Action Listener

The user clicks the Do It! button after entering numeric text into both JTextFields. The click generates an ActionEvent which is passed to the registered listener for the button. The listener (the JFrame) implements ActionListener by including an actionPerformed() method.

There is the usual problem: the getText() method returns a String reference, but we need a double value. The type wrapper Double has a static method parseDouble() that can help. If the text that getText() gets can be interpretted as a floating point value, parseDouble() can convert it to a double value.

double calories ;  // input: total calories per serving
double fatGrams ;  // input: grams of fat per serving
double percent;    // result: percent of calories from fat

  . . . . . .
  
public void actionPerformed( ActionEvent   evt )
{
  String userIn ;
  
  userIn    = inFat.getText() ;
  
  fatGrams  = Double. ;
  
  userIn    = inCal.getText() ;
  calories  = Double. ;
  
  calcPercent() ;
  
  outPer.setText( + " " ) ;
  repaint() ;                  
}