Custom JOptionPane Icon

Question

Java's "How to Make Dialogs" tutorial shows this code:

//custom title, custom icon
JOptionPane.showMessageDialog(frame, "Eggs are not supposed to be green.",
        "Inane custom dialog", JOptionPane.INFORMATION_MESSAGE, icon);

which would create the following dialog:

Why is JOptionPane.INFORMATION_MESSAGE needed when the icon will just be changed to the icon parameter?

Answer 1

The flag also indicates which message style to use on the window decorations, see http://nadeausoftware.com/node/91#Usinglookandfeelspecificwindowdecorations

from the JOptionPane class source code:

private static int styleFromMessageType(int messageType) {
    switch (messageType) {
    case ERROR_MESSAGE:
        return JRootPane.ERROR_DIALOG;
    case QUESTION_MESSAGE:
        return JRootPane.QUESTION_DIALOG;
    case WARNING_MESSAGE:
        return JRootPane.WARNING_DIALOG;
    case INFORMATION_MESSAGE:
        return JRootPane.INFORMATION_DIALOG;
    case PLAIN_MESSAGE:
    default:
        return JRootPane.PLAIN_DIALOG;
    }
}

and in the method showOptionDialog which is called by showMessageDialog...

int style = styleFromMessageType(messageType);
JDialog dialog = pane.createDialog(parentComponent, title, style);