Why are variables not local in case statements?

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暖寄归人
暖寄归人 2020-12-10 14:03

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 s

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  • 2020-12-10 14:07

    In additon to the other answers: here's the explanation from the java language definition:

    Whenever the flow of control enters a block [...], a new variable is created for each local variable declared in a local variable declaration statement immediately contained within that block [...] The local variable effectively ceases to exist when the execution of the block [...] is complete.

    A local variable scope is the block ({ ... }), which includes inner blocks. The actual block in your code is the block starting after the switch statement.

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  • 2020-12-10 14:09

    Others have explained what you should do, and that this is a Java language thing, not an Android specific thing.

    As to why the Java Language is defined this way, I haven't figured an entirely logical reason. The best I can think of is that if each of the case lists of a switch statement implicitly defined a scope, then the following could easily be misread:

    case foo:
       int var = ...
       // note drop through
    case bar:
                                  int var = ...
       var = var + 1;
       break;
    

    At least with the current definition of the scoping, all of the potentially confusing usages result in compilation errors.

    (IMO, it would have been better to eschew case drop-through in switch statements ... just like C# does. But design mistakes like that are much easier to spot in hindsight, and are hard to correct once made.)

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  • 2020-12-10 14:10

    Because you're not supposed to write much code / logic there - break these out into methods.

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  • 2020-12-10 14:11

    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();
    

    gets translated into

    if(a==1) jump ##1;
    if(a==2) jump ##2;
    jump ##3;
    ##1:
    dosometing();
    ##2:
    dosomemore();
    ##3:
    
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  • 2020-12-10 14:24

    You're right that at most one will execute, but a case does not create a new scope. You can manually create a block with its own scope.

    case foo:
        {
           int var = ...
        }
        break;
    
    case bar:
        {
           int var = ...
        }
        break;
    
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  • 2020-12-10 14:32

    Matthew is right - the whole switch statement has one scope for any variables declared directly within it. You can add more braces as per Matthew's answer - but it would almost certainly be better to extra the case bodies out as methods. It looks like they're doing rather a lot to be included "inline" like that.

    Note that the scoping rules here aren't specific to Android - they're Java's scoping rules.

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