CODEWITHSUNDEEP
javaswingjbuttonactionlistener
Leo Jiang
Leo Jiang

Reputation: 26075

Pass variables to ActionListener in Java

I have something like the code below:

    for(int i=0;i<10;i++){
        button=new JButton(buttons[i]);
        button.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e) {
                setPage(i);
            }
        });
        menu.add(button);
    }

However, the variable i isn't defined in the scope of the ActionListener class. How can I pass the variable?

Upvotes: 8

Views: 37586

Answers (3)

Robin
Robin

Reputation: 36601

A totally different approach would be to add a property to the button, and retrieve that property in your action listener. E.g. 

button=new JButton(buttons[i]);
button.putClientProperty( "page", i );
button.addActionListener(new ActionListener(){
   public void actionPerformed(ActionEvent e) {
      setPage((Integer)((JButton)e.getSource()).getClientProperty( "page" ));
   }
});

Upvotes: 22

JB Nizet
JB Nizet

Reputation: 691645

In addition to Hovercraft's answer, you should note that you're not forced to use anonymous classes for your listeners. The code of Hovercraft's answer is similar to the following one:

private class PageActionListener implements ActionListener {
    private int page;

    public PageActionListener(int page) {
        this.page = page;
    }

    public void actionPerformed(ActionEvent e) {
        setPage(page);
    }
}

...

for(int i = 0; i < 10; i++){
    button = new JButton(buttons[i]);
    button.addActionListener(new PageActionListener(i));
    menu.add(button);
}

Upvotes: 24

Hovercraft Full Of Eels
Hovercraft Full Of Eels

Reputation: 285405

The variable i is in fact in the scope of the ActionListener, but since you're trying to use a local variable in an inner class, the variable must be final. So, you could use a final variable for this:

for(int i=0;i<10;i++){
    final int index = i;
    button=new JButton(buttons[i]);
    button.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e) {
            setPage(index);
        }
    });
    menu.add(button);
}

Upvotes: 13

Related Questions