无法在ActionListener外部访问这些变量

Question

I have been stuck on this all weekend, i have looked everywhere and not a single solution. All help is appreciated.

    int iDayBirth = Integer.parseInt(jTextField_DoBDay.getText());
    int iMonthBirth = Integer.parseInt(jTextField_DoBMonth.getText());
    int iYearBirth = Integer.parseInt(jTextField_DoBYear.getText());

    int iDayCurrent = Integer.parseInt(jTextField_CdDay.getText());
    int iMonthCurrent = Integer.parseInt(jTextField_CdMonth.getText());
    int iYearCurrent = Integer.parseInt(jTextField_CdYear.getText());

    double iDaysAlive = 0;
    Calendar caBirthDate = new GregorianCalendar(iYearBirth, iMonthBirth - 1, iDayBirth);
    Calendar caCurrentDate = new GregorianCalendar(iYearCurrent, iMonthCurrent - 1, iDayCurrent);

    JButton btnCalculate = new JButton("Calculate");
    btnCalculate.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            iDaysAlive = (caCurrentDate.getTimeInMillis() - caBirthDate.getTimeInMillis());
            iDaysAlive = (iDaysAlive / (24 * 60 * 60 * 1000) + 1);
            // GET THE ERROR HERE: "Cannot refer to a non-final variable caBirthDate inside an inner class defined in a different method
            //get the same error for, caBirthDate and iDaysAlive
        }
    });
    btnCalculate.setFont(new Font("Calibri", Font.BOLD, 12));
    btnCalculate.setBounds(188, 187, 89, 23);
    frame.getContentPane().add(btnCalculate);

    jTextField_Result = new JTextField("Total Days Alive: " + iDaysAlive);
    jTextField_Result.setBounds(150, 233, 170, 20);
    frame.getContentPane().add(jTextField_Result);
    jTextField_Result.setColumns(10);

If i move the calender and integer variable to inside the ActionListener then i can't access the iDaysAlive variable in the final textfield.

Answer 1

As the error suggests:

Cannot refer to a non-final variable caBirthDate inside an inner class defined in a different method

You need to declare the variables final inorder to access them inside the ActionListener :

final int CURR_X = redSquare.getX();

But be careful:

Making a variable final means that it can/will not be modified by the program. It allows the compiler to optimize some of the code. Also it tells anyone reading the program the variables are CONSTANTS. For example in a program you would define the value of PI as a constant and use that variable in your program instead of typing in 3.14... everywhere you needed to have the value.

For this reason a good solution would be:

To create a class the implements ActionListener class. Then you can have any class access to the variables needed.

Another possible solution:

1. To wrap the needed variables inside a class as class variables.
2. Declare a variable from this class as final inside your main class.
3. Then use its class members to suit your needs

for example:

class Wrapper {
  public int iDaysAlive;

  public Wrapper (int iDaysAlive){
    this.iDaysAlive = iDaysAlive;
  }
}

Then:

int iDaysAlive = 0;
final Wrapper w = new Wrapper(iDaysAlive);

Then in ActionListener:

w.iDaysAlive = (caCurrentDate.getTimeInMillis() - caBirthDate.getTimeInMillis());

Answer 2

A solution is to create class the implements Action listener. Then you can give tha class access to the variable needed:

class ActionHandler implements ActionListener {
 public ActionHandler (MainClass mainClass) {
    //now getters of the main class can be called 
 }
    public void actionPerformed(ActionEvent e) { }
}

Answer 3

You declared an anonymous class of ActionListener. Refer to https://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html#accessing for oracle's documentation:

An anonymous class cannot access local variables in its enclosing scope that are >not declared as final or effectively final.

Refer to Why a non-final "local" variable cannot be used inside an inner class, and instead a non-final field of the enclosing class can? for why

A work-around for your situation: 1. declare iDaysAlive, caBirthDate, caCurrentDate and other variables you need to access inside the annoymous class in the class scope 2. refactor actionPerformed(ActionEvent e) to call a handler method

actionPerformed(ActionEvent e) {
     handleEvent(e);
}