access to variable within inner class in java

Question

I'm trying to create an array of JLabels, all of them should go invisible when clicked. The problem comes when trying to set up the mouse listener through an inner class that needs access to the iteration variable of the loop used to declare the labels. Code is self-explanatory:

    for(int i=1; i<label.length; i++) {
       label[i] = new JLabel("label " + i);
       label[i].addMouseListener(new MouseAdapter() {
          public void mouseClicked(MouseEvent me) {
             label[i].setVisible(false);   // compilation error here
          }
       });
       cpane.add(label[i]);
    }

I thought that I could overcome this by the use of this or maybe super instead of the call of label[i] within the inner method but I haven't been able to figure it out.

The compilation error is: local variable i is accessed from within inner class; needs to be declared final`

I'm sure that the answer must be something really silly I haven't thought of or maybe I'm making some small mistake.

Any help would be appreciated

Answer 1

This happens because label is not specified as final.

Declare the array of labels as:

final JLabel[] label;

instead of:

JLabel[] label;

Your MouseAdapter is not an inner class; it's an anonymous class. Anonymous classes can only refer to final variables of their enclosing code.

Answer 2

Anonymous inner classes may only access variables of the enclosing method that are final.

Answer 3

Your local variable must be final to be accessed from the inner (and anonymous) class.

You can change your code for something like this :

for (int i = 1; i < label.length; i++) {
    final JLabel currentLabel =new JLabel("label " + i); 
    currentLabel.addMouseListener(new MouseAdapter() {
        public void mouseClicked(MouseEvent me) {
            currentLabel.setVisible(false);   // No more compilation error here
        }
    });
    label[i] = currentLabel;
}

From the JLS :

Any local variable, formal method parameter or exception handler parameter used but not declared in an inner class must be declared final. Any local variable, used but not declared in an inner class must be definitely assigned (§16) before the body of the inner class.

---

Resources :

• JLS - Inner Classes and Enclosing Instances

Answer 4

If you're having a problem accessing i, make another variable outside the scope of your inner-class (e.g. before label[i].addMouseListener(...)):

for(int i=1; i<label.length; i++) {
   label[i] = new JLabel("label " + i);

   final int localI = i;
   label[i].addMouseListener(new MouseAdapter() {
      public void mouseClicked(MouseEvent me) {
         label[localI].setVisible(false);
      }
   });
   cpane.add(label[i]);
}