Question

I am using mouse listener to know when a user clicks on nodes of a JTree. Although when the user clicks on the arrow for expansion of a node(view childs) the following exception is thrown:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Core.ChannelView$1.mousePressed(ChannelView.java:120)
    at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:263)
    at java.awt.Component.processMouseEvent(Component.java:6370)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)

ChannelView listener :

MouseListener ml = new MouseAdapter() {

            public void mousePressed(MouseEvent e) {
                TreePath selPath = tree.getPathForLocation(e.getX(), e.getY());
                if (e.getClickCount() == 1) {
line 120>>>>>        System.out.println(selPath.getLastPathComponent());

                } else if (e.getClickCount() == 2) {
                    System.out.println("Double" +selPath.getLastPathComponent());
                }
            }
        };
        tree.addMouseListener(ml);

Any suggestions about how should I handle this case ? Should I simply try-catch inside the if statement? Also is this a good way to check for double-clicks or I should be doing it with a different method ? Thanks

Answer 1

Your listener tries to get the node at the mouse location. If there isn’t any node, null is returned by tree.getPathForLocation(). Just test if selPath is null before calling a method on it:

if (selPath == null) {
    System.out.println("No node at this location");
}
else {
    if (e.getClickCount() == 1) {
    ...
}

And yes, getClickCount() returns the number of clicks associated with the event, so it seems appropriate to check if it’s a double or a simple click.