CODEWITHSUNDEEP
javavariablesevent-handlinginner-classesjlabel
Oscar Wahltinez
Oscar Wahltinez

Reputation: 1195

access to variable within inner class in java

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

Upvotes: 17

Views: 44938

Answers (5)

Devesh Jadon
Devesh Jadon

Reputation: 7474

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();
      }
   });
}

Upvotes: 2

Colin Hebert
Colin Hebert

Reputation: 93197

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 :

Upvotes: 27

user153498
user153498

Reputation:

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]);
}

Upvotes: 4

meriton
meriton

Reputation: 70584

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

Upvotes: 0

Isaac
Isaac

Reputation: 16736

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.

Upvotes: 0

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