Switch语句：为什么我在不同的情况下不能拥有相同的变量名？ Java的

I still writing the code, and it doesn't make a very big difference in a project like mine, but if I was to make something bigger, it would be a pain. here it is:

我仍在编写代码，并且它在像我这样的项目中没有产生很大的不同，但如果我要做更大的事情，那将是一个痛苦。这里是：

        case 0:
            System.out.print("Insert the N: ");
            double N = in.nextDouble();
            double mol = N / Na;
            System.out.print("There are " + mol + " mol in that sample");
            break;

        case 1:
            System.out.print("Insert the m: ");
            double m = in.nextDouble();
            System.out.print("Insert the M: ");
            double M = in.nextDouble();
            double mol = m / M;
            System.out.print("There are " + mol + " mol in that sample");
            break;

        case 2:
            System.out.print("Insert the V: ");
            double V = in.nextDouble();
            double mol = V / Vm;
            System.out.print("There are " + mol + " mol in that sample");
            break;

The first "mol" has no problem, but in case 1 and case 2, it says "Duplicate local variable mol". If I use the If statement it works. Is Java just like this or there is a way around it?

第一个“mol”没有问题，但在案例1和案例2中，它表示“重复局部变量mol”。如果我使用If语句就可以了。 Java是这样还是有办法解决它？

Thanks

谢谢

2 个解决方案

#1

That's because a case doesn't create a scope. So, both the variables in the 2 cases are in the same scope. If you want to do this, you can add braces for each case, which will create a new scope for each case.

那是因为案例没有创建范围。因此，2个案例中的两个变量都在同一范围内。如果要执行此操作，可以为每个案例添加大括号，这将为每个案例创建一个新范围。

    case 0: {
        System.out.print("Insert the N: ");
        double N = in.nextDouble();
        double mol = N / Na;
        System.out.print("There are " + mol + " mol in that sample");
        break; 
    }

    case 1: {
        System.out.print("Insert the m: ");
        double m = in.nextDouble();
        System.out.print("Insert the M: ");
        double M = in.nextDouble();
        double mol = m / M;
        System.out.print("There are " + mol + " mol in that sample");
        break;
    }

But, ideally there is no need to declare a separate local variable for each case. If you are using a variable in all the cases, that clearly signifies that variable to be declared directly inside the switch statement:

但是，理想情况下，不需要为每种情况声明一个单独的局部变量。如果在所有情况下都使用变量，那么这清楚地表示要在switch语句中直接声明的变量：

switch (someVar) {
    double mol = 0.0;

    case 0: mol = n / Na;
            break;

    case 1: mol = m / M;
            break;
}

P.S.: Can I advice you to name your variables something apart from english alphabets - n, M, N?

P.S。：我可以建议你将变量命名为英文字母 - n，M，N吗？

#2

Because those variables are there in single block may be you would have written this switch statement in some method. In a single method you can't have dup variables.

因为那些变量存在于单个块中，您可能会在某些方法中编写此switch语句。在单一方法中，您不能拥有重复变量。

#1

    case 0: {
        System.out.print("Insert the N: ");
        double N = in.nextDouble();
        double mol = N / Na;
        System.out.print("There are " + mol + " mol in that sample");
        break; 
    }

    case 1: {
        System.out.print("Insert the m: ");
        double m = in.nextDouble();
        System.out.print("Insert the M: ");
        double M = in.nextDouble();
        double mol = m / M;
        System.out.print("There are " + mol + " mol in that sample");
        break;
    }