Java - How do you correctly add a ListSelectionListener to a controller?

Question

I've recently been struggling with a Java problem that has been driving me mad. I've been attempting to add a ListSelectionListener in my Controller to a JList in my View but when I have successfully managed to attach the listener to something, it has not been the JList that has been drawing on the screen.

The code below gives a basic idea of what I am attempting to do.

This is my class with my main method:

package Application;

public class Main {

/**
 * @param args
 */
public static void main(String[] args) {
    View v = new View();
    Controller c = new Controller(v);
}

}

This is my View:

package Application;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;

import javax.swing.BorderFactory;
import javax.swing.DefaultListModel;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JScrollPane;
import javax.swing.ListSelectionModel;
import javax.swing.ScrollPaneConstants;

public class View extends JFrame { 

/* Components for the JList */
public DefaultListModel<String> listModel = new DefaultListModel<String>();
public JList<String> selectedItems = new JList<String>(listModel);
JScrollPane scroll = new JScrollPane(selectedItems);

public View()
{
    // set the window title
    this.setTitle("JList Test");

    // set the window size
    this.setSize(new Dimension(400, 400));

    // set the window start position
    this.setLocation(25, 25);

    // set the window layout
    FlowLayout layout = new FlowLayout();
    layout.setHgap(0);
    layout.setVgap(0);
    this.setLayout(layout);

    // set the window background
    this.getContentPane().setBackground(Color.BLACK);

    // make the window non-resizable
    this.setResizable(false);

    // set the default close operation
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    // setup and add the JList
    initJList();
    this.add(scroll);

    // add elements to the list model
    listModel.addElement(" Item 1 ");
    listModel.addElement(" Item 2 ");
    listModel.addElement(" Item 3 ");

    // make the gui visible
    this.setVisible(true);
}

private void initJList() {
    selectedItems.setVisibleRowCount(8);
    selectedItems.setFixedCellWidth(300);
    selectedItems.setFixedCellHeight(40);
    selectedItems.setBorder(BorderFactory.createEmptyBorder(5, 10, 5, 10));
    selectedItems.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
    selectedItems = new JList<String>(listModel);
    scroll.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
    scroll.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
}
}

This is my controller:

package Application;

import javax.swing.event.ListSelectionEvent;
import javax.swing.event.ListSelectionListener;

public class Controller implements ListSelectionListener {

View gui;

public Controller(View v)
{
    gui = v;
    gui.selectedItems.addListSelectionListener(this);
}

@Override
public void valueChanged(ListSelectionEvent e) {
    System.out.println("Selection changed!");
}

}

It compiles OK and the list draws fine but the ListSelectionListener never fires when I select anything in the JList. Can somebody tell me where exactly I am going wrong? Because I have tried loads of things now and I am completely stumped! The thing I find strange is when I attempt to do what I am doing here with JButtons and an ActionListener it works absolutely fine.

Any help is appreciated.

Answer 1

you did a mistake there in initJList() function, you already have passed value of listModel before itself, and again you have passed this value here in this function , so commenting this line will solve the problem. the modified function will look like this here:

private void initJList() {
    selectedItems.setVisibleRowCount(8);
    selectedItems.setFixedCellWidth(300);
    selectedItems.setFixedCellHeight(40);
    selectedItems.setBorder(BorderFactory.createEmptyBorder(5, 10, 5, 10));
    selectedItems.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
    //selectedItems = new JList<String>(listModel);
    scroll.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
    scroll.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
}

Otherwise :

Declaring the JList like this 

public JList<String> selectedItems;

and keeping the function as it is will run fine.