Grouping Radio Buttons with ButtonGroup

Suppose a Swing window shows a row of checkboxes, and one of them looks wrong. It is still a box with a check mark, but the picture does not match the design you wanted. Before you can fix that, you need to know what the box on the screen actually is and which part of your program controls it. This article starts from that practical question and follows the behavior from the screen back to the mechanism that produces it.

Tracing means asking, at each step, which state the button is in and which icon belongs to that state. Start with a single checkbox that has been customized. Its unselected icon is the picture it shows when it is not checked. Its selected icon is the picture it shows once it becomes checked. A click moves the button from one state to the other, and the displayed icon follows the state.

The diagram shows the principle the source relies on. The icon is a function of the state, not of a separate drawing step. If the button is unselected, the unselected icon is the one to check. If it is selected, the selected icon is the one to check. Tracing a customized button is therefore a matter of following the state through each click or call and naming the icon that state uses.

This definition explains the appearance you see on screen. The checkbox does not decide, at drawing time, to add a check symbol. It holds two icons and uses whichever one matches its current selection state. The source describes this directly: you can change the look of the check box by substituting different icons for the standard ones.

The source names the methods that control each icon. The icon for the unselected state is the main icon of the JCheckBox. You can supply it when the component is constructed, or you can replace it later with setIcon(). The icon for the selected state is changed with setSelectedIcon(). Both methods belong to the JCheckBox object, so you call them on the particular checkbox you want to customize.

If you later want to change only the unselected appearance, call setIcon() on the same object. The selected icon set by setSelectedIcon() is a separate setting and is not replaced by that call.

The same two-icon customization applies to JRadioButton, JCheckBoxMenuItem, and JRadioButtonMenuItem. A radio button therefore has an unselected icon and a selected icon, and you customize them with the same two methods: setIcon() for the unselected appearance and setSelectedIcon() for the selected appearance.

The source points to ToolBarDemo.java as a concrete case. That program creates a set of radio buttons that use custom icons. It builds each button in a helper method whose documentation says it creates a JRadioButton and adds it to a specified button group. The button group is therefore part of the construction, so customizing a radio button happens during setup, alongside its grouping. The source excerpt does not explain how the button group behaves once its buttons are set up, so this article limits itself to the icon mechanism.

When a customized button looks wrong, the source gives a method: identify the state where the wrong appearance occurs, then check the call that sets the icon for that state. The question is not whether the appearance looks right in general. It is which step produced the first wrong picture.