Error: local variable referenced from inner class must be final or effective final

Question

JButton[] button = new JButton[noOfDays];

for(int j=0 ;j<studentNameList.size() ;j++) {
    for(int i=0 ;i<button.length ;i++) {
        button[i]=new JButton((i+1)+"");
        attendencepanels.add(button[i]);

        button[i].addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
        ---->  button[i].setBackground(Color.red); //Local variable refenced from inner class must be final or effective final

                //JOptionPane.showMessageDialog(null, "test");
                }
            });

How i can resolve this problem in button[i] within actionPerformed method

either make it global to class or final as suggested by @jens — Pavneet_Singh, Sep 14 '16 at 06:21
Possible duplicate of [Inner class and local variables](http://stackoverflow.com/questions/10338708/inner-class-and-local-variables) — default locale, Sep 14 '16 at 06:22
Possible duplicate of [Local variables refered from inner class must be final or effectively final](http://stackoverflow.com/questions/27165006/local-variables-refered-from-inner-class-must-be-final-or-effectively-final) — Ravindra babu, Sep 14 '16 at 06:59

score 3 · Answer 1 · answered Sep 14 '16 at 06:23

While I'm not sure about it, you may be able to obtain the JButton reference from the ActionEvent e argument. It seems that's what getSource() returns (The object on which the Event initially occurred) :

Instead of

button[i].setBackground(Color.red);

try

JButton button = (JButton) e.getSource();
button.setBackground(Color.red);

score 1 · Answer 2 · answered Sep 14 '16 at 06:42

The variable i gets modified in this context, so the compiler cannot cope with it. Give it a non-changing reference instead:

    for(int j=0 ;j<studentNameList.size() ;j++) {
        for(int i=0 ;i<button.length ;i++) {
            JButton btn = new JButton((i+1)+""); // effectively final in that scope
            button[i] = btn;
            attendencepanels.add(btn);
            btn.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    btn.setBackground(Color.red); //works now
                }
            });
        }
    }

Error: local variable referenced from inner class must be final or effective final

2 Answers2

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