为什么一个不完整的 switch 表达式编译成功

Why does an incomplete switch expression compile successfully

试用 JDK/12 EarlyAccess Build 20, where the JEP-325 Switch Expressions 已集成为预览功能。表达式的示例代码(在 JEP 中也是如此):

Scanner scanner = new Scanner(System.in);
Day day = Day.valueOf(scanner.next().toUpperCase());
int i = switch (day) {
    case MONDAY,TUESDAY, WEDNESDAY:
        break 0;
    default:
        System.out.println("Second half of the week");
        // ERROR! Group doesn't contain a break with value
};

我试图按照与上一个问题中所述相同的过程,关于如何 并使用命令行执行上述代码块:

java --enable-preview -jar target/jdk12-updates-1.0.0-SNAPSHOT.jar

出乎我的意料,我收到了以下错误:

Error: Unable to initialize main class
com.Whosebug.nullpointer.expression.SwitchExpressionMustComplete
Caused by: java.lang.VerifyError: Bad local variable type Exception
Details:   Location:
    com/Whosebug/nullpointer/expression/SwitchExpressionMustComplete.main([Ljava/lang/String;)V @66: iload   
Reason:
    Type top (current frame, locals[4]) is not assignable to integer   
Current Frame:
    bci: @66
    flags: { }
    locals: { '[Ljava/lang/String;', 'java/util/Scanner', 'com/Whosebug/nullpointer/Day' }
    stack: { }   
Bytecode:
    0000000: bb00 0259 b200 03b7 0004 4c2b b600 05b8
    0000010: 0006 4db2 0007 2cb6 0008 2eaa 0000 001f
    0000020: 0000 0001 0000 0003 0000 0019 0000 0019
    0000030: 0000 0019 0336 04a7 000b b200 0912 0ab6
    0000040: 000b 1504 3eb1                        
Stackmap Table:
    append_frame(@52,Object[#2],Object[#34])
    same_frame(@58)
    same_frame(@66)

我知道文档指出代码有误,将注释替换为 break 1; 即可解决,但我的问题是:

Q1。 为什么编译阶段会成功? 编译阶段本身不应该失败吗?

Q2。我看到如此详细的错误消息的原因是什么? --enable-preview 功能是否与此有关?

这是一个已知错误。参见 JDK-8212982 有关其状态的详细信息。

This code:

public class SwitchBug { 

    static String hold(String item) { 
        return switch(item) { 
            case String s -> { System.out.println(s); } 
            default -> "temp"; 
        }; 
    } 

    public static void main(String[] args) { 
        System.out.println(hold("bug")); 
    } 
}

compiles and produces:

bug 
temp 

This program should not compile, as the first case completes normally.