List Models and Dynamic Lists

A JList is a Swing component that displays a set of selectable items. Many programs use one to let a user choose from a set of options, such as a row of icons. Before looking at how a JList is built, try to predict what happens in a plain, unconfigured list. Suppose the user clicks one icon, and then clicks a different icon in the same list. Think about what you expect to see selected before you read the answer.

The rest of this article answers two questions. First, how does the array of objects you pass in decide what the list shows? Second, how do the selection rules come from code you write, not from the list itself? Keep the prediction in mind, because it is the behavior you will trace later.

A JList is built from an array. The objects stored in that array become the items the list displays. In the SillyStamper program, a Swing application, the images for the icons are read from resource files, placed into an array, and that array is then used to construct a JList of type Icon. The array is the source of the list's contents: whatever objects it holds are the objects the list is built to show.

Read the diagram as a question you can ask of any JList you build. The left column is the array that was passed in; the right column is what the list displays. Each displayed item traces back to one array object. If an icon is missing or wrong on screen, the array that was passed to the constructor is the first place to check, before looking at anything that happens afterward. The selection rules, which come later, do not change which objects appear.

Before changing anything, it helps to know what a JList does on its own. By default, the user can select any number of items in a list. The default gestures are listed below. The modifier key for the control-style gesture depends on the platform.

The SillyStamper program wanted only one icon to be chosen at a time. To impose that restriction, the program calls a method on the list that sets its selection mode. The call names a constant from the ListSelectionModel class, and the constant ListSelectionModel.SINGLE_SELECTION limits selection to one item at a time.

Notice what the second line does and does not say. It does not build the list, and it does not choose which icon is selected. It changes the rule governing selection on a list that already exists. Because the list is constructed first and configured second, a failure can come from either step, so the two steps should be checked separately.

The before-and-after comparison shows what the single-selection call removes. A plain click still selects one item. Shift-clicking and control-clicking, which in the default mode let the user build up several selected items, no longer produce a multiple selection once the mode is restricted to one item at a time.

When a JList behaves differently from what you expected, the goal is to find the line that caused it. The fastest method is to follow the order in which the program runs. The list is constructed from the array first, then its selection mode is set, and only then does user input reach the list. A flowchart of that order makes it easy to decide which line to inspect.

Several common errors make a JList look wrong when the real problem is in a different line. The items below are the ones most worth checking.