Local variable "i" is accessed within an inner class fix?

Question

I've seen some posts on this before, but I haven't been able to find one regarding actionListeners. I am trying to create tic-tac-toe using an array of JButtons. How do I add an action listener to them whilst using a for loop temporary variable if at all possible? Thanks for your help in advance.

JButton jb [] = new JButton[9]; int checkB [] = new int[9]; public SomethingSimple(){ JPanel p1 = new JPanel(); p1.setLayout(new GridLayout(3, 3, 5, 5)); p1.setBackground(Color.red); for (int i = 0; i < jb.length; i++){ checkB[i] = 0; } for (int i = 0; i < jb.length; i++){ jb[i] = new JButton(""); p1.add(jb[i]); jb[i].addActionListener(new ActionListener(){ @Override public void actionPerformed(ActionEvent e){ jb[i].setText("O"); } }); } add(p1); }

Thanks everyone for your help, you gave me some solutions!

Make it final. The fact that it's an ActionListener doesn't make any difference to the answer. — user207421
– user207421, Commented Apr 28, 2014 at 0:47
possible duplicate of Java: Anonymous inner class using a local variable — user207421
– user207421, Commented Apr 28, 2014 at 0:48

Rod_Algonquin · Accepted Answer · 2014-04-28 00:13:38Z

Create a final int inside the forloop; You cant access a local variable from your class to a anonymous class..

solution:

 for (int i = 0; i < jb.length; i++){ jb[i] = new JButton(""); final int index = i; jb[i].addActionListener(new ActionListener(){ @Override public void actionPerformed(ActionEvent e){ jb[index].setText("O"); } }); p1.add(jb[i]); }

I tried something similar, but it told me I couldn't change a final variable. I can't remember exactly what I did. Well I know now that I can do this. Thanks!

Alexey Malev · Accepted Answer · 2014-04-28 00:11:53Z

I think the best solution will be to move ActionListener to a separate class:

public class MyActionListener implements ActionListener { private final JButton button; public MyActionListener(JButton button) { this.button = button; } @Override public void actionPerformed(ActionEvent e){ button.setText("O"); } }

And change your code like this:

for (int i = 0; i < jb.length; i++) { jb[i] = new JButton(""); p1.add(jb[i]); jb[i].addActionListener(new MyActionLisener(jb[i])); }

Ian Roberts · Accepted Answer · 2014-04-28 00:14:49Z

0

Local and anonymous classes can only access final local variables, so do something like

final JButton btn = jb[i];

inside the for loop immediately after you create the new JButton and then you can refer to btn inside actionPerformed.

answered Apr 28, 2014 at 0:14

Ian Roberts

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3 Comments

matt Over a year ago

He still wants to use the iterator, though.

Ian Roberts Over a year ago

@Human I meant inside the for loop, I suppose it wasn't particularly clear the way I first wrote it.

matt Over a year ago

Oh, okay, that makes sense. Will btn still refer to the same object that jb[i] does?

Alexey Malev · Accepted Answer · 2014-04-28 00:40:40Z

This is coming from a C# programmer, who is 15, so take it with a grain of salt. First as I understand your question, you want to add the actionListener to all of your Jbuttons in the array jb, with a for loop. Basically in my java experience, you'll have to implement actionListener into your class(or into whatever class that'll act as the actual listener). Then your code should look something like this:

for(int i = 0; i < jb.length(); i++) { jb[i].addActionListener(class that implements the listener); }

Please tell me if I misunderstood your question.

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Local variable "i" is accessed within an inner class fix?

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