I can't seem to successfully add actionlisteners to JButtons in an ArrayList.

I have a class called PanelGenericButtons that creates a JPanel with some JButtons in it. I am attempting to keep this class as "dumb" as possible, so it only deals with the panel creation and layout, and it does not include the actionListeners. As the name suggests, it is intended to create a panel with an arbitrary number of buttons that is specified by a client-defined TreeMap (buttonInfo), as follows

		
ArrayList<JButton> buttons = new ArrayList<JButton>();
 
...
 
for (String s:  buttonInfo.keySet()){
			JButton b = new JButton();
			b.setText(s);
			b.setName(s);
			b.setMnemonic(buttonInfo.get(s));
			buttons.add(b);
			}

The buttons ArrayList can then be accessed by...

	public ArrayList<JButton> getButtonList() {
		return buttons;
		}

The client code then assigns actionListeners to the instance (called panel in this case) as follows...

		for (JButton b: panel.getButtonList()){
			b.addActionListener(buttonActionListener());		
			}

I have tested it as follows with an actionListener that changes buttonWasClicked to true, and the tests all pass

		for (JButton b: panel.getButtonList()){
			buttonWasClicked = false;
			b.addActionListener(runButtonActionListener());
			b.doClick();
			assertTrue(buttonWasClicked);
			}

...however, it doesn't work, because when I launch the panel, clicking the buttons does not yield the action specified.

The only thing I can think of is that JButton b is a copy or clone of the JButtons in buttons, but I'm struggling for an explanation.

Any suggestions would be welcome.

 

 

Note: This thread was originally posted in the Java Programming forum, but moved to this forum for closer topic alignment.

 

 

Hmmm, I can't tell where the problem is based on the code as presented, so perhaps the problem lies elsewhere. I think that your best bet here is to show us your problem in a way that we can actually see and work with. You should consider condensing your code into the smallest bit that still compiles, has no extra code that's not relevant to your problem, but still demonstrates your problem, in other words, an SSCCE (Short, Self Contained, Correct (Compilable), Example). For more info on SSCCEs please look here:

http://homepage1.nifty.com/algafield/sscce.html

Much luck!

 

 

To get better help sooner, post a SSCCE that clearly demonstrates your problem.

The variable in a for-each loop (aka enhanced-for loop) is a local variable. Assigning a new instance to the variable will have no impact on the object in the collection being iterated over; however, calling methods (as evinced in your code) will invoke those methods on the same instance.

db

edit For example, this SSCCE shows there is no problem with the parts of code you posted.

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.List;
import javax.swing.*;
 
public class ListOfButtons {
 
  List<JButton> buttons;
 
  public ListOfButtons(int count) {
    buttons = new ArrayList<JButton>(count);
    for (int i = 0; i < count; i++) {
      buttons.add(new JButton("Button " + i));
    }
  }
 
  public List<JButton> getButtons() {
    return buttons;
  }
 
  public void makeUI() {
    ActionListener listener = new ActionListener() {
 
      @Override
      public void actionPerformed(ActionEvent e) {
        JOptionPane.showMessageDialog(null, ((JButton) e.getSource()).getText());
      }
    };
 
    JPanel panel = new JPanel(new GridLayout(0, 1));
    for (JButton button : getButtons()) {
      button.addActionListener(listener);
      panel.add(button);
    }
 
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setContentPane(panel);
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }
 
  public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
 
      @Override
      public void run() {
        new ListOfButtons(6).makeUI();
      }
    });
  }
}

Edited by: DarrylBurke

 

 

Daryl,

Thanks for the reply.

I may give the SSCCE a try.

Before I do, I'll try to spot the difference between your code and mine. Has it got anything to do with the fact that your code adds the button to the panel after assigning the actionListener, but my code adds it to the panel before assigning the actionListener? Does the sequence make a difference?

 

 

It's trivial to try it and find out for yourself. Why do you think it might make a difference?

db

 

 

Aaarrgghh...

Found it.

I had called an "assemblePanel" method twice, so when I called a "getPanel" method, the code would recreate an exact duplicate of the panel, except for the assigned actionListeners.

Thanks for your attention anyway.