问题描述
作为 Java SE 12 的一部分，引入了 switch expressions，并且从 Java SE 14 开始，它们已经标准化。它们与 command = subprocess.Popen(source_cmd,stdout=subprocess.PIPE,stderr=subprocess.PIPE)
command.communicate()
语句有何不同？
解决方法
switch 语句：
与 switch 语句不同，if/else if/else 语句可以有多个可能的执行路径。 switch 与原始类型 switch、byte、short 和 char 一起使用，它们各自的包装类型（int、{{ 1}}、Byte 和 Short)、枚举类型和 Character type1。 Integer 语句用于测试基于值或条件范围的表达式，而 String 语句用于测试仅基于单个值的表达式。
演示
if-else输出：
switch
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);
}
}
表达式：
The order is partially paid. Some features will not be available. Please check the brochure for details.
表达式是在 Java SE 12 中引入的。但是，它在 Java SE 12 和 13 中仍然是预览功能，并最终在 Java SE 14 中标准化。{{3 }},switch 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 switch”标签，从而无需使用 switch 语句来防止落空。从 Java SE 15 开始，支持的数据类型没有变化（在上面的 case 语句部分中提到）。
演示
break输出：
switch带有 enum PaymentStatus {
UNPAID,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);
}
}
的 The order is partially paid. Some features will not be available. Please check the brochure for details.
表达式：
从 Java SE 13 开始，您可以使用 switch 语句代替箭头运算符 (->) 从 yield 表达式返回值。
演示
yield输出：
switch1 JDK 7 添加了对 enum PaymentStatus {
UNPAID,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);
}
}
的支持
写的不错！但我可能还会为单个 case 语句添加多个 case 的功能。下面的例子非常做作（有很多更好的方法来实现它）。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]);
印刷品
vowels - 4
digits - 7
consonants - 10
other - 5
添加到现有答案：yield 也可以与 -> 一起使用，其主要目的是在单个表达式不足以满足给定情况时允许使用块：
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 switch 表达式。
可以像往常一样在 Java Language Specification 中找到正式规范。