Temperature Conversion not working properly in JFrame

Question

I can not seem to figure out what I am doing wrong in my Event Handler; and I am not a good programmer. I can convert from Celsius to Fahrenheit but not the other way around. It usually shows 0.0 for Fahrenheit to Celsius conversions. I think it might have something to do with cfList which is my JComboBox to choose Celsius or Fahrenheit.

Also I want to be able to click the convert button without putting in a value, and the JTextField will automatically be set to zero and show a message that says "Value set to zero."

So how can I get this whole thing to work:

• Convert Celsius to Fahrenheit
• Convert Fahrenheit to Celsius
• Convert Celsius to Fahrenheit without a value(set to zero)

• Convert Fahrenheit to Celsius without a value(set to zero)

  class TCalcButtonHandler implements ActionListener { 
       public void actionPerformed(ActionEvent e) {
           double far, cel;
           String ff, cc;
           String inString;
  
           inString = tempField.getText();
           if(inString==null && cfList.getSelectedIndex() == 0)
           {
               degree.setText("F");
               cel=0;
               far = cel*(9/5)+32;
               ff = far+"";
               resultField.setText(ff);
               tempSysField.setText("Value set to zero");
           }
           if(inString==null && cfList.getSelectedIndex() == 1)
           {
               degree.setText("C");
               far=0;
               cel = (5/9)*(far-32);
               cc = cel+"";
               resultField.setText(cc);
               tempSysField.setText("Value set to zero");
           }
           if(cfList.getSelectedIndex()==0 && inString!=null)
           {
               degree.setText("F");
               cel = Double.parseDouble(inString);
               far = cel*(9/5)+32;
               ff = far+"";
               resultField.setText(ff);
               tempSysField.setText("");
           }
           if(cfList.getSelectedIndex()==1 && inString!=null)
           {
               degree.setText("C");
               far = Double.parseDouble(inString);
               cel = (far-32)*(5/9);
               cc = cel+"";
               resultField.setText(cc);
               tempSysField.setText("");
           }
       }
   } 
  

Answer 1

Your question title states:

Temperature Conversion not working properly ...

Almost always this is due to your doing int division unknowingly, and in fact, that's exactly what is going on:

far = cel*(9/5)+32;

Change 9/5 to 9.0/5.0

far = cel * (9.0 / 5.0) + 32;

Why is this important? int division (dividing one int by another) always returns an int, with the fractional part truncated away. So 9/5 returns 1 and 5/9 returns 0. If you want the calculation to retain the fractional portion, then at least one double has to be part of the division equation, so in fact 9.0 / 5 would work just fine as would ((double)9/5)