为什么变量在case语句中不是本地的？ [英] Why are variables not local in case statements?

问题描述

我最近在android java应用程序中添加了另一个菜单项，并且很惊讶Eclipse说来自前一个案例的变量：break不是本地的（所以我刚刚添加了一个后缀来获取）。

I recently add another menu item to an android java app and was suprised that Eclipse said that variable from the previous case:break were not local (So I've just added a suffix to get by).

我有点困惑，在我看来，第一组案例：如果选择了第二个选项，则根本不会执行中断。有人可以解释我的错误想法吗？

I'm a bit confused as in my mind, the 1st set of case:break would not be executed at all if the 2nd option was chosen. Could someone explain my faulty thinking please?

        case R.id.menuDebugMode:
            debugMode = !debugMode;
            if (debugMode){
                Toast.makeText(mainActivity.this, "Debug Mode on - NOT TO BE USED WHILST DRIVING", Toast.LENGTH_LONG).show();           
            } else {
                tvDebug.setText("");
                tvInfo.setText("");
            }
            SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0);
            SharedPreferences.Editor editor = settings.edit();
            editor.putBoolean("debugMode", debugMode);
            editor.commit();
        break;

        case R.id.menuSpeedMode:
            speedSignMode = !speedSignMode;
            if (speedSignMode){
                Toast.makeText(mainActivity.this, "SpeedSign Mode in use", Toast.LENGTH_LONG).show();           

            } else {
                    Toast.makeText(mainActivity.this, "MapSpeed Mode in use", Toast.LENGTH_LONG).show();            
            }
            SharedPreferences settings2 = getSharedPreferences(PREFS_NAME, 0);
            SharedPreferences.Editor editor2 = settings2.edit();
            editor2.putBoolean("speedSignMode", speedSignMode);
            editor2.commit();
        break;`

推荐答案

与C一样，在Java中，switch语句不是人们在查看它时所期望的。压力使得很难理解未创建范围。这一切都归结为C，其中一个开关只是语法糖。编译器将切换转换为许多条件跳转。这使得语言能够使用直通，这是在设计C期间的特征（中断仍然是可选的）。此Java功能仍与C兼容。

As in C, in Java a switch statement is not what one would expect when looking at it. The indendation makes it difficult to understand that a scope is not created. This all boils down to C, where a switch is just syntactic sugar. The compiler transforms a switch into a number of conditional jumps. This enables the language to use fall-through, a feature that during the design of C was intended ("break" remained optional). This Java feature remained compatible to C.

switch(a):
    case 1:
        dosomething();
    case 2:
        dosomemore();

被翻译成

if(a==1) jump ##1;
if(a==2) jump ##2;
jump ##3;
##1:
dosometing();
##2:
dosomemore();
##3:

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