break和switch似乎执行所有case语句 [英] break and switch appears to execute all case statements

问题描述

在Java和Eclipse（Kempler）的最新稳定版本中，输入以下代码并执行它，假设包和类名存在：

In the latest stable release of Java and Eclipse (Kempler), entering the following code and executing it, assuming the package and class names exist:

package some_package;

public class what_the_heck {

    public static void main(String[] args) {
        int p = 2;
        int x = 1;
        switch(p){  
            case (1):
                x--;
            case (2):
                x = 2;
            case (3):
                x = 3;
            default:
                x++;
        }
        System.out.println(x);
    }
}

这打印值4.原来，我认为它应该打印2因为我认为即使没有break语句，每段代码仍然保存在case语句中。现在我认为问题在于如何编译。例如，我目前的信念是内部布尔值跟踪case语句是否等于该值。如果是，则布尔值为true，所有case语句将被视为true ，直到找到中断为止。这是有道理的，但我仍然想知道这背后是否有其他原因，或者我的假设完全错误。

This prints the value 4. Originally, I thought it should print 2 because I thought that even if there were no break statements, each piece of code is still held in a case statement. Now I think that the issue lies in how it is compiled. For example, my current belief is that internally a boolean keeps track of whether or not a case statement was equal to the value. If it was, then the boolean is true and all case statements will be treated as true until a break is found. This makes sense, but I am still wondering if there are other reasons behind this, or if I am entirely wrong in my assumption.

推荐答案

开关正常工作的原因是：

The reason why switch works as it does is that this:

switch(p){  
        case (1):
            x--;
        case (2):
            x = 2;
        case (3):
            x = 3;
        default:
            x++;
    }

真的只是语法糖（基本上）：

is really just syntactic sugar for this (basically):

if (p == 1)
    goto .L1;
else if (p == 2)
    goto .L2;
else if (p == 3)
    goto .L3;
else
    goto .L4;

.L1:
    x--;
.L2:
    x = 2;
.L3:
    x = 3;
.L4:
    x++;

Java没有 goto 语句，但C确实如此，而这就是它的来源。因此，如果 p 为2，它将跳转到 .L2 并执行该标签后面的所有语句。

Java doesn't have a goto statement, but C does, and that's where it comes from. So if p is 2, it jumps to .L2 and executes all the statements following that label.

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