IF ELSE语句输出

Question

Why does message5 output even when courseNum is equal to 103 or 206? I think it has to do with where the last else statement is placed, but I've already tried moving it around and have gotten results where message5 would output in all the if statements.

      if (courseNum == 103)
      {
         JOptionPane.showMessageDialog(null,message1);
      }                  
      if (courseNum == 206)
      {
         JOptionPane.showMessageDialog(null,message4);
      }      
      if (courseNum == 106)
      {
         answer = JOptionPane.showInputDialog("Have you passed IT 102?");
         if (answer.equals("yes"))
         {
            JOptionPane.showMessageDialog(null,message2);
         }
         else if (answer.equals("no"))
         {
            JOptionPane.showMessageDialog(null,message3);
         }
      }
      else
      {
         JOptionPane.showMessageDialog(null,message5);
      }

Answer 1

You should use ... else if ... instead of just if :

  if (courseNum == 103)
  {
     JOptionPane.showMessageDialog(null,message1);
  }                  
  else if (courseNum == 206)
  {
     JOptionPane.showMessageDialog(null,message4);
  }      
  else if (courseNum == 106)
  {
     answer = JOptionPane.showInputDialog("Have you passed IT 102?");
     if (answer.equals("yes"))
     {
        JOptionPane.showMessageDialog(null,message2);
     }
     else if (answer.equals("no"))
     {
        JOptionPane.showMessageDialog(null,message3);
     }
  }
  else
  {
     JOptionPane.showMessageDialog(null,message5);
  }

Or, even better, a switch :

switch (courseNum) {
case 103:
    JOptionPane.showMessageDialog(null,message1);
    break;

case 206:
    JOptionPane.showMessageDialog(null,message4);
    break;

case 106:
    answer = JOptionPane.showInputDialog("Have you passed IT 102?");
    if (answer.equals("yes")) {
        JOptionPane.showMessageDialog(null,message2);
    } else if (answer.equals("no")) {
       JOptionPane.showMessageDialog(null,message3);
    }
    break;

default:
    JOptionPane.showMessageDialog(null,message5);
}

Answer 2

It's an else statement; it's what's executed if nothing else matches.

This is how your code looks (in terms of evaluating conditionals):

• if
• if
• if...else

That dangling else is going to be caught every time if its leading if is false.

What you likely wanted was to use else if statements instead.

Code below:

if (courseNum == 103) {
    JOptionPane.showMessageDialog(null, message1);
} else if (courseNum == 206) {
    JOptionPane.showMessageDialog(null, message4);
} else if (courseNum == 106) {
    answer = JOptionPane.showInputDialog("Have you passed IT 102?");
    if (answer.equals("yes")) {
        JOptionPane.showMessageDialog(null, message2);
    } else if (answer.equals("no")) {
        JOptionPane.showMessageDialog(null, message3);
    }
} else {
    JOptionPane.showMessageDialog(null, message5);
}

Answer 3

Please use a switch statement instead of an if else chain, it's going to keep the code more clear and you will find bugs in your code with more ease.

switch (courseNum) {
    case 103:
        JOptionPane.showMessageDialog(null, message1);
        break;

    case 206:
        JOptionPane.showMessageDialog(null, message4);
        break;

    case 106:
        answer = JOptionPane.showInputDialog("Have you passed IT 102?");
        if (answer.equals("yes")) {
            JOptionPane.showMessageDialog(null, message2);
        } else if (answer.equals("no")) {
            JOptionPane.showMessageDialog(null, message3);
        }
        break;

    default:
        JOptionPane.showMessageDialog(null, message5);
}