为什么我不能在case语句java中返回值

Question

我正在制作一个简单的程序，它使用1 到 10之间的随机数生成一个随机数学问题。运算符也将在+,- 和 *之间随机。当我尝试使用 case 语句并返回操作值并打印问题（最后）时，它说没有操作变量。    int number1 = (int)(Math.random()* 10) + 1;    int number2 = (int)(Math.random()* 10) + 1;    int operator = (int)(Math.random()* 3) + 1;        switch (operator){            case 1: {                String operation = "+";                int correctResult = number1 + number2;                break;            }            case 2: {                String operation = "-";                int correctResult = number1 - number2;                break;            }            case 3: {                String operation = "*";                int correctResult = number1 * number2;                break;            }        }    System.out.print(number1+operation+number2+": ");    String studentAnswer = scanner.next();    

Answer 1

您需要在 od switch 块之外声明操作：

int number1 = (int)(Math.random()* 10) + 1;

int number2 = (int)(Math.random()* 10) + 1;

int operator = (int)(Math.random()* 3) + 1;

String operation = null; // move outside of switch block

int correctResult; // move outside of switch block

    switch (operator){

        case 1: {

            operation = "+";

            correctResult = number1 + number2;

            break;

        }

        case 2: {

            operation = "-";

            correctResult = number1 - number2;

            break;

        }

        case 3: {

            operation = "*";

            correctResult = number1 * number2;

            break;

        }

    }

System.out.print(number1+operation+number2+": ");

String studentAnswer = scanner.next();    

Answer 2

在外部声明参数并在 switch case 中设置。所以这段代码会是这样的；

int number1 = (int) (Math.random() * 10) + 1;

int number2 = (int) (Math.random() * 10) + 1;

int operator = (int) (Math.random() * 3) + 1;

//initalize value which is changing in swich case statement and set initializing value

String operation = "";

int correctResult = 0;

switch (operator) {

    case 1: {

        operation = "+";

        correctResult = number1 + number2;

        break;

    }

    case 2: {

        operation = "-";

        correctResult = number1 - number2;

        break;

    }

    case 3: {

        operation = "*";

        correctResult = number1 * number2;

        break;

    }

}

System.out.print(number1 + operation + number2 + ": ");

String studentAnswer = scanner.next();

Answer 3

问题是您没有遵循变量可见性范围。您需要计算括号 {} 这是一个通用示例。

    public void exampleScopeMethod  {

     String localVariable = "I am a local Variable"; 

     {

        String nextScope = " NextScope is One level deeper";

        localVariable += nextScope

      }

      {

         String anotherScope = "Is one level deeper than local variable, still different scope than nextScope";

         //Ooops nextScope is no longer visible I can not do that!!!

         anotherScope +=nextScope;

       { one more scope , useless but valid }

}

      //Ooopppsss... this is again not valid

      return nextScope; 

      // now it is valid 

      return localVariable;

     }

    }