access to variable within inner class in java

问题

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

回答1:

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 parameter, or exception 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

回答2:

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]);
}

回答3:

you can also use getSource in your program. After this you can access your component with the help of typecasting. it will reduce extra lines of code, your code will look like this

for (int i = 1; i < label.length; i++) { 
   currentLabel.addMouseListener(new MouseAdapter(e) {
      public void mouseClicked(MouseEvent me) {
         JLabel label = (JLabel) me.getSource();
      }
   });
}