有多个可能的执行路径。 switch 适用于原始类型 byte、short、char 和 int，它们各自的包装类型（Byte、Short、Character 和 Integer），枚举类型和String 类型¹。 if-else 语句用于测试基于值范围或条件的表达式，而switch 语句用于测试仅基于单个值的表达式。

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);
    }
}

输出：

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

switch 表达式：

switch 表达式是在 Java SE 12 中引入的。但是，它仍然作为 Java SE 12 和 13 中的预览功能，最终在 Java SE 14 中实现了标准化。Like any expression,@ 987654341@ 表达式计算为单个值，并且可以在语句中使用。它还引入了“箭头case”标签，无需break 语句来防止失败。从 Java SE 15 开始，支持的数据类型没有变化（在上面的 switch 语句部分中提到）。

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);
    }
}

输出：

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

switch 表达式与yield：

从 Java SE 13 开始，您可以使用 yield 语句而不是箭头运算符 (->) 从 switch 表达式返回值。

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);
    }
}

输出：

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

---

¹ JDK 7 添加了对String 的支持

Answer 2

写得真好！但我也可以为单个案例语句添加多个案例的能力。下面的例子非常做作（有很多更好的方法可以实现）。它对字符串中的元音、数字、辅音和其他字符进行简单的频率计数。

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]);

打印

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

Answer 3

添加到现有答案：yield 也可以与 -> 一起使用，其主要目的是允许在单个表达式不足以满足给定情况时使用块：

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

我还要提到JEP-354，其中提出和描述了开关表达式。
可以在Java Language Specification 中找到正式规范。