[Java] Variables declared inside the `for statement` cannot be used outside the` for statement block`

  int classNum = 3; //Number of classes
  int stuNum = 4; //The number of students

  for (int i = 1; i <= classNum; i++)
  { //Repeat 3 times
      System.out.println(i + "Class");

      for (int j = 1; j <= stuNum; j++)
      { //Repeat 4 times
          System.out.print(j + "Please enter the grade of the person: ");
          sum += input.nextInt();
          int allStuNum = i * j;← This ↑ This ↓
          avg = sum / allStuNum;← This ↑ This ↓
      }
  }

Line 12 ʻint allStuNum = i * j; , Line 13 ʻavg = sum / allStuNum; When trying to use it outside the for block "Variable not found" error

Impressions

Maybe it's too common sense, so it's a point that isn't written in any introductory book, If you don't know it, you will be confused when you try to reuse the variables ʻi, j, ʻallStuNum in the for statement like yourself.