java - 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，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语句:
与if/else if/else语句不同，switch语句可以具有许多可能的执行路径。 switch使用原始类型byte，short，char和int，它们各自的包装器类型(Byte，Short，Character和Integer)，枚举类型和String type1起作用。虽然if-else语句用于基于值或条件的范围来测试表达式，但是switch语句用于仅基于单个值来测试表达式。
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，它保留为Java SE 12和13中的 预览功能，并最终通过Java SE 14进行了标准化。陈述。它还引入了“arrow switch”标签，从而消除了对 case语句的预防需求。从Java SE 15开始，支持的数据类型没有变化(在上面的 break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.

带有 switch的 switch表达式:
从Java SE 13开始，您可以使用 yield语句而不是箭头运算符(->)从 yield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中添加了对 switch的支持