什么是 switch 表达式，它们与 switch 语句有何不同？

声明：switch

与该语句不同，一个语句可以有多个可能的执行路径。A 适用于基元类型、、、 和 、它们各自的包装类型 （、 、 和 ）、枚举类型和类型¹。语句用于基于值或条件的范围测试表达式，而语句用于仅基于单个值测试表达式。if/else if/elseswitchswitchbyteshortcharintByteShortCharacterIntegerStringif-elseswitch

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.

表达式：switch

该表达式是在 Java SE 12 中引入的。但是，它仍然是Java SE 12和13中的预览功能，并最终与Java SE 14一起标准化。与任何表达式一样，表达式的计算结果为单个值，并且可以在语句中使用。它还引入了“箭头”标签，消除了对语句的需要以防止掉落。从 Java SE 15 开始，支持的数据类型没有变化（在上面的语句部分提到）。switchswitchcasebreakswitch

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.

表达式为 ：switchyield

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

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 在 JDK 7 中添加了 对 的支持String

写得不错！但是，我还可以添加为单个案例语句设置多个案例的功能。下面的例子是非常人为的（有很多更好的方法可以实现它）。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]);

指纹

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